ENERGY VALUE

OF FOODS

• • • basis and

derivation

UNITED STATES DEPARTMENT OF AGRICULTURE

Agriculture Handbook No. 74 {^^^ ¡ ^ B
887524

ENERGY VALUE OF FOODS

. . . basis and derivation

by

Annabel L. Merrill

Bernice K. Watt

Human Nutrition Research Branch

AGRICULTURAL RESEARCH SERVICE

UNITED STATES DEPARTMENT OF AGRICULTURE

Agriculture Handbook No. 74—March 1955 ^^B^HB

For sale by the Superintendenf of Documents, U. S. Government Printins Office, Washinston 25, D. C. - Price 55 cents
CONTENTS

Introduction

Part I. Sources of food energyFatDetermination of fat content

Heat of combustion

Carbohydrate

Determination of carbohydrate content

Heat of combustion

Protein

Determination of protein content

Heat of combustion

Determined versus calculated gross energy values

of foods

Other sources of energy

Organic acids

Alcohol

Part II. Digestibility and available energy of foods._

Definition of terms _.

Digestibility of fat, carbohydrate, and protein

Availability of energy from digested nutrients

Fat

Carbohydrate

Protein

Alcohol

Page

1

1

2

2

2

2

2

3

4

4

4

6

7

8

8

8

9

9

10

10

18

Part III. Derivation of current calorie factors _ _ _ -

Physiological fuel values of foods of animal

origin

Physiological fuel values of plant products

Products of wheat

Products of grain other than wheat

Legumes

Nuts

Vegetables

Fruits

Miscellaneous foods

Part IV. Application of calorie factors _-

Comparison of calculated and determined available calories for diets

General factors and more specific factors for

calculating calories in individual foods

Application of general factors to national food

supplies

Conclusions

Literature cited

Appendix. Tabular summary of experiments on

digestibihty of foods of plant origin by human

subjects

Apparent digestibility and available energy

Composition and heat of combustion of foods—

Text Tables

1. Average determined heats of combustion of

fats and oils and assumed factors for fat of

different groups of food materials

2. Average determined heats of combustion of

different carbohydrates and assumed factors

for carbohydrates of different groups of food

materials

3. Factors for calculating protein from nitrogen

content of food

Average determined heats of combustion of

proteids and nonproteids and calculated heat

of combustion of protein

5. Comparison of calculated heats of combustion

with results of direct determinations

6. Fresh fruits classified as to organic acid content.

7. Factors for heats of combustion and fuel values

of nutrients in different groups of food materials and in mixed diet

8. Summary of data showing calorie-nitrogen ratio

of urine based on early studies of energy

metabolism and digestibility

Daily food intake in the experiments from which

Atwater originally obtained the calorienitrogen ratio of 7.9 for urine

Daily nutrient intake in the experiments from

which Atwater originally obtained the calorienitrogen ratio of 7.9 for urine

Comparison of data for available energy

obtained by direct determination only and

in part by calculation

4.

9.

10.

11

10

12

16

17

19

12. Effects on energy metabolism of replacing

portions of dietary carbohydrate and fat by

alcohol

13. Data used for calculating energy values of foods

or food groups by the Atwater system

14. Energy values of wheat flours calculated by use

of specific energy factors for protein, fat, and

carbohydrate.

15. Apparent digestibility and physiological fuel

value of wheat flours

Coefficients of apparent digestibility for grain

products

Comparison of determined and calculated gross

energy values of potatoes

Summary of steps for checking available

energy values calculated by factors from

table 13

Comparison of determined and calculated

available energy values of various types of

diets

20. Factors for digestibility, heats of combustion,

and physiological fuel values of nutrients in

food groups as used in present-day mixed

diets

21. Comparison of energy values for different

dietary patterns calculated with specific and

with general calorie factors

16.

17.

18.

19.

Page

24

26

26

27

31

36

36

37

40

40

43

44

48

48

51

51

58

58

100

22

25

28

29

32

38

44

46

49

50

Appendix Tables

22.

23.

Use of digestibility data to determine coefficients of apparent digestibility and available

energy 59

Apparent digestibility and available energy of

foods of plant origin for human subjects.- 60

Grains, grain products 60

Legumes and nuts 87

23.

24.

Apparent digestibility, etc.—Continued

Vegetables, vegetable products

Fruits

Miscellaneous

Composition and heat of combustion of food

items used in experiments on human digestibility (table 23)

94

98

100
Energy value of íoods

. . . basis and derivation

INTRODUCTION

Accurate evaluation of the energy value of foods

is essential for dealing with problems of normal

nutrition, undernutrition, or obesity. The classic

investigations of Professor W. 0. Atwater and his

associates at the Storrs (Conn.) Agricultural Experiment Station some 50 years ago provided the

basis used in this country for measuring the energy

values of food. The general calorie factors 4, 9, 4

developed from that work gained widespread

acceptance, and until recently they were used for

calculating the calories shown in official food composition tables. Properly applied, these general

factors provide a satisfactory measure of available

energy in average diets and food supplies in this

country. Following Atwater's period little attention was given to methods of calculating food

energy and to the details of Atwater's procedure.

However, in recent years attention has again

turned to the important problems of determining

and meeting man^s energy needs. In attempts to

alleviate food shortages experienced duiing and

following World War II consideration was given

first to meeting energy needs in stricken areas.

Maynard; who represented this government in

various interallied food-planning groups, pointed

out the necessity of understanding the bases of

the different methods for estimating energy values

in use in Canada, the United Kingdom, and in

this coimtry. On several occasions he called attention to the correct application of the general

calorie factors 4, 9, 4 and pointed out their limitations and misuse when apphed to individual foods

and diJSFerent tjipes of diets {114, 115)}

The Food and Agriculture Organization, faced

with the -urgency of assessing energy values of food

supplies in various countries and population groups,

convened an ad hoc committee of experts in 1947

to study the problems involved and to make

recommendations. While endorsing the Atwater

method as one that in the light of present knowledge is suitable if properly used, the committee

pointed out the limitations of the use of general

factors and the need for more specific calorie

factors {65) when dealing with individual foods.

These developments have pointed to the need

for summarizing the kinds of information Atwater

used, the steps followed in his procedure for determining fuel values of food, and the need for revising

calorie data for foods to take account of additional

research accumulating since his time. This publication has been prepared to provide more background information on food energy data than that

given in current textbooks and food tables and to

show the basic data drawn upon in deriving the

revised calorie factors now used in tables of food

composition in this country. Except for a few

recent revisions, factors derived as shown in this

publication have been used in U. S. Department of

Agriculture Handbooks No. 8 {185) and No. 34

{100) and in various other sources, including food

tables published by the lood and Agriculture

Organization {36),

PART I. SOURCES OF FOOD ENERGY

The chief food sources of energy to the human

body are fat, carbohydrate, and protein. Fats

and carbohydrates contain carbon and hydrogen

which can be oxidized to their end products, CO2

and H2O, both in the bomb calorimeter and in the

body. In addition, protein contains nitrogen.

This nitrogen together with some carbon and

hydrogen leaves the body chiefly in the form of

^ The authors express appreciation to Mildred Adams

for her review of the manuscript and her invaluable suggestions; to William Kunerth for his generous help in

translating numerous articles from German; and to Blanche

C. Spears for her collaboration in various phases of the

study.

urea. Thus protein is incompletely oxidized in

the body, whereas it can be completely oxidizedfin

the calorimeter. The heat released by oxidation

of food in the bomb calorimeter is its heat of

combustion and is a measure of its gross energy

value.

Rubner {147), as early as 1885, realized that

each of these broad groups of energy-yielding

components of foods consisted of substances of

more or less unlike composition and that the heat

values for pure protein, neutral fat, and pure

2 Italic figures in parentheses refer to Literature Cited,

p. 51.

1
carbohydrate might not be applicable to foods.

He also recognized that methods of determining

how much of each is present in a food were not

entirely satisfactory. Innumerable improvements

in methods and techniques for separating and

determining the fractions making up these three

main sources of energy in food have been made in

the intervening years, but many of the limitations

of determining and dealing with the main sources

of energy in food that were pointed out in 1890

by an ad hoc Committee of the Association of

Official Agricultural Chemists {5) still remain.

In the following sections the terms as they are

used today in tables of food composition are

discussed so that their meaning and limitations

will be better understood.

Fat

Determination of fat content

The fat content of foods usually is determined

by one of three general methods: (1) simple extraction with a solvent, (2) acid hydrolysis with

extraction, and (3) saponification with extraction.

The fat content reported for foods in American

tables of composition refers as a rule to the weight

of crude fat and is obtained by simple extraction

with a solvent, usually ether. Included with the

fatty acids and the true fats (triglycérides) thus

extracted are other materials having similar

solubility, such as the sterols, and chlorophyll and

some other pigments. Special precautions are

necessary to insure complete extraction; carbohydrate-containing foods, particularly those high

in starch, present additional problems {61, 66, 105,

A method based on acid hydrolysis before extraction gives, in addition to substances listed

above, fats which are in combination or which for

other reasons are not removed by the usual fat

solvents. Egg and yeast have been showQ to

contain appreciable amounts of fat not extracted

without preliminary hydrolysis (75).

The third procedure used in determining the

fat content of a food, saponification, is usually

followed by extraction and titration of the fatty

acids. The data obtained by this method are

translated into terms of total fat on the assumption that all the fatty acids are present as

triglycérides.

The determination of fat in foods is fraught

with complications. Particular care is necessary

to avoid oxidation of fat during sample preparation aod analysis, loss of volatile fatty acids, and

the possible formation of esters of fatty acids with

alcohol.

Heat of combustion

The heat of combustion of the ether extract

from a food depends on the particular fatty acids

making up the triglycérides and on the components and proportions of the other ether-extractable materials present. The triglycérides of beef.

mutton, and pork fat have been found to have

heats of combustion of 9.50 or 9.51 calories per

gram; butterfat, 9.27; and the fats from several

common plant sources, about 9.3. Lower figures

for heats of combustion have been found for total

ether extract, indicating that the extractable

matter other than the glycerides has a lower heat

value than the glycerides alone.

Atwater {17) applied the heat of combustion

factors determined for triglycérides to crude fat

on the assumption that the error resulting from

the use of the higher heat of combustion factors

would in some measure offset the error resulting

from the incomplete extraction of fat in the determination of the fat content of the food. The

table containing the data which Atwater assembled from the literature and from his own

work and the heat values he considered best

suited to apply to the fat content (ether extract)

has been reproduced here as table 1.

TABLE 1.—Average determined heats oj combustion

offats and oils as assumed factors forfat of different groups offood materials

Heat of combustion per gram

Kind of material

Determined Assumed or

calculated

Beeffat._ .__ _.. ___ __

Calories

9.50

9.24

9. 51

9.32

9.50

9. 13

9.59

9.32

9.27

9.36

9.07

9. 32

9.20

9.28

8.93

9.07

9. 49

9. 47

9.07

Calories

Beef "ether extract" __ _ Mutton fat

Mutton, ''ether extract".

Pork fat. .__ __ _ __ ___ .

Pork, ''ether extract". __ _ _

Lard

Cottolene.

Butterfat- _.

Wheat oil

Wheat, "ether extract" _

Rye oil-_

Rye, "ether extract"- _ Maize oil

Oats, "ether extract"

Barley, "ether extract" .-

Nut oil (except cocoanut) _ _

Olive oil _ _

Cocoanut oil

Fat of meat, fish, eggs, etc__ _ . 9 50

Fat of dairy products- _ _- 9 25

Fat of cereals- ___ _ 9 30

Fat of vegetables and fruits__ 9. 30

NOTE.—This table appears as table 7 in The Availability and Fuel Value of Food Materials (17).

Carbohydrate

Determination of carbohydrate content

The difference between 100 and the sum of the

crude protein and fat, moisture, and ash is caUed

''total carbohydrate'' or ''carbohydrate by difference,'' a practice used by Atwater in his food

tables and contiaued in this country. In addition

to the true carbohydrates, this "difference" fraction may include such compounds as organic acids.
Foods of animal origin, except the milk products, contain little carbohydrate. Foods of plant

origin, on the other hand, have a variety of carbohydrates. The principal ones are starch, sugars, and cellulose, but appreciable amounts of

pentosans, dextrins, gums, and other carbohydrates also may be present. It has been generally assumed that the starches, at least when

cooked, and the monosaccharides and disaccharides are well used by the body. Much less is

known about the utilization of cellulose, pentosans, and other of the more complex carbohydrates.

^^Carbohydrate by difference'' has been shown

to be generally satisfactory for estimating energy

values of foods (17). However, for certaia purposes, such as dietary planning for the diabetic,

carbohydrate values are needed which exclude the

fractions that are not potential glucose formers.

For these purposes nitrogen-free extract, '^carbohydrate by difference" minus fiber, may be calculated. As the digestibility of fiber may be very

low, nitrogen-free extract is considered a much

closer estimate of the sum of potential glucose

formers than the ''carbohydrate by difference.''

''Nitrogen-free extract," sometimes abbreviated to

NFE or Nifext, has been used for classifying fruits

and vegetables into different carbohydrate groups

(2,37).

Another approach has been the determination of

the sum of the sugars, starches, and dextrins

measured as total reducing sugars but exclusive of

pentoses and hemicelluloses. In such cases it is

fairly common to report total reducing sugars expressed as glucose based on analyses in which copper was used. For routine determinations, this

procedure is not entirely satisfactory since the extent of the reduction of the copper reagent differs

for the various sugars, and mixtures of sugars may

be present. In addition the determination may

be complicated by the presence of noncarbohydrate reducing substances. Improvements have

been made in procedures involving the use of copper reagents, and progress is also being made in

the development of totally different methods which

may some day provide the specific information

needed. For example, differential fermentation,

Chromatographie separation, and differential spectrographic analysis give promise of quantitative

determinations for specific carbohydrates.

Heat of combustion

Atwater assumed that 97 percent of the carbohydrate in flours and meals was composed of starch

with a small amount of fiber, about 2 percent

dextrin, and 1 percent sugar. As the heats of

combustion of dextrin, 4.11, and of sucrose, 3.96,

are not greatly different from that for starch, 4.2,

he considered 4.2 calories per gram the suitable

factor to use for carbohydrate in cereal foods. He

also applied this figure to the carbohydrate content

of foods consisting largely of starch, such as cornstarch and tapioca, and to dried legumes because

he considered that the carbohydrate'portion of the

latter consisted mainly of starch.

In many vegetables the carbohydrate is largely

starch and cellulose with more or less sugar.

Atwater suggested the same calorie factor for

vegetables that he had used for cereals and for

legumes, 4.2 calories per gram. He thought that

vegetables had a higher proportion of pentosans

than the cereals and that the higher heat of combustion of pentosans as compared with polyhexoses

might offset the lower heat value of the sugars.

In fruits a large proportion of the carbohydrate

is present as sugar, especially monosaccharides,

but some starch, cellulose, and pentosans also are

present. Combining the lower heat of combustion

of the sugars with the higher value for starch,

Atwater considered that 4.0 calories per gram was

probably not far from a correct figure for carbohydrate in fruits.

The main carbohydrate of animal source is mük

sugar. Atwater found that figures on record for

its heat of combustion were not in agreement and

he used 3.9 calories per gram. Muscle meats and

fish contain traces of glycogen, which in ordinary

analyses is not taken into account. Oysters, other

shellfish, and liver, however, may contain an appreciable amount of glycogen, which has a heat of

combustion of 4.2 calories per gram. Since the

amounts of these foods contained in ordinary diets

were small, Atwater used 3.9 calories per gram of

carbohydrate in all foods of animal origin for general dietary calculations.

The table prepared by Atwater summarizing

data on heats of combustion to apply to carbohydrate is reproduced here as table 2.

TABLE 2.—Average determined heats of combustion

of different carbohydrates and assumed factorsfor

carbohydrates of different groups offood materials

Kind of material

Pentoses ^

Dextrose

Lévulose

Cane sugar

Milk sugar

Cellulose

Starch

Dextrin

Glycogen

Carbohydrates

foods, meats,

ucts, etc

Carbohydrates of cereals

Carbohydrates of legumes

Sugars

Starches

Carbohydrates of vegetables.

Carbohydrates of fruits

of animal

dairy prodHeat of combustion per gram

Determined

Calories

3. 72 to 4. 38

3. 75

3. 76

3.96

3. 86

4.20

4.20

4. 11

4. 19

Assumed or

calculated

Calories

3.90

4.20

4.20

3.95

4.20

4.20

4.00

1 Apparently includes not only the simple pentoses but

also the pentosans.

NOTE.—This table appears as table 8 in The AvailabiUty

and Fuel Value of Food Materials {17).
Protein

Determination oí protein content

It is customary in this country to calculate the

protein content of a product from the nitrogen

present by applying a factor considered suitable

for converting nitrogen to the protein in the

particular food. The factors used are based on

the nitrogen content of the predominating protein present in various foods. As a great many

commonly occurring proteins contain approximately 16 percent nitrogen, 6.25 is the factor often used

for general purposes. In the course of extensive

investigations, however, Jones (76) found rather

wide variation in the nitrogen content of different

kinds of protein, for example, 13.4 percent for an

alcohol-alkali-soluble protein preparation from

avocado and 19.3 for amandin in almonds. He

therefore prepared special factors for converting

nitrogen to protein in those foods for which he

considered there was sufficient information to justify their derivation. Table 3 lists these factors

along with others obtained from him through

personal communication.

TABLE 3.—Factors ior calculating protein from

nitrogen content offood ^

Food Factor Food Factor

Animal origin: Plant origin—Con.

Eees 6. 25 Legumes—Con.

Gelatin 5.55

6.25

Beans—Con.

Meat Soybeans 5.71

Milk. - 6.38 Velvetbeans_

Peanuts

6. 25

Plant origin: 5. 46

Grains and cereals: Nuts:

Barley 5.83 Almonds 5. 18

Corn (maize) __ 6.25 Brazil 5.46

Millets 5.83 Butternuts 5. 30

Oats 5.83

5. 95

5.83

b. 25

Cashew

Chestnuts

Coconuts

Hazelnuts

5. 30

Rice 5. 30

Rye 5. 30

Sorghums 5. 30

Wheat: Hickory 5. 30

Whole kernel- 5.83 Pecans 5.30

Bran 6.31 Pine nuts 5. 30

Embryo 5.80 Pistachio 5.30

Endosperm. _ 5. 70 Walnuts 5.30

Legumes: Seeds:

Beans: Cantaloup 5. 30

Adzuki 6.25 Cottonseed 6.30

Castor 5.30 Flaxseed 5.30

Jack 6.25 Hempseed 5.30

Lima 6.25 Pumpkin 5.30

Mung 6.25 Sesame 5. 30

Navy 6.25 Sunflower 5.30

1 Adapted from table 5 of U. S. Department of Agriculture Circular 183, revised edition, February 1941 (76) and

from unpublished data obtained by personal communication with the author. For groups of foods not included

here, the conventional factor 6.25 should be used until

more is known regarding their proteins.

The figures commonly reported in American

tables of composition for protein actually represent crude protein, since as a rule the figures are

derived by applying the appropriate factor to the

total nitrogen present. This procedure mvolves

the assumption that all of the nitrogen present is

in the form of protein, which is not wholly valid

because in this procedure counted with the true

protein may be other nitrogenous compounds,

such as nitrates, nitrites, purine bases, cholme,

and free amino acids.

Heat of combustion

The heat of combustion of the nitrogenous

portion of food depends on the kinds of protein

present and on the proportion of protein and nonprotein nitrogenous material—the latter usually

having lower heat of combustion than the former.

Atwater's procedure for obtaining a figure for

the heat of combustion for the total nitrogenous

portion of a food may be illustrated by his figures

for cereal grains having 17.5 percent nitrogen in

their proteins. The protein would therefore be

computed by multiplying the nitrogen by the

factor 5.7. He assumed, from analyses of Teller

(4), Snyder (163), and Wiley (183), that not less

than 96 percent of the nitrogen of the seeds of

cereals was in the form of protein and not over

4 percent as nonprotein material. One gram of

cereal nitrogen, then, would be equivalent to

5.47 grams of protein (0.96 gm. N X 5.7) and,

using asparagin (21.2 percent N) as a model of

the nonprotein nitrogenous fraction, 0.19 grams

of asparagin (0.04 gm. N X 4.7).

Applying to the protein portion the heat of

combustion of the principal proteins in the cereals,

about 5.9 calories per gram according to Atwater's

data, and to the nonprotein portion, the heat of

combustion of asparagin, 3.45 calories per gram,

the total heat of combustion for the nitrogencontaining compounds in cereals was calculated

as follows:

5.47 gm. protein X 5.9 cal./gm. =32.27 calories

.19 gm. asparagin X 3.45 caL/gm.= .655 calories

5.66 gm. nitrogenous compounds=32.9 calories

1.0 gm. nitrogenous portion=5.8 calories

For the heat of combustion of the nitrogenous

portion of meat, Atwater felt the most satisfactory

procedure was to use the value for the fat-free

muscle tissue including the nonprotein extractives,

as quantitative data on creatin and other nonprotein compounds were lacking. The heat of

combustion for fat-free muscle meat was about

5.65 calories. He used this same factor for the

protein of milk. He estimated the heat of combustion for the nitrogenous portion of egg to be

5.75 calories per gram, based on data for proteins

in the white and yolk, assuming that very little

nonprotein nitrogen is present.

Table 4 is a reproduction of one prepared by

Atwater showing average determined heat of combustion of '^proteids" and ^^nonproteids'^ and

calculated heat of combustion of "protein.'' Atwater used the term "proteid" to cover the true

proteins, and the term "protein" to cover both the
food can be shown by using potatoes as an example—a food known to contain a considerable

portion of nonprotein nitrogen. If 60 percent of

potato nitrogen is attributed to protein and 40

percent to asparagin, the heat of combustion of the

nitrogenous matter equivalent to 1 gram of nitrogen should be 28.2 calories and the heat of combustion per gram of nitrogenous compounds, 5.01

calories, as shown by the calculations below:

0.6 gm. NX6.25=3.75 gm. protein

0.4 gm. NX4.7=^1.88 gm. asparagin

1.0 gm. N=5.63 gm. nitrogenous compounds

3.75 gm. proteinX5.8 cal./gm.=21.75 calories

1.88 gm. asparaginX3.45 cal./gm.^6.49 calories

5.63 gm. nitrogenous compounds=28.24 calories

1.0 gm. nitrogenous compounds=5.02 calories

Kind of material

Determined

nonprotein compounds, the extractives, amides,

etc., and the true proteins. If Atwater's heat of

combustion values for protein (as defined by him)

is applied to protein as currently determined, that

is, total N times a factor, some error will result

because the heat of combustion of the true proteins

is usually higher than that of other nitrogenous

compounds. It has become the custom in this

country, however, to apply heat of combustion

factors to total nitrogen treated as protein without

weighting the composition data according to the

proportion of the different nitrogen-containing

compounds present. This has been done because

of the limited information available on the partition of nitrogen in foods between true protein and

other forms. Although this procedure may result in an appreciable error in the calorie value of

the protein of a food, the error in the total energy

value is generally small, as most foods having a

large proportion of their nitrogen as nonprotein jf however, all of the nitrogen is assumed to be

lutrogen (mostly vegetables and fruits) contam p^^^^-^^^ (^ 25 gm. protein) and to this is appUed

relatively small amounts of total mtrogen. ^^^^ f^^^^^. 5 Q2 calories (corrected as shown above

.rn A A j • 7 I r r ' fo^ thc lowcr hcat of combustion for the nonproTABLE 4..--Average determined heats of combustión ^^;^^ portion), an energy value of 31.4 calories per

ofproteids and nonproteids and calculated heat of ^^^^ nitrogen results \l X 6 25 X 5.02) This

combustion of protein ^^3^1^ -g ^bout 11 percent higher than that obtained in the first calculation because the content

Heat of combustion per gram ^^ protcln is Overestimated. If no correction is

A.um d r made for the presence of nonprotein nitrogenous

cScSated"^ compouuds and if the higher heat of combustion

of potato protein, 5.8 calories per gram, is applied,

Calories an energy value equivalent to 36.25 calories per

gram of nitrogen would result (1 X 6.25 X 5.8).

This result is nearly 30 percent higher than the

first calculation because there has been overestimation in both the content of protein and the heat

^' ^^ of combustion of the nonprotein fraction. This

[lllllllll illustration shows that we should have data on

the actual partition products, but until we do, it

S- 75 seems best to continue the rather arbitrary procedure shown here as the second calculation,

5.'65 namely, to apply a weighted calorie factor to total

nitrogen treated as protein.

Determined versus calculated gross

energy values or roods

5.80

Gross energy may be determined directly by

^' ^^ burning a sample of food, or it may be calculated

5 00 by applying previously determined heats of combustion to composition data on the energy-yielding

5- 20 components of food and obtaining the sum.

In view, however, of the diversity within the

fractions of the so-called protein, fat, and total

carbohydrate components of food pointed out m

preceding paragraphs, and in view of the assumptions made in deriving heat of combustion values

Note.-This table appears as table 6 in The Availability to apply to each fraction, Atwater recognized the

and Fuel Value of Food Materials (17). importance of checking the gross energy values

calculated for foods. He compared results ior

The effect of method of calculation on estimated calculated and determined gross heats of combusenergy values for the nitrogenous compounds in a tion for 276 samples mcludmg foods ot animal

Beef, fat-free muscle

Beef, fat-free muscle, extractives removed

Teal, fat-free muscle

Mutton, fat-free muscle

Protein of meat

F]gg albumin

Egg, protein of yolk

Yitellin

Protein of egg

Milk casein

Milk protein

Protein of dairy products

Gliadin

-Glutenin

Gluten of wheat

Legumin

Plant fibrin

Protein of cereals (96% proteids)

Protein of legumes (96% proteids)

Protein of vegetables (60%

proteids)

Protein of fruits (70% proteids)

Gelatin

Greatin, as type of non-proteids of animal foods

Asparagin, as type of nonproteids of vegetable foods..

Calories

5. 65

5.73

5. 65

5. 60

5.71

5.84

5. 76

5. 63 to 5. 86

5. 67

5.92

5.88

5. 95

5.79

5.89

5.27

4. 27

3. 45
origin as well as a variety of plant products. He

found that when he applied the heats of combustion he had worked out for protein (actually

nitrogenous compounds), fat (as ether extract),

and carbohydrate (usually determined by difference) to amounts present, the results were in good

agreement with those obtained by bomb calorimeter. Although in a few cases discrepancies

were as much as 5 or 6 percent, agreement was

very much closer in most cases and justified the

use of the calculated values.

The table in which Atwater summarized these

comparisons has been reproduced here as table 5.

Possibly the difficulties in getting satisfactory

composition data for dried samples of high original

water content was responsible for the larger

discrepancies observed between the calculated

and determined gross heats of combustion for

fruits and vegetables. Differences might be

expected for milk likewise and may have been

observed for individual samples, but the averages

for the 37 samples are in excellent agreement.

With the improved techniques in moisture determinations now available, we would expect even

better agreement between the gross heats obtained

by calculation and the determined values.

TABLE 5.—Comparison oj calculated heats oj

combustion with results oj direct determinations

Kind of food material

Beef

Beef, canned

Mutton

Pork

Poultry

Fish

Eggs

Butter

Milk

Breakfast foods

Bread, crackers, etc

Corn (maize) meal and corn

preparations

Rye preparations

Barley preparations

Rice

Oatmeal (rolled oats)

Oatmeal, cooked

Wheat, pastry

Legumes, fresh

Legumes, cooked

Vegetables, fresh

Vegetables, cooked

Vegetables, canned

Fruits, fresh

Fruits, canned

Average 276 samples

Number

of

analyses included

in

average

55

7

10

10

5

3

10

20

37

3

36

Average heat of

combustion per

gram of waterfree substance

Determined

Calories

6507

6197

7146

7835

6310

6317

7103

8832

5437

4367

4536

7 4580

6 4353

2 4352

5 4390

2 4834

6 4488

8 4579

8 4367

5 4312

10 4195

3 4057

2 4264

12 4389

4 4078

Calculated

Calories

6619

6268

7316

7944

6508

6427

7160

8918

5413

4360

4513

4624

4343

4365

4474

4811

4480

4605

4361

4343

4051

4277

4102

4123

4056

Calculated

results

in percentages

of those

determined

Percent

101. 7

101.2

102. 4

101. 4

103. 1

101. 8

100.8

101.0

99. 6

99.8

99. 5

101.0

99.8

100.3

101. 9

99.5

99. 8

100. 6

99. 9

100.7

96. 6

105.4

96.2

93.9

99.5

100.3

jQ-ote.—This table appears as table 9 in The Availability

and Fuel Value of Food Materials {17). Figures for heat

of combustion are in terms of small or gram calories rather

than large or kilogram calories customarily used for foods.

We have compiled gross calorie data for a

number of samples of wheat and flours produceü

in this country. For 15 samples of wheat or whmewheat flour reported in the literature {164^ P^y

168,183,194, 195) differences between determined

and calculated gross heats varied from 0.1 percent

to 1.9 percent and averaged only 0.6 percent. J^or

16 additional samples of wheat flour of varymg

degrees of refinement the average difference

between gross calories obtained the two ways was

slightly higher, 1.3 percent.

Other sources of energy

Two other sources of energy—organic acids and

alcohol—are noted below since in some circumstauces one or both may be important.

Organic acids

Occurrence of organic acids.—Organic acids

are widely distributed in foods but for the most

part in small concentrations. Among the various

acids that have been identified are: Malic, citric,

isocitric, ascorbic, oxalic, lactic, succinic, acetic,

quinic, tartaric, benzoic, glyoxalic, sahcylic, aconitic, and malonic. As explained earlier, figures for

the total carbohydrate content of a food, that is,

''carbohydrate by difference,'' include organic

acids. In a very few foods the acids are suflEiciently abundant that they should be determined

separately for estimations of energy values of

those foods, inasmuch as they are distinctly different chemically from carbohydrates and their heats

of combustion are lower than for carbohydrates

generally. Total free acid is commonly determined by titration against standard alkali and

expressed as the predominant acid in the food.

To the extent that the organic acids may be

present in bound form the total acid value may be

underestimated, but this error is ordinarily considered of little importance.

Fruits contain organic acids in more significant

amounts than other food groups. In table 6 a

number of fruits have been classified according to

the total free organic acid content as reported in

the literature. Citric and malic acids predominate in all fruits listed except grapes and tamarind.

Tartaric acid accounts for most of the total in

these two fruits. Other organic acids have been

found present in small amounts in fruits. Of the

fruits listed in table 6 only 7 have been reported to contain more than 2 percent organic

acid; 15 contain from 1 to 2 percent; and more than

35 contain less than 1 percent. However, in proportion to the total solids, the organic acids may

provide an appreciable percentage of the total

energy value of some fruits. For lemon juice, it

would amount to over half, but for peaches, only

about a twentieth.

Less information is available on the acid constituents of vegetables, but the amounts in most

vegetables tend to be less than 0.5 percent.
TABLE 6.—Fresh fruits classified as to organic acid content ^

3 percent and over 3 2 to 3 percent 1 to 2 percent 0.5 to 1 percent Less than 0.6 percent

Lemons (C). Cranberries (C). Apricots (M). Apples (summer) (M). Apples (fall) (M).

Limes (C). Currents, red, black, Carissa or natal plums Blackberries (C). Apples (winter) (M).

Tamarind (T). and white (C). (C). Blueberries (C). Bananas (M).

Gooseberries (C). Cherries, sour (M). Cherries, sweet (M). Cherimoya (C).

Grandillas, purple, or Grapefruit, all (C). Crab apples (M). Feijoa (C).

passion fruit (C). Groundcherries (in- Grapes, pulp or juice, Figs (C).

cluding poha and American type, all Jujubes (C).

cape-gooseberry) (C). (T). Limes, sweet (C).

Kumquats (C). Grapes, European type, Muskmelons (C).

Loganberries (C). all (T). Papayas (C).

Loquats (C). Guavas (C). Pears, all (C).

Nectarines (M). Mamey or Mammee Persimmons, Japanese

Oranges (C). apple (C). or Kaki (M).

Plums, excluding Mangos (C). Persimmons, native

prunes (M). Mulberries, black, (M).

Pomegranates (C), white, and red (M). Prickly pears (M).

Raspberries, red and Peaches, all (M). Roseapples (C).

black (C). Pineapples (C). Sapodilla or sapota (C).

Strawberries (C). Plantains (M). Sapote or Marmalade

Tangerines, other Prunes (M). plum (C).

Mandarin type Quinces (M). Sugar apples or

oranges (C). sweetsop (C).

Watermelons (M).

1 Total free acid expressed in terms of the predominating acid as malic (M), anhydrous citric (C), or tartaric (T) in the edible portion of fruit.

2 Lemons and limes, 6 percent; tamarind, ripe, 13 percent.

Hartman and Hillig (63) ^ reporting results from

analyses of organic acids in a large number of food

products, included a table of 29 vegetables which

showed a total malic and citric acid content (free

and combined) ranging from 0.1 to 0.8 percent.

Only for lima beans, cauliflower, white potatoes

(Idaho), and tomatoes were the values above 0.5

percent.

In certain types of processing by fermentation

the total acidity of the product is increased several

fold over the original content of the food. Cabbage, for example, contains only a fraction of a

percent of acid as malic and citric, while sauerlo-aut has around 1.5 percent lactic acid. Similarly apples contain less than 1 percent acid

expressed as malic, but vinegar made from apples

averages about 4.5 percent acetic acid.

Some of the acid constituents of food are

available to the body as a source of calories;

others are known to be unavailable or of doubtful

availability. Oxalic acid is probably excreted in

the form of its insoluble calcium salt; tartaric

acid is thought to be either excreted unchanged

or destroyed by micro-organisms. Little is known

about the availability of such acids as glyoxalic,

malonic, and aconitic, but since they occur in

insignificant amounts they would make a negligible

contribution to the total energy value of the foods

in which they are found.

Heat of combustion.—For the several acids

found in appreciable amounts and considered

available, the heats of combustion or gross calorie

values per gram of acid calculated from grammolecular weight data are as foUows:

Calories

Acid: per gram

Acetic 3. 488

Citric 2. 471

Lactic 3. 620

Malic 2. 388

Organic acids contribute a very small portion

of the total daily calorie intake, but in a few

foods they are present in amounts that should not

be overlooked as potential sources of energy.

The gross energy value of organic acids in 100

grams of a few foods has been estimated as follows:

Food: Calories

Lemons, limes 15

Currants, gooseberries 6

Fruits, 1-2 percent group (see table 6) 2.5 to 5

Apple vinegar 16

Sauerkraut 5

Alcohol

Alcohol, with a gross energy value of 7.07

calories per gram, is another source of energy

which may be important in the diet of sonie

individuals or some population groups. It is

discussed in connection with the availability of

energy from the various sources (p. 18) since the

availability of its fuel value is the point of uncertainty.

305774—55-
PART II. DIGESTIBILITY AND AVAILABLE ENERGY OF FOODS

Définition oí terms

Meanings of some of the terms necessary in a

discussion of energy value of foods have changed

over a period of years. In the following paragraphs terms of most importance are explained

and attention is called to differences in past and

present connotations.

Digestibility was the term Atwater used for

the proportion of food material actually digested.

If there had been a way to measure the undigested

residue in the feces, digestible food would have

been computed as the difference between the total

food eaten and the undigested residue. However,

as he pointed out, methods for distinguishing

between metabolic products in the feces and

undigested residue from the food were not sufficiently accurate to permit the determination of the

undigested residue separately and he did not

compute digestibility.

Availability was the term Atwater used to

designate the quantity or proportion of the food

or of the nutrients which could be used for building and repair of tissue and the yielding of energy.

Some of the absorbed nutrients are used to form

digestive juices and returned to the tract in the

form of hue and other digestive secretions. Inasmuch as these metabolic products are not used

for tissue building or as fuel, they are not available in the sense in which Atwater employed the

term. He computed the amounts of available

nutrients (protein, fat, carbohydrate) by subtracting the amounts in the feces from the amounts

in the food. Availability as Atwater used the

term is the same as apparent digestibility in more

recent years and in current usage. He calculated

the coefficient of availability, using nitrogen for

illustration, as foUows:

N in food—N in feces

N in food XI00=coefficient of

availability.

According to present usage this would be called

the coefficient of digestibility, meaning of course

apparent digestibility, and it corrects only for

total fecal losses.

Heat of combustion data are obtained by burning

samples of food in a bomb calorimeter. The heat

of combustion is a measure of the gross energy

value of the food.

Available energy of a food takes into account

both fecal and urinary losses. The total available

energy of the food is its heat of combustion less

that of the lu-inary and fecal residues. For fat

and carbohydrate the available energy is the gross

energy of the amounts absorbed (intake—fecal

fat and carbohydrate) since each nutrient is

assumed to be completely oxidized. The incompletely oxidized matter of the urine is assumed to

be of protein origin and the available energy of

protein is the gross energy of the absorbed protein

(intake—fecal protein) less the gross energy of

the urine. Available energy of a food may be

obtained entirely from data on heat of combustion

or it may be calculated in part from analytical

data on nitrogen according to the foliowing|procedures:

1. Gross energy of food— (gross energy of urine+

feces).

2. Gross energy of food — (gross energy in feces+

net absorbed grams NX7.9).

3. Gross energy of food— (gross energy in feces+

urinary N in gramsX7.9).

If the subject is in nitrogen balance, no difference

would be expected in the deduction for urinary

loss between procedures 2 and 3. A discussion>f

the extent of the differences resulting from ^these

methods of calculation under other conditions

follows the section on calorie-nitrogen ratio of the

urine, page 18.

Atwater distinguished between physical and

physiological fuel values, the latter being the actual

benefit gained by the body from the use of fuel for

the different purposes served. This distinction

was made in recognition of the possibility that the

energy value of a gram of fat, for example, might

be different for mechanical work from what it

would be if used only for maintaining body heat.

Atwater used the term fuel value as obtained by

method 1, 2, or 3 described above to mean physical

fuel value, not physiological fuel value. The

latter term, however, has since been applied to his

data and to his method of obtaining fuel values

{65, 111, 169). Likewise, in the present publication physiological fuel value is the term used to

connote energy value of a food obtained by subtracting energy lost in the excreta (feces and urine)

from the total energy value of the food, no consideration being given to the specific functions

served in the body.

Digestibility of fat, carbohydrate,

and protein

On any diet some ether extractable matter,

nitrogenous matter, and other organic matter are

lost in the feces and must be taken into account in

calculating the energy value of foods. The nitrogenous matter present in the feces may be due in

part to undigested food residues, bacteria and their

products, the residues of digestive juices, and

mucus or particles of epithelium mechanically

separated from the walls of the digestive tract.

Numerous studies have been made to determine to

what extent the nitrogenous matter in the feces

under different kinds of dietary conditions is

metabolic and to what extent it is undigested or

unabsorbed food material. Some investigators

have concluded that all the nitrogenous matter in

the feces results from metabolic processes but that

8
some foods cause greater loss than others {104,

106, 147). Other workers, including Murlin and

coworkers {4.0,127,128) and Bricker, Mitchell, and

Kinsman (Sl), as a preliminary step in obtaining

biological values of proteins, have estimated the

digestibility of foods with the assumption that

part of the fecal nitrogen is metabolic in origin and

part is from food eaten.

Since this publication is concerned primarily

with estimation of energy value no attempt has

been made to distinguish between metabolic and

undigested food nitrogen appearing in the feces,

because neither is available to the body as a source

of energy. Actually, level of N intake may appreciably affect the apparent digestibility of protein;

on low levels of protein intake the fecal N may

represent chiefly metabolic N which, when charged

against a specific test food, leads to low values for

apparent digestibility of this food. Results reported in the literature in which digestibilities of

test foods were measured under conditions of

extremely low protein intake are therefore not

satisfactory for application to a more normal level

of protein intake. Even under conditions of

higher protein intake, losses attributed to the

protein of the test food by this method of calculation may actually be due to the influence of the

test food on the digestibility of the entire diet.

Similar problems occur in calculating the energy

factors for carbohydrate and fat {188, 190, 191).

More information or possibly an entirely different

approach is needed to relate fecal losses directly to

the test food.

Atwater assembled results of many digestion

experiments on men in which the apparent digestibility of a food was studied. In some experiments a single food was fed and in others the test

food was fed as part of a simple mixed diet.

From these findings he developed tentative coefficients of digestibility. As they had been

based largely on the digestibility of single foods

in very simple diets, Atwater tested these tentative

coefficients by applying them to the several foods

in experiments in which ordinary mixed diets

were eaten. In these latter experiments the

amount of protein, fat, and carbohydrate in the

feces was compared with that in the total food so

that the '^ availability'' measured applied to the

whole mixed diet and not to nutrients in individual

foods. The results found for these actual experiments were then compared with the calculated

results in which the various tentative coefficients

for each kind of food had been applied to the

quantities of the respective foods in the diet.

Atwater reported that some adjustments in the

tentative coefficients were necessary and he altered

them slightly in the way he considered most

probable. The resulting average coefficients of

apparent digestibility (availability as Atwater

used the term) for the nutrients in different food

groups and for nutrients in a mixed diet were as

follows:

Food group Protein Fat Carbohydrate

Animal foods _ _ _

Percent

97

85

78

Percent

95

90

90

Percent

98

98

97

98

Cereals

Legumes, dried

Sugars and starches

Vegetables 83

86

84

90

90

90

95

90

Q7

Fruits

Vegetable foods__

Total food Î ._ 92 95 97

1 Weighted by consumption statistics based on a survey

of 185 dietaries.

When these coeiBcients were applied to data in

93 digestion experiments on ordinary mixed dieta

very good agreement was found between calculated

values and the results of actual determination.

The calculated coefficient for protein in the whole

diet was 93.6, and that found by actual determination, 93,3; for fat the calculated value was 94.5

and that found by determination, 95.0; for carbohydrate the calculated value was 98.1 and the

actual value, 97.7. From this Atwater concluded

that for average mixed diets the calculated

coefficients were close enough to the determined

so that the calculated could be used. But he

pointed out that the calculated coefficients might

not be applicable under all circumstances and

might not apply to all foods in one class.

Digestibility studies published since his time have

indeed shown rather wide differences among foods

within these groups.

A review of the literature shows that in most of

the experiments very simple diets have been used

in which the test foods made up a large proportion

of the total diet. In experiments where the test

foods were fed alone or contributed essentially all

of the nutrients tested, the supplemental action

of one food upon another cannot be observed.

Woods and Merrill {193) reported that some of

their early digestion experiments with men showed

milk and bread to be more completely assimilated

when fed together than when eaten separately.

A similar conclusion was reached by Bryant {32)

regarding milk and oatmeal when fed together and

separately to infants. Unfortunately there is not

adequate basis at this time for estimating how

significant the differences in digestibility are under

different conditions of diet intake.

Availability of energy from digested

nutrients

Fat

Atwater illustrated his method of estimating

the fuel value of fat (ether extract) with the fat

of meat. The coefficient of digestibility (current

usage) had been determined to be about 95 percent. As its heat of combustion was about
9.5 calones per gram, its fuel value was 9.0 calories

per gram (9.5X.95=9.02).

Carbohydrate

The fuel value of carbohydrates was determined

in like manner. For example, cereal carbohyhydrate was considered about 98 percent available

(absorbed) for use in the body, and using the heat

of combustion of 4.2 calories per gram, the fuel

value was 4.1 calories (4.2X.98=4.12).

Protein

For protein (nitrogenous products), in addition

to the use of the coeflScient of digestibility, it was

necessary to correct for the loss of incompletely

oxidized nitrogen from the body. To do this

Atwater determined the ratio of the nitrogen in

the urine to the heat of combustion of the urine.

The average of 46 determinations showed that for

TABLE 7.—Factors jor heats oj combustion andjuel

and in

every gram of nitrogen present in the urine there

was sufl&cient unoxidized matter to yield 7.9 calories, the equivalent of approximately 1.25 calories

(7.9-^6.25) per gram of available (absorbed) protein. The heat of combustion of a gram of absorbed protein (nitrogenous compounds) was

therefore reduced by 1.25 calories per gram to

allow for incomplete metabolism. In the case of

digestible meat protein, for example, the heat of

combustion per gram is 5.65 calories. Of this

number, 1.25 would be deducted for the heat of

combustion of the unoxidized products in the

urine. This figure was derived from the ratio of

the calorie value of the urine to the nitrogen content of the urine on the assumption that the subjects were in N-equilibrium and that all of the

nonmetabolized part of the available N was recovered in the urine. The fuel value, 4.40 calories, would then be applied to each gram of protein

available as a source of fuel.

values oj nutrients in different groups offood materials

mixed diet

Kind of food material

Nutrients

furnished by

each group

per 100 grams

Total

Heat of combustion per

gram

Proportion

of total

nutrient

actually

available

Total energy

per gram in

available

nutrients

Fuel value

Per gram

available

nutrients

Per gram total nutrients

Protein

Meats, fish, etc

Eggs

Dairy products

Animal food

Cereals

Legumes

Vegetables

Fruits

Vegetable food

Totalfood

Fat

Meat and eggs

Dairy products

Animal food

Vegetable food

Totalfood

Carbohydrates

Animal food

Cereals

Legumes

Vegetables

Fruits

Sugars

Vegetable food

Totalfood

D

= (B X C) F2

Orams

43. 0

6.0

12.0

Calories

5.65

5.75

5.65

0.97

.97

.97

Calories

5. 50

5. 60

5.50

Calories

4.40

4. 50

4.40

Calories

4. 25

4. 35

4.25

61. 0 5. 65 97 5. 50 4.40 4. 25

31. 0

2.0

5.5

. 5

5.80

5. 70

5.00

5. 20

, 85

, 78

83

85

4.95

4. 45

4. 15

4. 40

4.55

4. 45

3.75

3.95

3.70

3.20

2.90

3. 15

39.0 5. 65 85 4.80 4. 40 3. 55

100.0

60. 0

32.0

5. 65

9. 50

9. 25

. 92

95

95

5. 20

9. 00

8. 80

4. 40

9.50

9. 25

4.00

9.00

8. 80

92.0

8.0

9. 40

9.30

95

90

8.95

8. 35

9. 40

9.30

8.95

8.35

loa 0

5.0

55. 0

LO

13. 0

5.0

21.0

9. 40

3.90

4.20

4. 20

4. 20

4.00

3. 95

95

,98

98

97

95

90

98

8. 90

3.80

4. 10

4.05

4.00

3. 60

3.85

9.40

3.90

4. 20

4.20

4.20

4.00

3.95

8. 90

3.80

4. 10

4.05

4. 00

3.60

3.85

95. 0

100. 0

4. 15

4. 15

97

97

4. 00

4.00

4. 15

4. 15

4.00

4.00

F revised '

Calories

4.27

4.37

4.27

4.27

3.87

3.47

3. 11

3.36

3.74

4 05

9.03

8. 79

8.93

8. 37

8.93

3.82

4. 11

4.07

3.99

3.60

3.87

4. 03

4.03

1 Values for fats and carbohydrates, same as corresponding values in column B. Values for protein, same as

corresponding values in column B minus 1.25.

2 Values for fats and carbohydrates, same as corresponding values in column D. Values for protein, same as

corresponding values in column D minus 1.25.

3 Proportion of total nutrients available (column C)

applied to heat of combustion values (column B). (Heat

10

of combustion values for protein adjusted for energy loss in

the urine by deduction of 1.25.)

NOTE.—This table appears as table 10 in The Availability and Fuel Value of Food Materials {17) with the

exception of column F, revised. The figures in this column

appear in tabular form in Investigations on the Nutrition

of Man in the United States {98, p. 18).
The basic data needed for computing fuel value

of a diet were brought together by Atwater and

Bryant in a table, reproduced here as table 7.

They presented two sets of factors for use in estimating energy values. In column E of their table

they listed the factors to apply to a gram of available protein, fat, and carbohydrate in each of the

various food groups and the average calorie factors

per gram, 4.40, 9.4, and 4.15, to apply to the total

amounts of protein, fat, and carbohydrate available in a mixed diet. The factors in column E

were therefore to be appHed to absorbed nutrients.

The fuel value factors listed in column F included a correction for digestibihty loss and were

to be applied to grams of ingested protein, fat, and

carbohydrate in each of the food groups; the

average factors rounded to 4.0, 8.9, and 4.0 calories

per gram were to be applied to the total amounts

of the nutrients in mixed diets. These then were

the factors that they considered could be applied

directly to representative data on the chemical

composition of foods.

For some time after the publication of this work

of Atwater and Bryant, apparently no consistent

pohcy was followed with respect to the factors

used to estimate energy values of foods {6, 8, 10,

18, 19, 20, 68, 89, 157, 169, 171). For a period of

time the Atwater and Bryant general factors appeared in the literatm-e as 4, 8.9, 4; then a reference to a further rounding of the factors to 4, 9, 4

was made in the 1910 revision of Farmers' Bulletin 142 (11), The 4, 9, 4 factors later came into

widespread usage in estimating calorie values of

food and not only were applied to the total

amounts of protein, fat, and carbohydrate (by

difference) of a mixed diet as Atwater and Bryant

had originally intended but also were used in

assessing the fuel value of individual foods.

Following the publication of the 1899 report, it

was realized that for protein the number of calories

calculated by applying factors in column E to

absorbed nutrients was not identical with the

number derived by applying factors in column F

to total nutrients. Results obtained by the latter

were too low. The error resulted from the misuse

of the factor 1.25 derived from a gram of protein.

It had been applied to protein which, after digestion loss was taken into account, was less than 1

gram. To illustrate: If a subject ingests 1.0

gram of protein the gross fuel value of which is

5.65 calories, and if only 0.97 gram is digested,

the gross available calories are 0.97X5.65, or 5.48.

Since only 0.97 gram is available from each gram

of ingested protein, only 0.97X1.25 or 1.21 calories should be deducted. Thus for 1 gram of

ingested protein, the available energy value would

be 5.48—1.21, or 4.27 calories. This is the same

as 0.97 (5.65-1.25).

Corrected values for column F were written in

file copies 3 of the report and have been included

as column F revised here in table 7. The corrected values were also published by Langworthy

and Milner in 1904 in a summary of investigations

on the nutrition of man in this country (98). This

publication may not have had wide circulation

and has seldom been cited. The revised values

make for consistency in the use of columns E and

F. It should be pointed out that the revised

figures for column F were unrounded in contrast

to the values in columns D and E in the original

table.

The calorie value per gram of urinary nitrogen.—

Several questions have been raised on the advisability of applying 7.9, the calorie-nitrogen ratio

in urine pubhshed by Atwater (12, 17), to energy

calculations for which dietary conditions may be

greatly different. Lusk (101) summarized data

showing that the ratio was affected by the proportion of dietary protein, fat, and carbohydrate.

Other questions have been raised concerning the

effect of negative or positive nitrogen balance,

and of high-fruit diets having more than the usual

amount of organic acid.

Unfortunately, at the present time no record is

at hand showing the specific experiments from

which Atwater derived the ratio of 7.9 calories per

gram of urinary nitrogen and from it concluded

that 1.25 calories per gram of available protein

should be subtracted for loss of incompletely

oxidized material in the urine.

As early as 1897 Atwater and Benedict in the

Storrs Agricultural Experiment Station report for

that year (12,. p. 167), stated, '' . , . the heat of

combustion of the water-free substance of the

m-ine will be 1.25 calories for each gram of digested

(available) protein. This factor is the average

found in a number of experiments in this laboratory, in which the heat of combustion of the

water-free substance of the urine was determined."

At the time this statement was published, results

probably were available from the first 16 of a

series of 55 experiments on the metabolism of

matter and energy in the human body conducted

under Atwater's supervision. We found the ratio

of the heat of combustion of urine to urinary nitrogen when calculated for these 16 experiments to

average 7.9 calories, or the equivalent of 1.26 calories per gram of absorbed protein (7.9-^6.25=

1.26).

The study that included the 55 metaboHsm

experiments was made at Middletown, Conn.,

during the years 1896-1902 under the auspices

of the U. S. Department of Agriculture in cooperation with the Storrs (Conn.) Agricultural Experiment Station and Wesleyan University. The

subjects were normal healthy men of similar

weight, around 65 to 79 kg.

3 A note on one of the marked copies on file in U. S.

Department of Agriculture reads, ''A copy showing corrections as made on slips sent to Magnus Levy in letter'^of

July 6, 1904."

11
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14
A complete outline of the work, and the procedures followed were given in considerable detail

by Atwater and Benedict (16). Those aspects of

the study thought to have a direct bearing on the

calorie-nitrogen ratio of the urine will be referred

to here. Some additional details are given in the

section on alcohol (p. 18). Selected data from

the 55 experiments have been summarized in

table 8 along with data from a later series in which

Benedict and Milner continued the study.

Atwater designated the first foiu* experiments

of the series as "respiration experiments;^' for

these, analyses were made of food intake, drink,

feces, urine, and respiratory products. No determinations were made of the heat given off from

the body nor of the heat equivalents of external

work in these experiments. He called the remaining experiments. Nos. 5 through 55, ''metabolism

experiments.'^ They included measurements

of energy in addition to the data obtained in the

respiration experiments. Each metabolism experiment had two parts, a digestion experiment in

which the subject lived under ordinary conditions

and the metabolism experiment proper in which

the subject lived in a respiration chamber. Digestibility data were available from the second

part as well as from the first part of each metabolism experiment.

The respiration calorimeter, described in detail

in U. S. Department of Agriculture Bui. 63 (21),

was especially designed for this series of experiments. It included among other equipment a

bed and a stationary bicycle with an ergometer for

measuring external muscular work, thus providing

for the study of metabolism of matter and energy

under conditions of rest and strenuous activity.

In the so-called work experiments, the activity

of the subject varied but in most cases he rode

the stationary bicycle for 8 hours daily. During

the preliminary digestion period, prior to the work

periods within the calorimeter, the activity of

the subject was sometimes comparable to that

during the work period and sometimes was only

his normal activity with some additional light

exercise. This latter activity was designated as

''hght'^ in table 8 to differentiate it from the

more strenuous activity of pedaling the stationary

bicycle for 8 hours daily, referred to as a ''work''

experiment. In the "rest" experiments the subject

remained quiet, avoiding all muscular activity as

far as it was practical.

Certain precautions were taken to minimize

errors in the nitrogen and energy determinations.

The urinary nitrogen was determined in 6-hour

intervals throughout the day, using the Kjeldahl

method. A portion of each collection was reserved

as part of a composite sample for the day. Nitrogen and the heat of combustion were determined

on a portion of this composite and the remainder

was preserved by adding formalin or thymol.

This became a part of the composite sample for

the whole period of usually 3 to 4 days. The

analysis of the total composite sample checked

closely with results obtained when the urine was

analyzed each day. This assured the investigators

that no significant error occurred from nitrogen

or energy loss in the urine during storage.

The heats of combustion were determined by

the Kellner method. A weighed absorption block

of cellulose of known heat of combustion was

saturated with a known amount of urine, dried

at about 60° C, and burned in a bomb calorimeter. The results were corrected for the heat

of combustion of the absorption block. This latter

factor was an average of determinations for a

number of similar blocks. The method was given

in detail by Atwater and Snell, 1903 (22).

^ The investigators took into account the possibility that a lag in nitrogen excretion by the

subjects would introduce some error in urinary

estimations in the relatively short experimental

periods of 3 to 4 days. This possible error was

reduced by having periods on the same diet run

consecutively. In addition to the incompletely

oxidized matter lost iu the urine the perspiration

losses should be recognized. However, as nitrogen

losses have been shown by 25 work experiments of

Atwater and Benedict (16) to be small, averaging

only 0.29 gram per day, and as data on compa-^

rabie energy loss in the perspiration were lacking,,

the data in table 8 apply to urinary losses only.

The series was plaimed to study metabolism (1)

while fasting, (2) when the proportions of fat and

carbohydrate of an ordinary diet were varied,,

and (3) when a moderate amount of alcohol

replaced fat and carbohydrate isocalorically. In

the first 16 experiments rather simple mixed diets

were used as shown in table 9. For these experiments the amounts of protein, fat, and carbohydrate, and the gross calories found by determination were reported. The amounts of other

nutrients present in the diet have been calculated

from tables of nutrient composition; these calculated values are shown in table 10.

In the annual report for 1899 (17) Atwater continued to use the same factor, 1.25 calories per

gram of digested protein, in his calculations of

available energy, although he recognized that this

deduction was not accurate for all foods. Some

error is introduced when this correction, based on

the factor 6.25 to convert nitrogen to protein, is

used with proteins or with nonproteins containing

more or less than 16 percent nitrogen. In the

same publication he mentioned briefly the derivation of the basic figure 7.9 calories per gram of

urinary nitrogen. He stated that the figure was

based on the average of 46 determinations. They

were mainly from his laboratory with a few from

Chas. D. Wood of the Maine Experiment Station.

In addition to the first 16 experiments conducted

prior to the 1897 report, the next 25 of the series

may have been completed before the 1899 report

was prepared. Possibly these 41 experiments,

together with 5 unpublished from the Maine

Experiment Station, made up the 46 experiments

to which Atwater referred in the 1899 report.

15
TABLE 9.—Daily food intake in the experimentsfrom which Atwater originally obtained the calorie-nitrogen

ratio of 7.9 for urine

Respiration experiments Metabolism experiments Digestion experiments

Food item

1 2 3 41 6 6 7 8 37 39 41 43

Beef, fried

Qrams

121

Grams

121

Grams

96

Grams

96

Grams

120

25

Qrams

100

""50'

54

850

Grams

169

25

Qrams

150

Qrams

121

25

Grams

100

""52'

52

850

Qrams

170

25

Grams

150

Beef, dried _ _ __ _. _

Ham deviled

Eggs _ __ ""98"

1,000

75

35

'"iôr

500

75

35

""iöö'

660 ~'65Ô"

95

775

141

575

95

850

107

775

144

575

103

Milk, whole (assumed raw)

Cheese

850

Butter. . 20 45

250

150

35 75 15 35 34 75 15 35

Bread, brown

Bread white

"325"

450 45Ö

Bread, rye _ _ _ 250

100

228

100

275 150 325 316 150 328

Crackers, milk

Oatmeal 40

6

38

125

Sugar - __ """2Ó" "~'4Ö' ""46" ""20'

120

100

125

35

125

50

125

45

125

40

125

50

125

45

125

40

Beans, baked 125

Potatoes, boiled in skins

Apples

150 150 270

85

140

210

200 200

Peaches

Pears, canned _ _ _ 150 300 150

72. 5

150 300 150

Alcohol 72. 5

Includes experiments 4P, 4A, 4B, 4C, and 4S. (See table 8.)

In the 25 additional published experiments in

this series conducted prior to 1900, the diet was

modified somewhat as compared with the first 16.

It consisted of beef, whole or skim milk, butter,

bread, cereal breakfast foods, graham crackers,

ginger snaps, and sugar. The estimated nutrient

intake was similar to that of the preceding experiments except that the ascorbic acid content was

lower, probably only between 10 and 20 milligrams

per day. The average calorie-nitrogen ratio for

the 41 experiments, 7.88 (table 8), is not different

from that found for the first 16 alone, 7.86.

The calorie-nitrogen ratio of the urine in these

41 experiments showed a wide variation with a

range from 5.22 to 10.54. As the number of

experiments under any one set of conditions was

limited, it is scarcely feasible to conclude from this

series how different factors such as level of intake,

extent of digestibility, type of diet, and degree of

activity influenced the calorie-nitrogen ratio of the

urine. To the data in table 8 aheady mentioned,

we have added data selected or calculated from the

rest of the 55 metabolism experiments completed

after 1899, and data from a series of metabolism

experiments, numbers 56-67 by Benedict and

Milner {27), which was actually a continuation of

the earlier series of Atwater and Benedict. Benedict and Milner resumed the investigations of

matter and energy in 1903. We have included

data from these studies for reference since copies

of the various publications in which the experiments were reported are no longer readily available

and they furnish much valuable basic data.

The diets of the experiments conducted in 1900

and later showed very wide variations in gross

calories and in the levels of protein, fat, and carbohydrate. The urinary calorie-nitrogen ratio for

these experiments varied from 6.44 to 10.36. Both

extremes were within those observed for experiments conducted prior to 1900; the average was

8.32, a little higher than for the preceding

experiments.

Many other studies have been made in which

data on urinary nitrogen and energy have been

reported. To facilitate further study of this

problem, some of these are noted below.

Rubner {H8) determined the calorie-nitrogen

ratio in urine on a variety of mixed diets, reporting

an average ratio of 8.5. But a number of years

later in a paper with Thomas {151) he reported

that the ratio was between 7 and 8, although he

had found variations outside this range. Among

other problems Rubner {H8) studied the influence

of level of fat, single foods, and periods of rapid

growth on calorie-nitrogen ratio of the urine and

summarized the results as follows:

Food Calories per

gramN

Duration of

experiment

Mother's milk^ • _ 12. 10

6.93

7.71

8. 57

a 33

8.87

8.44

6. 42

7.50

7. 69

7. 85

Days

Cow's milk, infants 7

Cow's milk, adults ___ .__ ___ __. 7

Diet poor in fat _ _ _ 2

Do ~ 4

Diet rich in fat _ _ 2

Do __ 4

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Eubner and Thomas {151) found the urinary

calorie-nitrogen ratio for a subject on a diet solely

of potatoes to be 9.04, 11.92, and 10.09 for the 1st,

4th, and 6th days respectively—^ratios which were

much higher than Eubner had observed in earlier

experiments except for the infant on mother's

milk. Sherman (l56) reported a series of metabolism experiments on very simple diets of crackers

and milk and in some cases butter. In one series

periods of restricted and liberal intakes were alternated. Experimental periods of 3 to 5 days followed consecutively, two series for 12 days each

and a third series for 20 days, to provide a better

basis for following and hiterpreting changes in the

composition of the urine. There was no apparent

difference in the calorie-nitrogen ratios found for

the periods on restricted and liberal intakes. The

range was 7.39-8.00. In general the ratio was

somewhat lower than that found by Atwater and

coworkers for subjects on mixed diets.

Benedict made an extensive investigation of

nitrogen and energy losses in the urine under

fasting conditions, reporting his results in two

pubhcations {25, 26), When body material is

metabolized the calorie-nitrogen ratio appears to

be even more variable than that foimd for different kinds of mixed diets but the average ratio is

higher. He reported ratios in the range of 8 to

10 for the first day of fasting, increasing with each

successive day until after several days some were

in the range of 14 to 18.

Several investigations have been made in which

calories and nitrogen in the urine of children have

been reported, notably those of Macy {Illy 112).

Her studies provided data on a group of children

ranging from 4 to 12 years of age over an extended

period of time. From the composition of the urine

reported the calorie-nitrogen ratio has been calculated for each child. The ratio does not appear

to differ appreciably from that obtained by

Atwater for adults. Related problems have been

studied by Folin {54) j Rubner {14^), Rubner and

Heubner {150), and Tangl (i^O).

In view of the wide variation observed for the

calorie-nitrogen ratio of the urine, the use of an

average calorie value per gram of nitrogen may

be questioned. Data providing a measure of the

magnitude of the discrepancies when the available energy of the whole diet is calculated by the

three procedures outlined on page 8 have been

brought together in table 11. The experiments

selected represent the more extreme conditions on

record as foUows: (1) Those in which the actual

calorie-nitrogen ratio of the urine was considerably

above or below the average; (2) those in which the

subject was in different states of N-balance; and

(3) those in which the subjects had diets of widely

different composition with respect to proportions

of calories from fat, protein, and carbohydrate.

The data show that although the amount of

energy lost in the urine is highly variable, on the

whole it is small compared with the gross energy

of the food eaten. This is not surprising since

less than a third of the gross calories from the

digestible protein is involved, and digestible

protein makes up only 10 to 25 percent of the

total calories in these diets. Consequently, an

error introduced by the use of an average calorie

value per gram of either urinary nitrogen or net

absorbed nitrogen does not affect •greatly the

calculated available energy of the whole diet.

For individual high-protein foods such as lean

meat and some defatted nut and legume products,

urinary loss might be a much more significant

factor in determining available energy. If suitable

data were available not only for foods of high

nitrogen content but for all foods having some

nonprotein nitrogen, and if data were available

on the digestibility and utilization of the various

nitrogenous compounds, a more accurate procedure

for calculating available energy could be developed.

Such data are not available and we are continuing

to use Atwater^s correction of 1.25 calories per

gram of available protein (nitrogen content 16

percent).

For purposes for which the calculation of urinary

energy loss from nitrogen in the usual way is not

satisfactory, attention is called to the work of

Rubner {149) and of Benedict {25, pp. 490-492).

Benedict found less variability in the ratios of

calories to either carbon or organic matter than

in the ratio of calories to nitrogen. He found

closer relationship when he related the energy to

carbon but in view of the difficulty in determining

carbon he suggested as a more feasible procedure,

using the somewhat less constant calorie-organic

matter ratio of the urine. The latter was largely

proportional to the carbon content of the inline

and far more readily determined. Benedict's

suggestion for making the urinary energy deduction on this basis rather than using the more

variable calorie-nitrogen ratio in estimating available calories in foods should be given further

consideration.

Alcohol

The perplexing subject of the energy value of

alcohol has been investigated from time to time

for more than 50 years. Investigations have

included such problems as the extent to which

alcohol can spare protein for building or maintenance of body tissue, and the use of alcohol for

muscular activity, deposition of fat, and generation

of heat for maintenance of body temperature.

Particularly controversial has been the question

of the body's use of alcohol for muscular work.

The gross energy value of alcohol is 7.07

calories but its physiological energy value has

been assessed variously by different groups of

investigators. Daniel, 1951 {45) suggested using

about 5.0 calories per gram, since from animal

experiments and various biochemical studies it

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appears that only 65 to 70 percent is available for

muscular work. The Food and Agriculture

Organization Committee on Calorie Conversion

Factors and Food Composition Tables did not

publish a review of the literature but suggested

in their report {55) that alcohol be omitted in

computations of energy value of diets for two

reasons—^it is seldom possible to estimate alcohol

consumption accurately, and little is known

regarding its physiological energy value.

The gross potential energy value of alcoholic

beverages was estimated by Atwater and Benedict

to be as much as 500 calories per day for an

individual's consumption described as ''moderate.''

Statistics based on alcohol tax receipts in this

country indicate that the average per capita

consumption (man, woman, and child) of alcoholic

beverages in recent years is approximately equivalent to 76 calories (gross value) per day. Were

children and all other nonusers eliminated, the

average consumption for users would be much

higher. For numerous purposes, therefore, some

assessment must be made of the energy value of

alcohol.

The question is still being debated whether the

energy of alcohol can be used for various physiological processes to the same extent as the organic

constituents of food or whether its use is limited

entirely or partially to providing heat. Reviews

of the different aspects of alcohol utilization have

been pubhshed by Carpenter {35), Mitchell and

Curzon {122), Keys {80), and Klatzkin and others

To be accurate, the assessment of food energy

should take into account the site of the energy

conversion and physiological destination of the

nutrients. Up to the present no such additional

refinements have been attempted in any common

method of estimating energy values of foods.

Attempts to do so with a view to obtaining calorie

values to apply to foods under real life situations

would be very complex. To illustrate, Keys {81)

pointed out that when starch is hydrolyzed to

glucose in the gastro-intestinaltract, approximately

14 calories per 100 grams are released in the body.

This is available only as body heat and for no

other purpose. If the hydrolysis occurs during

cooking, these calories are lost before ingestion.

In estimating calorie values of starch this type of

difference is not taken into account. Of more

importance, he pointed out, is the demonstration

that, calorie for calorie, fat is about 12 percent

less efficient for production of external muscular

work than carbohydrate, yet there is no difference

if calories are needed solely to maintain body

temperature.

The potential energy of moderate amounts of

alcohol may have a more limited usefulness in

body metabolism than energy from proteins, fats,

and carbohydrates. In view of the fact that ia

estimating calorie values, differences in availability and efficiency of use of the energy from

these common sources are not considered, it does

not seem necessary at present to discount txie

energy value of alcohol, particularly since there

is considerable evidence that when intake is

moderate a large part may be available for muscular work and that aU of it may serve as a source

of body heat.

The calorie factor for alcohol used in the tables

in this pubhcation is the one proposed by Atwater

and Benedict (15), 6.9 calories per gram based

on respiration calorimeter studies m which they

found that 98 percent of the heat of combustion

(7 07 calories per gram) was utihzed by the human

body in its combined needs for energy m muscular

work, building tissue, and mamtaming body

temperature.

Altogether Atwater and Benedict conducted 26

experiments, each lasting 2 to 4 days, m which

they compared the metabohsm of man on diets

with and without alcohol. These expermients

were part of the long series akeady referred to

and are included in table 8.

In three groups of the first experiments, the

periods with and without alcohol were not as

directly comparable as in the six groups of later

experiments, owing largely to lack of means for

providing a food supply of uniform composition.

By the time the later experiments were conducted,

ways of preserving considerable quantities of food

by canning and by cold storage had been devised.

Three men in good health and with apparently

normal digestion served as subjects in these 26

rest and work experiments. The respiration

calorimeter used (see p. 15) was so constructed

and equipped as to permit measurement and

sampHng for analysis of ventüathig air, food, and

excreta, and also for measurement of heat given

off and external work performed by the subject.

The experimental plan for these 26 experiments

was in general the same as that for the otber experiments ÍQ the entire series. One difference

was that in this group of experiments aU periods

were more than 1 day.

A prelimiaary digestion experiment of 3 or 4

days preceded each metabolism experiment. Each

subject was on the experimental diet he was to

have ia the following period in the respiration

chamber. During this preliminary period the

subject made adjustments considered necessary

in the diet and controlled his activity as much as

possible to that he would have during the metabolism experiment in the calorimeter. The amounts,

heats of combustion, and composition of food,

feces, and urine were determined.

Duriag the metabolism experiment these determinations were continued and in addition determinations were made of the water and carbon

dioxide content of the ventilating air entering

and leaving the respiration chamber, the heat

given off by the body, and the heat equivalent of

the muscular work performed during the work

experiment. These data made it possible to

determine the carbon, nitrogen, and hydrogen

20
balance, the potential energy of food and unoxidized excreta, the kinetic energy of heat given off,

and the external work performed.

Each subject had a rather simple diet consisting of such ordinary foods as meat, milk, bread,

cereals, butter, sugar, and in some cases, coffee.

During the rest experiments the diet supplied about

2,500 calories and during the work experiments,

about 3,900 calories. In the rest experiments the

subject performed as little activity as possible in

addition to the necessary motions of dressing, undressing, handling of samples, recording of data,

and the daily setting up and taking down of his

cot. Most of the day was spent sitting, reading,

or writing. In the work experiments he rode a stationary bicycle for a total of 8 hours a day.

Alcohol was substituted for either carbohydrate

or fat or a mixture of both in 13 experiments, including rest and work experiments. About 72

grams (about 500 calories) were given in 6 small

doses, 3 with meals and 3 at regular intervals

between meals. Thus it furnished about a fifth

of the calories during the rest experiments and

between a seventh and an eighth of the calories

during the work experiments.

The data showed that alcohol had no practical

effect on digestibility except possibly in the case

of protein. The coefficient of apparent digestibility of protein was a little larger in the experiments when the diet included alcohol than in

comparable experiments without alcohol, 93.7

percent as compared with 92.6 percent.

The amounts of unoxidized alcohol given off by

the kidneys, lungs, and skin were measured and

deducted from the amount ingested. The difference was taken as the amount of alcohol oxidized

in the body. Previous research by another

worker had indicated that alcohol was not excreted by way of the intestine even when considerable quantities were taken. Therefore, no

analysis of feces for alcohol was made. In these

experiments only small amounts of unoxidized

alcohol (0.7 to 2.7, averaging 1.3 grams) were

recovered. The authors concluded that not more

than about 2 percent would be given off unoxidized

when taken in amounts comparable to those in

these experiments. They suggested using 98

percent of the gross heat of combustion as the

value of alcohol.

From these results Atwater and Benedict compared the energy of the daily net income and the

outgo for subjects on diets with and without

alcohol. The net income was the energy of the

material actually oxidized in the body, and was

determined by adjusting the available energy

(gross food energy minus total calories in urine

and feces) for calorie equivalent of loss or storage

of body protein and fat. The total heat outgo

was the energy measured by the apparatus as the

heat given off plus the heat equivalent of the work

performed by the subject. Whether or not the

diet contained alcohol, the average energy outgo

was equal to the average amount of energy of the

net income. Atwater and Benedict concluded

that the energy of alcohol oxidized was transformed completely into kinetic energy and appeared either as heat or as muscular work, or both.

Atwater and Benedict made some deductions

concerning the protecting effect of alcohol on

body material, based on the carbon, nitrogen, and

hydrogen balances of the subjects. From these

balances they estimated the daily gains and losses

of body fat and protein, assuming that the glycogen stores for each individual at the beginning and

end of the experiment were the same. They

found some gains and some losses of body fat on

either kind of diet but on the average there was

a gain. This gain was slightly larger when the

subjects were on the diets including alcohol than

when on the ordinary diets; 2.4 grams daily as

compared with 1.1 grams of fat in comparable

experiments with and without alcohol. Storage

and loss of body protein also was calculated. Comparisons made between the ordinary and alcohol

periods indicated that alcohol was slightly inferior

to carbohydrate or fat in protecting body protein;

that is, a larger average daily loss, 6.9 grams, of

body protein occurred in the alcohol periods than

the average loss, 3.5 grams per day, for the ordinary periods.

Loss of the energy of alcohol by radiation of

heat seemed to account for only a small proportion

of the calorie value. Atwater and Benedict found

that the radiation of heat from the body was only

slightly greater with the alcohol diet than with

the ordinary diet, and amounted to not more than

6 percent of the energy of alcohol.

Some of the results of the six groups of experiments (totaling 15 balances on 2 men) in which

the alcohol and nonalcohol periods were more

nearly comparable are shown in table 12. As the

protein intake within each group of comparable

experiments with and without alcohol is nearly

constant, these data indicate approximately the

effect of alcohol on both the apparent digestibility

of protein and on the retention or loss of digested

protein. There was a small increase in apparent

digestibility and also some increase in urinary

nitrogen excretion when the diets included alcohol.

The heats of combustion which Atwater and

Benedict applied in experiments 9 and 10 to

changes in body protein and fat were 5.65 and 9.54

calories per gram, respectively. In their later

experiments they changed the figure for body fat

as they considered 9.4 calories per gram more

nearly correct. The net effect of alcohol on gain

or loss of body protein and fat in terms of total

energy change is shown in column 13. The last

two columns of the table show excellent agreement

between energy expenditures obtained in two

entirely different ways: by adjusting available

energy of food intake for changes in amounts of

body protein and fat, and by direct measm^ements

of the heat given off by the body plus the heat

equivalent of the muscular work performed (m

the work experiments).

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23
A comparison of the experiments with and

without alcohol (column 13) indicates that within

each group, when 72 grams of alcohol (509 calories)

replaced an approximate calorie equivalent of

fat and/or carbohydrate, the calculated net gain

or loss of energy value in the form of body tissue

was sometimes a little larger and sometimes a

little smaller than the change calculated for the

comparable experiments when no alcohol was

included. In these six groups of experiments

the calculated calorie change of body tissue

varied from an additional gain of 83 calories to a

larger loss of 95 calories as compared with the

corresponding nonalcohol experiment. Under tne

conditions of these experiments Atwater and

Benedict concluded that alcohol must have been

used by the body about as efñciently as the nutrients from ordinary food it replaced. In the most

extreme case, alcohol calories were only about

four-fifths as well used as the food calories which

alcohol replaced. These experiments do not

prove but suggest that under the conditions

comparable to those in these experiments much

of the energy of alcohol can be used m the body

for internal or muscular work.

PART III. DERIVATION OF CURRENT CALORIE FACTORS

Since Atwater first proposed his individual food

group factors and his general factors for estimating

the fuel value of mixed diets as a whole {17) ^

enough data for a number of foods have accumulated to make possible some revisions and additions. For other foods more data are urgently

needed.

Prior to 1947 the Bureau of Human Nutrition

and Home Economics had summarized the

available information on digestibility by man of

bread made from wheat of three levels of extraction and had compiled preliminary material for

potatoes. Since then, study of the scientific

literature has been continued, permitting the

addition of coefficients of digestibility for many

more items. Data from the digestibility studies

reviewed are given in appendix tables 23 and 24.

The resulting summary of data on human digestibility and heat of combustion needed for deriving

specific calorie values of individual items of food

or of small food groups is given in table 13.

Where further information was lacking, Atwater's

data were taken from the revised figures for column F in table 10 of his report cited above and

reproduced as table 7, page 10. The figures in

columns 4, 7, and 10 are the specific factors to be

applied to the grams of protein (nitrogenous

material), fat (usually ether extract), and carbohydrate (determined by differences) in the food

to obtain the physiological energy value.

Before discussing the derivation of the specific

calorie factors shown in table 13, a few general

observations should be made regarding the basis

of the data.

The basis for the coefficients of digestibility in

table 13 could have been broadened greatly, if the

large volume of work with experimental animals

in the literature had been included. Some work

has been done to compare digestibilities of man

and experimental animals. Brierem and Nicolaysen {SO) compared utilization of protem and

dry matter in wheat and rye brans by man with

utilization by sheep and swine. Later Crampton

and others {^S) compared man^s use of several

grain products with that of rats, sheep, and swme.

However, there is insufíicient evidence at present

for concluding that digestibility of nutrients by

experimental animals can be used to predict that

of man. If a relationship could be established,

research in this field could proceed more economically and more rapidly.

The energy factors shown in table 13 do not

rest on equally rehable information. The number

of subjects for different foods varied considerably.

In general, no information was available on the

possible departure of the test diet from the previous dietary pattern. Lack of uniformity was

observed in the experimental procedures used,

including lengths of the preliminary and experimental periods, choice of marker, and the relative

proportion of the diet furnished by the test food.

The foods tested, expecially in the early digestion

experiments, were not always adequately described, nor was the chemical composition of the

sample always reported. In some cases, reasonable assumptions could be made as to the identity

of the samples. For a few foods neither descriptive

nor composition data were reported by the

investigators, and energy factors derived from

digestibility data in those experiments may be

shown by future work to need considerable

revision. Grain products of various degrees of

milling as described some 50 years ago have

presented particularly knotty problems. Although

the products were identified by extraction and

other recognized milling terms, composition data

in addition were necessary to classify them in

terms of the most nearly comparable products on

the market today.

24
TABLE 13.—Data used for calculating energy values of foods or food groups by the Atwater system

Food or food group

(1)

Eggs, Meat products, Milk products:

Eggs

Gelatin

Glycogen

Meat, fish

Milk, milk products

Eats, separated:

Butter

Other animal fats

Margarine, vegetable

Other vegetable fats and oils

Fruits:

All (except lemons, limes)

Lemons

Limes

Grain products:

Barley, pearled

Buckwheat flour, dark

Buckwheat flour, light

Cornmeal, whole ground

Cornmeal, degermed

Dextrin

Macaroni, spaghetti

Oatmeal, roUed oats

Rice, brown

Rice, white or polished

Rye flour, dark

Rye flour, whole grain

Rye flour, medium

Rye flour, light

Sorghum (kaoliang) ^ whole or nearly whole meal__

Wheat, 97-100 percent extraction

Wheat, 85-93 percent extraction

Wheat, 70-74 percent extraction

Wheat, flaked, puffed, rolled, shredded, whole

meal

Wheat bran (100 percent)

Other cereals, refined

Wild rice

Legumes; Nuts:

Mature dry beans, cowpeas, peas, other legumes;

nuts

Immature lima beans, cowpeas, peas, other legumes

Soybeans, dry; soy flour, flakes, grits

Sugars:

Cane or beet sugar (sucrose)

Glucose

Vegetables :

Mushrooms

Potatoes and starchy roots

Other underground crops *

Other vegetables

Miscellaneous foods:

Alcohol 5

Chocolate, cocoa

Vinegar

Yeast_ '

Protein

Coeflacient of

digestibility

(2)

Pet.

97

97

97

97

97

'97"

85

85

85

78

74

78

60

76

86

76

75

84

65

67

71

75

20

79

83

79

40

85

78

78

78

78

70

74

74

65

42

80

Heat of

combustion loss

1.252

(3)

Cal./(jm.

4. 50

4. 02

4. 40

4. 40

4. 40

4 4Ö'

3.95

3. 95

3.95

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4.55

4. 55

4.55

4. 55

4. 55

4. 55

4. 55

4. 55

4. 55

4.45

4.45

4. 45

3.75

3.75

3. 75

3.75

4. 35

3.75

I'actor

to be

applied

to ingested

nutrients

(4)

Cal.lgm.

4. 36

3.90

4.27

4.27

4.27

'4.'27'

3.36

3. 36

3.36

3.55

3.37

3. 55

2.73

3. 46

3. 91

3. 46

3.41

3.82

2.96

3.05

3. 23

3.41

.91

3. 59

3.78

4.05

3. 59

1. 82

3. 87

3. 55

3.47

3.47

3.47

2. 62

2. 78

2. 78

2. 44

1. 83

3.00

Fat

Coemcient of

digestibility

(5)

Pd.

95

95

95

95

95

95

95

95

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

90

'90'

Heat of

combustion

(6)

Cal./gm.

9. 50

9.50

9. 50

9.25

9.25

9. 50

9.30

9. 30

9.30

9. 30

9.30

9. 30

9.30

9. 30

9.30

9.30

9.30

9. 30

9.30

9.30

9.30

9. 30

9.30

9.30

9.30

9. 30

9.30

9.30

9. 30

9.30

30

30

30

30

30

'30'

Factor

to be

applied

to ingested

nutrients

(7)

Cal.jgm.

9.02

9.02

9.02

8.79

8.79

9.02

8.84

8.84

8.37

8.37

8.37

8. 37

8.37

8. 37

8.37

8. 37

8. 37

8. 37

8.37

8. 37

8. 37

8.37

8. 37

8. 37

8. 37

8. 37

8. 37

8.37

8. 37

8.37

8. 37

8.37

8.37

8. 37

8.37

.37

.37

, 37

37

37

"37'

Carbohydrate

Coefficient of

digestibility

m

Pet.

98

98

"98'

98

90

98

98

94

90

94

96

99

98

98

98

98

99

90

92

95

97

96

90

94

98

90

56

97

97

97

85

96

96

85

32

98

80

Heat of

combustion

(9)

Cal.jqm.

3. 75

4. 19

3.'96"

3.95

3.'95"

4.00

2. 75

2.75

4.20

20

20

20

20

11

20

20

4.20

20

20

20

20

20

20

20

4.20

4.20

4.20

4.20

4.20

4. 20

4.20

4.20

4.20

3. 95

3.75

4. 10

4.20

4.00

4.20

4. 16

2. 45

4.20

1 In a few cases values in columns 4, 7, and 10 are slightly

different from those shown in table 7, column F, revised,

because of different methods of rounding figures.

2 The correction, 1.25 calories, has been subtracted from

the heat of combustion. This gives values applicable to

grams of digested protein and identical with Atwater's

factors per gram of available protein.

heart, kidney,

parsnips,

3 Carbohydrate factor, 3.87 for brain,

liver; 4.11 for tongue and shellfish.

4 Vegetables such as beets, carrots, onions,

radishes

5 Coefficient of digestibility, 98 percent; heat of combustion, 7.07 calories per gram; factor to apply to mgestea

alcohol, 6.93 calories per gram.

25
Physiological Fuel Values of Foods of Animal Origin

For determining physiological fuel values of

foods of animal origin, Atwater's factors for the different categories are still being used with only slight

changes. His coefficient of digestibility of 97

percent for the protein of meat, fish, eggs, and

dairy products has been used without change in

this pubUcation (see table 7).

Many items of animal origin contain small

amounts of carbohydrate to which Atwater

applied the energy factor 3.82 calories per gram.

He obtained this using 3.90 calories as heat of

combustion and a coefficient of digestibility of 98

percent. Some small revisions in this factor are

indicated in view of current information on the

form of carbohydrate predominating in the different kinds of foods of animal origin. These

revisions, which are very minor, are noted in the

following paragraph.

For the carbohydrate of milk and milk products,

we have used for the physiological energy factor,

3.87 calories per gram. This is based on the heat

of combustion for lactose, 3.95 calories per gram,

and Atwater's coefficient of digestibility, 98

percent.

Eggs contain a small amount of carbohydrate,

chiefly glucose, bound in a large complex. The

energy factor used in this publication, 3.68 calories

per gram, was obtained by applying the coefficient

of digestibility, 98 percent, to 3.75, the heat of

combustion of glucose. Perhaps this figure is too

low for simple sugars which require no digestion.

Appreciable amounts of glucose and glycogen

have been found in tissue of brains, heart, and

glandular organs, the relative amounts varying

according to metabolic conditions at the time of

slaughter and conditions of storage. For heat of

combustion, 3.95 calories per gram, a figure intermediate between the heats of combustion of

glucose and glycogen, has therefore been selected,

resulting in a physiological energy factor of 3.87

calories per gram.

Analyses have shown glycogen to be the main

carbohydrate constituent of tongue and some

kinds of shellfish. Hence, to derive an appropriate

energy factor, we applied the coefficient of digestibility, 98 percent, to 4.19, the heat of combustion

of glycogen, and the resultant factor was 4.11

calories per gram.

For animal fats we have used Atwater's energy

factors, 8.79 calories per gram for fats in dairy

products and 9.02 for fats from other animal

sources.

Physiological Fuel Values of Plant Products

Separated fats of plant origin are important

items today but were practically imknown 50

years ago. For them we have used the digestibility coefficient, 95 percent, that Atwater used

for butter and other animal fats. For the heat of

combustion of fat in plant products, whether or

not separated, we have continued the use of

Atwater's factor, 9.3 calories per gram.

Margarine as manufactured in the United

States of America may be made of either animal

or vegetable fats, and a few States have laws

requiring a specified high proportion of animal

fats. However, as most margarine in this country

is made with vegetable oils, the factor 8.84 calories

per gram for fat in margarine shown in table 13

was based on heat of combustion of 9.3 calories

per gram and a coefficient of digestibility of 95

percent. Margarine of either type and butter

contain small amounts of protein and carbohydrate carried over from the milk in which the

fats were blended or chxu-ned. The calorie factors

for protein and carbohydrate of milk were used

for those constituents of margarine.

For fat as it occurs in cereals and other plant

sources Atwater assumed the apparent digestibility to be 90 percent and we have continued this

practice. The energy factor for fat in plant foods

is therefore assumed to be 8.37 calories per gram.

The revisions we have made in the Atwater

factors for the physiological fuel values of protein

and carbohydrate of plant products have resulted

26

mainly from adding data from a comprehensive

review of digestibility studies reported since 1875.

For a few foods the revisions result from changes

in the heat of combustion factors used. More

specific energy factors, together with the average

coefficients of digestibility and heats of combustion

from which they were derived, are presented in

table 13. The basis for the differences in these

figures as compared with Atwater's factors for

food groups is discussed in the remainder of this

section. In some instances the values may prove

to need further revision as the result of future

research, but we believe them to be better approximations for individual foods than either the

general, overall factors or the food group factors

that were developed in 1899. The basic data on

digestibility from which the factors were obtained

have been compiled in table form (appendix

table 23).

This compilation is not entirely complete for

studies reported in foreign languages, but we

beheve it covers the bulk of early and recent

research m which apparent digestibility of the test

food was measured. Articles in which apparent

digestibihty was not reported or could not be

calculated for the test food alone were not included. By this criterion, digestion experiments

such as those of F. Erismann (52) were excluded.

His coefficient of digestibihty for protein has been

quoted by various authors in early pubhcations as

applying to peas, but a translation of the original
article shows that the coefEcient was applied to

bread made of 50 percent pea meal and 50 percent

rye flom^ without adequate basis for calculating the

digestibility of the pea meal alone. Included in

the compilation are several experiments which

contribute useful information although for various

reasons they have been excluded from the data

used for obtaining average coefficients.

The derivation of the energy factors in table 13

is discussed in the following paragraphs by food

groups since foods within a group have certain

characteristics and problems in common. Where

no mention is made of the derivation of factors,

Atwater's data considered most appHcable to the

particidar item or small food group have been used.

Products of wheat

By far the largest proportion of the digestion

experiments reported have been concerned mth

foods of the cereal group. Of the cereal foods

wheat has been studied in most detail.

Flours

Digestibility of wheat flour was studied first by

Eubner and other European scientists during the

latter part of the 19th century. As milling

practices and the terms used have changed over

the years, we encountered problems in deciding

how to combine and group the large volume of

data on the digestibility of wheat flours. Wheat

flours milled commercially and experimentally

have been studied extensively since 1900, particularly in the United States and in Great Britain.

Por a great many of these flour samples, enough

information is available so that the flours can be

arranged in three groups according to degree of

extraction from the kernel. These three groups

were described by United States scientists in the

early part of the 20th century as graham, entire

wheat, and as straight or standard patent. Not

all the terms used then still apply but the data are

usable.

Graham was essentially whole-wheat meal, but

may have had a very small amount of coarse

material removed. The straight and standard

patent flour group contained the first and second

patent flours and the first clear flour and made up

about 70 to 72 percent of the wheat kernel, which

is in line with modern-day yields of straight

grade flour. Data on the composition of the

straight patent flours used in the early experiments

when reported also indicated that from the standpoint of proximate constituents the straight

patent flours were similar to those produced in

recent years. In some instances the standard

patent was blended with small amounts of the

low-grade flours, second clear and red dog.

More variation existed in the flours included in

those of the intermediate extraction. The socalled entire-wheat flom* as described by Woods

and Merrill {194) included patent, first clear.

second clear, red dog, and shorts, indicating that

It may have been somewhat more than 85 percent

of the kernel. For comparison, the average miflmg yields from several commercial milHngs of

cleaned wheat reported in 1941 by Sherwood and

others {160) have been included here as follows:

Milled fraction: yield

Patent flour ^3 Q

First clear flour 7Q

Second clear flour 4 5

Red dog flour ..._ ¿0

Germ '_ * 2

Shorts _ 12*3

Bran V\V.V__ ao

If it could be assumed that the sum total of the

fractions comprising the entire-wheat flour of the

early experiments was comparable to the fractions

reported by Sherwood, theoretically the yield of

entire-wheat flour would have been close to 91

percent. UsuaUy the entire-wheat floiu* was referred to as being of about 85-percent extraction.

Probably this was a little low. Woods and Merrill {194) stated that 100 pounds of cleaned No. 1

wheat would make 85 to 88 pounds of entire-wheat

flour, that the large mifls gave rather larger yields

than small mills, and that a starchy wheat yielded

1 to 3 pounds more than a hard wheat. They

also stated that the ash content of the entire-wheat

flour was about half that of the whole-wheat flour.

Suyder {164, lp6, 168) also conducted experiments in that period and used entire-wheat flours,

but the indications are that the flours he used

were of somewhat longer extraction than those

described by Woods and MerriU. The flours of

longer extraction were obtained by removal of

part of the coarser brau by screening and the

inclusion of fine bran, shorts, and germ. The

amount of coarse bran removed varied from a

small proportion to over half the total amount

present. Data he reported showed that the ash

content of the entire-wheat samples ranged from

51 to 92 percent of that in the wheats from which

they were milled. In view of this kind of information it is likely that entire-wheat flour represented about 90 percent of the cleaned wheat.

The latter figure was also arrived at independently

by a milling expert in the United States Department of Agriculture who estimated the probable

extractions of a number of samples of entire-wheat

flour from their ash contents in relation to the

ash content of the whole-wheat from which each

was milled.

Since such estimates may be more or less in

error, and since the information on the muling

and composition of the fiours suggests that the

entire-wheat flour was not always of uniform

extraction, we have considered it preferable in

this publication to assume that the data applied to

flours within the range of 85- to 93-percent extraction. Likewise for flours designated as standard

patent we have assumed that the data applied to

flours of 70- to 74-percent extraction. With re27
gard to the whole-wheat or graham flours used in

the early studies, it has been assumed that flour

so designated may have been of from 97- to 100-

percent extraction, since there was some evidence

that a small amount of the outer portion of the

kernel may have been removed.

The average coefficients of digestibility of protein and of carbohydrate for wheat flours of these

three extraction ranges are based on more than

70 digestibility trials on whole-wheat and near

whole wheat flours, more than 50 trials on wheats

of interniediate extractions, and over 100 digestibility trials on straight and patent flours. The

average coefficients of digestibility are shown in

table 14. The variation in digestibility found for

the protein of wheat was much greater than that

for the carbohydrate, and there was greater variation in the digestibility of longer extraction flours

than for refined flours of shorter extraction.

The wheat samples used in the digestibility

studies were largely hard wheats, both sprmg

and winter varieties; a few soft wheats were

included. Data on proximate composition were

available for the whole-wheat flours used in 18

digestibility trials, for the flours of intermediate

extraction in 22 trials, and for the straight and

patent flours in 28 trials. Average values for

the flours of known composition are shown in the

first column of table 14. Within each of the three

groups there was much variation. The protein

content of the whole-wheat samples, for example,

ranged from 8.5 to more than 15 percent, the

majority containing over 12 percent. At present,

data are inadequate to determine to what extent

variation observed in digestibihty within groups

and between groups may have been due to differences in protein and carbohydrate content of the

flours.

TABLE 14.—Energy values oj wheat Hours calculated by use of specific energy factors for protein, fat^ and

carbohydrate

Type of flour

(1)

Composition '

(2)

Coefficient of

digestibility

(3)

Energy value

of available

nutrient

(4)

Energy factors to be

applied to

ingested

nutrients

Ccol. 3 X col. 4)

(5)

Available

energy of food

using specific

factors (col. 2

X col. 5)

(6)

Essentially whole wheat ^ (97-100 percent extraction) :

Protein (N X 5.83) _._

Percent

12.6

1.9

71.8

Percent

79

' 90

90

CaL/gm.

34.55

9.30

4.20

Cal.lgm.

3.59

8.37

3.78

Cal.jlOO gm.

45.2

Fat _ __

Carbohydrate by difference 271.4

Total 332.5

Intermedíate extraction (85-93 percent) : ^

Protein (N X 5.7) 12. 0

1.8

73.0

83

4 90

94

3 4. 55

9. 30

4.20

3.78

8.37

3.95

45.4

15.1

288.4

Fat

Carbohydrate by difference.

Total 348. 9

Patent ^ (70-74 percent extraction) :

Protein (N X 5.7) __ 11.7

1.3

74. 5

89

< 90

98

34.55

9.30

4. 20

4.05

8. 37

4. 12

47.4

10.9

306. 9

Fat _-. _

Carbohydrate by difference

Total. 365. 2

^ Composition data are calculated to a 12-percent

moisture basis.

2 The ash content found for the wheat flours are 1.7

percent for essentially whole wheat, 1.2 percent for wheat

of intermediate extraction, and 0.5 percent for patent

flour.

The average digestibility of the protein and

of the carbohydrate for any one type of flour of

known chemical composition showed differences

of less than 1 percent from average coejEcients of digestibility obtained by using data from all of

the samples of that type. The coeJQBcients based

on all the samples within the group, therefore,

were used for deriving the energy factors shown

in table 14. It is unlikely that the heat of combustion value would be the same for the nitroge28

3 1.25 calories have been deducted from the heat of

combustion of 1 gram of protein to correct for loss of

incompletely metabolized products in the urine.

* Assumed coefficient of apparent digestibüity for fat in

plant products, 90 percent, used because actual data forwheat were unsatisfactory.

nous portion of flours of different extraction rates.

The relative proportions of protein and nonprotein

nitrogen compounds and the composition of the

protein fraction itself are each known to vary.

These changes would be expected to affect the

heat of combustion and hence the energy factors

also. However, in preparing table 14, no attempt

!^^s made to adjust the heat of combustion data

for differences in the heat values of the protein or

fat mixtures in the flours of different extractions.
Energy values for flours at each of the three

extractions have been worked out from the composition data of the known samples. The results,

shown in the last column of table 14, are considered suitable for flours of the extractions and

compositions specified.

From time to time calorie values for flours of

other extractions are needed. To estimate coefficients of digestibility for protein and carbohydrate in such flours, the digestibility data

estimated at the three extraction rates, 100, 90,

and 70 percent, for the wheat flours just discussed

were used. This was done inasmuch as the few

scattered digestion experiments reported on flours

of other specified extractions of wheat were so

varied in conditions and methods that the results

were not usable for this purpose. The relationship between the percent extraction and coefficient

of digestibility of wheat can be expressed in the

form of the equation y=a-\-bx+cx^, where a:=percent extraction and ?/= coefficient of digestibility.

By the method of least squares the constants for

this equation, fitting the three points based on

average digestibihty data for wheat of 100-, 90-,

and 70-percent extraction, were as follows for^

protein and carbohydrate:

Equation constant

Value or constant

Protein Carbohydrate

a - 0.890

. 00233

-. 0000333

0. 700

h . 00867

c —. 0000667

Digestibility coefficients (y) for other extraction

percentages (x) were computed by solving the

equation.

From this equation, coefficients of digestibility

of protein and carbohydrate were calculated for

flours of 95-, 85-j 80-, and 75-percent extraction.

The values found are shown in table 15. Additional intervening points may be determined from

the equation or read from a curve. For convenience the energy factors to apply to the protein

and carbohydrate in wheat flours of the specified

extractions are also included in table 15.

Degree of extraction has been accepted here as

the most important influence on digestibility of

wheat flour. To test this assumption an estinaate

was made of the extent to which rate of extraction

was associated with coefficients of digestibility.

The relationship of these two, based on those

subjects for whom data were available at each of

three extraction rates, was found to be represented

by an equation of the form y=a-\-bx-\-cx^j where

the extraction rate is the independent variable.

B^, the variance, was 44 percent, showing that

almost half of the variation in the coefficient of

digestibility was associated with variation in

extraction. This indicates that the assumption

was warranted since 56 percent of the variation

remamed to be distributed among such factors as

level of protein intake, level and nature of carbohydrate, length of experimental period, the variation characteristic of each subject, and fineness of

grind of the flour.

TABLE 15.—Apparent digestibility and physiological

fuel value of wheatflours

Perci^nf Evtraptinn

Coefficient of digestibility Specific energy factor

Protein Carbohydrate ProteiQ Carbohydrate

100

95

Percent

79

(81)

83

(85)

(86)

(88)

89

Percent

90

(92)

94

(96)

(97)

' (98)

98

Cal/gm.

3. 59

3. 69

3.78

3. 87

3. 91

4.00

4. 05

Cal./gm.

3. 78

3. 8a

3.95

4.03

4.07

4. 10

4. 12

90- -

85

80

75

70

1 97.5 by calculation, from equation, page 29.

Since 1945, when the relationship between

coefficient of digestibility and degree of extraction

were worked out rather hastily for wheat of 100-^

90-, and 70-percent extractions, some additional

data have been located in the literature and others

have become available as the result of more

recent research. The inclusion of these additional

values did not warrant any change in the coefficients of digestibility published earlier (55, 118)^

and we have continued to use them.

Methods of determining energy values used in

Great Britain.—McCance and his associates have

given special attention to assessing the energy

values of cereals {102, 104, 108). They differentiated between available and physiological

calories and as both procedures differ from the

one used in this country and as certain of the

terms have different meanings we include here a

brief discussion of their work—particularly as it

relates to wheat.

The consideration given carbohydrate has

been an important difference between the systems

for estimating energy used by English and

American scientists. Data on carbohydrate in

British tables of food composition are based on

^^available carbohydrate,'' which includes sugars,

starches, and dextrins—carbohydrates assumed to

be fully utilized by the body, but exclude as unavailable, fiber and nonfermentable sugars or

pentoses. The analytical determination of available carbohydrate has been discussed by McCance

and Lawrence {103).

Available calories as calculated by McCance are

the sum of the gross calories in the available

carbohydrate fraction of the food, plus the

calories from fat after deduction for fecal lipids,

plus the calories from protein after deduction for

fecal and urinary losses of nitrogenous matter,

In principle this procedure is similar to that used

29
in this country but the method for determining

available calories from carbohydrate is different;

also there are some differences in the actual

calorie factors selected for protein and fat.

To illustrate how the calorie factors for the

calculation of available calories are obtained,

data are presented here from a study of McCance

and Walsham {lOJj). Digestion and utilization of

calories of two samples of whole-wheat flour, one

made from a low-protein English wheat and the

other from a high-protein Canadian wheat were

determined for adult subjects. The calorie factors

for protein were found by applying the coefficients

of apparent digestibility of protein found in these

digestion experiments to the factor 4.35 calories

per gram (5.65 less 1.3 calories per gram for urinary

loss) used by Sherman {158) for the heat of combustion of protein in a mixed diet. The apparent

digestibility of protein was found from the experiment to be 84.9 percent for the Canadian flour

and 74.2 percent for the English flour.

The calorie factor for fat was obtained by applying 58 percent, the coefficient of apparent digestibility found for fat, to 9.45 calories the heat of

combustion per gram of fat in a mixed diet {158).

From these experimental data the calorie factors

to be applied to analytical data on the content

of protein and fat to obtain the calories from these

nutrients in the two samples of flour were thus

found to be 3.65 and 5.5 calories per gram,

respectively, for the Canadian and 3.21 and b.t>

calories per gram for the English wheat flours.

Calories from carbohydrates were obtained by

applying the gross calorie factor for starch, 4.2,

to data on content of '^available carbohydrate^'

expressed as starch for each wheat.

Physiological calories as calculated by McCance

include only the gross calories from those fractions

of the nutrients which definitely may be considered usefully available. Provision is made for

excluding urinary calories, a loss resulting from

incomplete combustion of protein, but no deduction is made for nitrogenous matter in feces; likewise no deduction is made for fecal fat. The

fraction of the carbohydrate measured is considered completely available. It is treated in the

same way for determining physiological calories

as in the determination of available calories—the

fraction of questionable value as a source of energy

is excluded and a gross heat value is used with the

data on available carbohydrate content. For use

in food tables physiological calories are the values

McCance considers most suitable {102, 104y 108) j

because, as he has pointed out {107), it is unusual

to make allowance for losses in the feces in presenting data on composition of foods.

Physiological calories based on data from the

experiment with the two whole-wheat floinrs were

calculated to be the sum of the calories from

carbohydrate obtained in the same way as for

available calories above, plus the gross calories

from fat (9.4Xgrams of fat in the wheat sample)

present in the food, plus the gross calories from

protein present with a deduction for urinary l<^ss

(4.35Xgrams of protein content in the wheat)

Data reported by McCance and Walsham irom

this study for gross, available, and physiological

calories for the two samples of whole-wheat flour

(15 percent moisture) were as follows:

Type of data

Canadian

wheat

English

wheat

Gross calories:

Fnnr! Cbomb) - -

Calories

372

-45

-17

Calories

350

Fecal (bomb) -40

Urinary loss (LSXnet absorbed

N^ -8

Available calories (from above)

Available calories calculated by

A/TpCancp Drocedure

310

299

320

302

304

Physiological calories calculated by

A/rp(~î*iTirp TiTOCpdure - 320

Additional calculations of energy values.—

Using data from the same study, McCance and

Walsham also made two additional energy calculations—one attributed to the Atwater procedure and one in which energy factors for wheat

used by Food and Agriculture Organization

were applied. We have not included results for

these two calculations for two reasons. The

application of the general calorie factors for protein, fat, and carbohydrate of a mixed diet to a

specific food is not a procedure used by Atwater.

Moreover, McCance and Walsham do not report

values for ash content and thus it is not possible

to determine the total carbohydrate (by difference),

a value needed to use the Atwater procedure

correctly. When assumed figures for ash suggested

by the authors were used, gross calories calculated

from composition were observed to differ from

bomb determinations by 15 calories per 100 grams

in the case of Canadian wheat and 24 calories per

100 grams in the case of English wheat. In

view of the close agreement previously reported

(p. 6) between calculated values for gross calories

and bomb calorimeter determinations, the discrepancies of 4 percent for the Canadian and 7

percent for the English wheat seem too large to

warrant the use of these assumed figures for a

comparison between the two methods of calculation.

We have not located any reports in the literature

in which adequate data are given for evaluating

the two methods of assessing available calories

in foods—the McCance procedure outlined above

and the Atwater procedure as we use it. For a

correct appraisal of the two methods by means of

digestion experiments, the following data are

essential: (1) Chemical composition of the food

samples, which should include moisture content,

nitrogen compounds, fat, ash, carbohydrate by

difference, and available carbohydrate (starch

and sugar); (2) bomb calorimeter determinations

of the foods and excreta. Such an evaluation, if

30
made for other types of food as well as wheat,

would provide very useful information.

Alimentary pastes; other flour mixtures

Macaroni and other alimentary pastes usually

are made Avith semolina or durum flour as the

principal ingredient. This type of wheat has

characteristics different from wheats used for

preparing wheat flours for bakers and homemakers.

Digestion experiments were carried out in 1905

by Snyder (168) and at an earlier date by several

European scientists to determine the digestibility

of macaroni and other kinds of alimentary pastes.

Only in the study reported by Snyder was the

flour used in making the pastes described in any

detail. In this latter study the flour milled from

durum wheat represented a somewhat larger

portion of the kernel and was more granular in

appearance than the patent flour used for bread

making purposes. The average coefficients of

digestibility found for macaroni, 86 percent for

protein and 98 percent for carbohydrate, seemed

reasonable in view of other studies indicating that

both degree of extraction and coarseness of grind

affected the apparent digestibility, particularly of

the protein, of flours.

The specific energy factors for protein and carbohydrate calculated by use of these coeiBcients of

digestibility and the heats of combustion of

wheat flours were 3.91 and 4.12, respectively.

The factors are considered applicable to the various

ahmentary pastes made from flour and water.

Digestibility of those containing eggs or milk might

be somewhat different, but as only small amounts

of these optional ingredients are present separate

factors are not proposed.

The digestibility of some other products in which

flour is the main ingredient has been studied also.

Deuel published results found for a variety of

baked products including yeast breads, baking

powder biscuits, cakes, cookies, crackers, and

others (45). The coefficient of digestibility of

protein ranged from 85 to 94 percent and that for

carbohydrate from 97 to 99 percent.

For flour mixtures that vary considerably with

respect to ingredients, such as cookies, energy

factors were calculated for each product, weighting the energy factors of each ingredient in proportion to the amounts present in the product.

This procedure is explained in more detail on

page 42.

Bran

Differences in apparent digestibility between

whole-grain flours and those of short extraction

suggested that bran might have a low coefficient

of digestibility. Some investigations have been

undertaken to determine the digestibility of bran

alone, and while the results were variable, the coefficients were in every case very low. The average apparent digestibility found for the protein of

bran was 40 percent based on 14 digestion experiments, and for carbohydrate, 56 percent based on

16 experiments. These coefficients were applied

to the heats of combustion of protein (nitrogenous

matter) and carbohydrate of wheat, and the energy

factors obtained for the two nutrients after customary deduction for urinary nitrop-en loss were

1.82 and 2.35.

Wheat breakfast foods

Foods in this group have been studied in some

detail in digestion experiments, but as the experimental diets were low in protein the apparent

digestibility as determined may have been too

low. Hence, rather than use these data in estimating digestibility, we used the coefficients of

digestibility of whole-wheat flour (table 13) to

obtain tentative estimates for whole-wheat meals

and other whole-wheat cereals. Likewise for

farina and other breakfast foods made from the

endosperm we used the coefficients of digestibility

of patent flour. These factors would result in

some overestimation of energy values for foods

subjected to special processing that reduces

utilization of any of the organic nutrients and

possibly for meals of a coarser grind than that of

wheat flour. For breakfast foods that are mixtures, we derived weighted energy factors if we

knew the kind and approximate proportions of

ingredients. _; ! \:^\Mm ..^ÉúlSi^MM

^Productslo^Srain^other thanlwheat^

The digestibility and physiological fuel values of

corn, oat, rice, and rye products have been studied

less than wheat but more than most of the remaining grains.

Although information on the various cereals is

not strictly comparable, on the whole it indicates

that differences in digestibility may be expected

among grains and that neither the fiber content

nor the level of protein intake alone appears to be

adequate basis to explain differences observed

among cereals when fed to human subjects.

Spriggs and Weir (172) found the digestibility

of the protein in a mixed diet containing bread

made with white flour to be 91 percent, but when

only a third of the flour was replaced successively

by oatmeal, barley flour, fine sifted corn flour or

rice flour the digestibility was 87.6, 81.5, 86.7,

and 89.7 percent, respectively. Jones and Waterman {77) observed significant differences in the

extent to which proteins may be digested in vitro

by pepsin and trypsin. They found that arachin,

casein, and cooked phaseolin were 48, 61, and 58

percent digested and suggested that the order in

which the amino acids were united to form the

proteins might be responsible for the incomplete

digestion; some linkages are more resistant than

others to the hydrolytic action of digestive

enzymes.

Other workers studying the physiological availability of purified proteins have observed mde

differences in the proportions of a given amino

305774—55- 31
acid in the feces of experimental animals, depending on the protein fed. These and other studies

indicate that among the various kinds of cereals

considerable difference in digestibility and physiological fuel values may be expected. Digestibility data from the literature for the various kinds

of cereals have been summarized separately. The

average coefficients of digestibility and the number

of experiments on which each was based are shown

in table 16. As more research becomes available

these data will no doubt need revision. Data on

wheat have been included in the table for ease in

comparison.

TABLE 16.—Coeßcients of apparent digestibility for

grain products

Grain Product Coefficient of digestibility

Cornmeal, wholeground, bolted

Cornmeal, degermed.

Oatmeal, rolled oats

Kice, brown

Rice, white or milled.-.

"Rye flour:

Dark

AVhole-grain

Medium

Light

Wheat flour:

Essentially whole

wheat, 97 - 100

percent extraction

Intermediate, 85-

93 percent extion

Straight or patent,

70 - 74 percent

extraction

Protein

Percent

60

76

76

75

84

65

67

71

75

79

83

Experiments

Coefficient of digestibility

Number

3

21

48

22

119

0)

0)

0)

Q)

72

53

104

Carbohydrate

Percent

96

99

98

98

99

90

92

95

97

90

94

Experiments

Number

3

21

24

22

119

(0

(0

(0

72

53

104

1 Coefficients of digestibility for rye flours were derived

less directly as explained on pages 33 and 34. Therefore,

the number of experiments is not indicated.

Cornmeal

The cornmeals studied have been of two general

types, whole meal, sifted through a 16-mesh

sieve which may have removed a small amount of

the bran, and degermed cornmeal which probably

had most of the bran, as well as the germ, removed. In addition to the meals, digestibility

data for several other corn products have been reported. Although most of these other studies

were not used directly they have been included in

appendix table 23 and were helpful in that the

digestibility coefficients tended to confirm the

averages for the two cornmeals shown in table 16.

Data for only three digestion trials were found in

which sifted whole-ground cornmeal was used.

The average coefficient of digestibility of the meal

eaten in the form of cornbread by three subjects

was close to 60 percent for protein and 96 percent

for carbohydrate. These values are in line witn

digestibility coefíicients for field corn (pressurecooked) and for hulled corn (hominy), which,

judging by their fat and fiber content, must have

contained the germ and more of the branny portion than degermed corn products. Degermed

cornmeal prepared as mush or as cornbread has

been used in several studies. Average coefficients

based on 21 digestibility trials were found to be

76 percent for protein and 99 percent for carbohydrate. Hence, the energy factors suggested

and used here for protein and carbohydrate m

whole grain corn products are 2.73 and 4.03

calories per gram, and in degermed corn products,

3.46 and 4.16.

Data on apparent digestibdity of frozen raw

cornmeal and toasted corn breakfast products also

were found in the hterature or could be calculated

from the information reported by the authors.

They have been recorded in appendix, table 23,

for reference but for various reasons were not used

in assessing the energy value of corn products.

Oatmeal

Several studies have been made of oat products,

mainly oatmeal or rolled oats, which were either

cooked and used as a porridge or baked as oat

cakes. Also recorded in this appendix table but

not used in obtaining average digestibility figures

were a few experiments on ready-to-eat cereals.

Based on 48 digestibility trials, the average

coefficient of digestibility for protein was found to

be 76 percent and that for carbohydrate, based on

24 trials, 98 percent. As was the case with most

of the cereals there was more variation in the

coefficients of digestibility for protein than for

carbohydrate. Using the average coefficients of

digestibility, the energ}^ factors for protein and

carbohydrate are 3.46 and 4.12 calories per gram,

respectively.

In most cereals the quantity of fat present is so

small that the use of Atwater's assumed digestibility coefficient of 90 percent for fat in plant

foods introduces very little error in the total

energy value of the food. Oatmeal, which contains 7 to 8 percent fat on an average, has more

fat than most cereals and its digestibility is of

more significance. In 28 experiments in which

the digestibilit}^ of fat in oatmeal has been reported

or could be calculated from data given, the results

have varied from 56 percent to complete digestibility. These values were largely from two studies,

a recent report b}^ McCance and Glaser, 1948

(102), and a much earlier article by Harcourt and

Fulmer, 1907 (62). The range in digestibility of

fat found by McCance and Glaser was 62.5 to

77.6 percent. In their experiments, six subjects

were fed a mixture of two oatmeals, having an

average fiber content of 0.9 percent, supplemented

with a little bramble jelly or skup. The daUy

intake of 34 to 67 grams of fat was supplied

entirely by the oatmeal.

32
A very wide range, 57.6-97.9 percent, in the

coefficients of digestibility for fat in oatmeal was

calculated from data in 16 experiments published

by Harcourt and Fulmer (62). These investigators tested four oatmeals with fiber contents of

1.94, 1.15, 1.12, and 1.04 percent. Seven subjects

participated but not all subjects ate each of the

four meals. The daily intake of 58-120 grams of

fat was supplied by oatmeal and either milk or

cream, with about one-fifth to one-third of the

total contributed by oatmeal. For the purpose

of calculating the digestibility of the fat in oatmeal from data supplied we assumed the digestibility of the fat of milk and cream to be 95 percent.

To the extent that this average figure may not be

applicable for these specific experiments there may

be some error in our calculated results for oatmeal.

The da ta from Harcourt and Fulmer suggest that

there may be some relation between digestibility

and fiber content of oatmeal. When the oatme-als

of both 1.94 percent fiber and lower fiber contents

were fed to the same subjects under similar experimental conditions, the fat digestibility of the oatmeal of 1.94 percent fiber was lower than that of

samples with the much smaller percentages of

fiber. Of interest in this connection are results

reported by McCance and Glaser (102) who

found that the substance or mixture of substances

which they estimated as fiber in oatmeal passed

through the gut almost without change. The

results raise several questions. Is intestinal motility related to the fiber content of oatmeals and if

so does this affect the digestibility of fat? To

what extent do metabolic products account for fat

(ether-extractable matter) in the feces? The experimental evidence is inadequate either to interpret the widely divergent coefficients found or to

determine which part of the range would be closer

to the true value. The Atwater energy factor,

8.37 calories per gram of fat in cereals and other

plant foods in general, therefore has been used

without change for fat in oat products.

Rice

The most extensive study on digestibility of rice

was carried out in a series of experiments by Sugimoto, Higuchi, Momyeda, Tonaka, Yasuda, and

others and was published by Saiki in 1926 (178).

The subjects, all Japanese men, ate rice as part of

a mixed diet. The rice used was of four categories

which were described in one of the articles as

follows:

(a) Unpolished rice.—Rice from which the

husk had been removed, but which still retained

the outer layer, or silver skin, and the embryo

or germ.

(b) 50% polished rice.—Rice which had been

milled and polished but which retained half of

the outer layer and germ.

(c) 70% polished rice.—Rice which still retained about 30 percent of the outer layer and

germ.

(d) Polished rice (white rice).—Rice which

had been pohshed perfectly, so that the germ

was almost entirely rubbed off in the milling.

The actual amounts of the rice kernel removed in

the polishing differ somewhat with the variety of

rice, but about 4 percent of the unpolished rice

kernel was removed in making the half-pohshed rice,

nearly 6 percent in making the 70-percent polished

rice, and approximately 7 to 8 percent in making

the fully pohshed while rice. According to one

article in the series no polishing powder was used.

Judging from other information given, the unpolished rice may have been the same as brown rice;

the ha If-polished rice may be considered as an

extraction of about 96 percent of the brown rice;

the 70-percent polished, as an extraction of about

94 percent; and the polished rice, about a 92-percent extraction. On the basis of paddy or rough

rice, which is a more common basis for expressing

milling yields, the extraction rates, assuming the

loss in removing the hulls to be 21 percent, would

be about 79 percent for brown rice, 76 percent for

half-polished, 74 percent for 70-percent polished,

and 73 percent for polished white rice.

Rubner (14-4) ^^^ í^lso Snyder (167) studied the

digestibility of rice when eaten by men subjects

as part of very simple diets. Probably the rice

used was ordinary white rice but it was not described in either study. The number of experiments in which Europeans and Americans were

subjects was too small to permit a good comparison

but there appeared to be no marked difference in

digestibility of the rice for the Japanese men who

very likely were accustomed to eating it as a major

item in their diet and for the German and American

subjects who probably had it only occasionally.

The average coefficients of digestibility for the

unpolished (brown) rice, based entirely on the

Japanese studies, were 75 percent for protein and

98 percent for carbohydrate; for half-polished rice

the coefficients were 82 and 99 percent, respectively, for the two nutrients, and for 70-percent

polished rice, 83 and 99 percent. For white rice

the average coefficients of digestibility based on

data reported by Saiki, Rubner, and Snyder were

84 percent for protein and 99 percent for carboh3^drate.

The energy factors we suggest for the protein

and carbohydrate of bro^vn rice are 3.41 and 4.12

calories per gram respectively, and for white rice,

3.82 and 4.16.

Rye

Many of the digestion experiments on rye flours

were conducted years ago in Germany. In some

studies the diet was simple rye bread or rye bread

and beer. Some of the subjects who were unaccustomed to eating large quantities of rye

bread experienced pain in the digestive tract and

in some cases diarrhea, a factor that might vitiate

the digestibihty figures. But on the basis of the

data reported there was no clear-cut means of

33
eliminating this factor. In other experiments

there was more variety in the diet but on the

whole the diets were simple. Kesults indicate no

appreciable differences in digestibility when subjects had rye bread alone or as part of simple

mixed diets.

Rye flours have been described as dark, wholegrain meal, medium, and light, and the following

data on their composition have been cited as

being representative for these products {100, 185) :

Constituent Dark

Wholegrain

meal

Medium Light

Water

Percent

11. 0

16.3

2. 6

2. 0

2. 4

68.1

Percent

11. 0

12. 1

1.7

1.8

2.0

73. 4

Percent

11. 0

11.4

1.7

1. 1

1.0

74. 8

Percent

11.0

Protein 9.4

Fat 1.0

Ash - _____

. 7

Fiber . 4

Carbohydrate, total by difference 77. 9

As there are no standards of identity for these

products, much variation may be expected in

appearance and composition of samples of a given

designation or grade. For example, light rye has

been described as ranging from white to mediuinlight fiour with a comparatively wide range in

proximate composition. In these circumstances,

it was difficult to determine the type of ffour used

in the early digestibility studies. However, there

is a marked decrease in fiber and ash content

with increasing degree of refinement, and the ash

content reported for the rye flours studied in the

digestion experiments served as a criterion for

deciding in which of the above four categories—

dark, whole-grain, medium, or light—to include

the data.

Little information is available on digestibility

of medium and dark rye flours. However, the

relationship of the digestibilities of either protein

or carbohydrate observed between the light and

whole-grain rye flours was similar to that observed

between the straight patent and whole-wheat

flours. On the assumption that this similarity in

ratios can be extended to include intermediate

extractions of rye and wheat flours, digestibihty

values for other extractions of rye flour were

imputed from ratios for wheat flours where data

on rye flour were lacking. Ash content was taken

as a general index of the degree of extraction of

the flour. Thus for rye flours of the composition

shown in Agriculture Handbook No. 8 (185) and

described as dark, whole-grain, medium, and

light we estimated the digestibility of protein as

65, 67, 71, and 75 percent and the corresponding

digestibility of the carbohydrate as 90, 92, 95,

and 97 percent, respectively. Using these coefficients of digestibility and Atwater^s heats of

combustion for cereals, the energy factors to

apply to dark, whole-meal, medium, and light rye

flour are, respectively, 2.96, 3.05, 3.23, and 3.41

calories per gram of protein and 3.78, 3.86, 3.99,

and 4.07 per gram of carbohydrate.

Other grains

For grains and grain products not included in

table 16 very few digestibihty data are available.

Amon,c^ the reports on these products is one by

Woods and Snyder (196) which summarizes the

results of the digestion experiments on cereal

foods at the Connecticut (Storrs), Mame, and

Minnesota Agricultural Experiment Stations.

For pearled and flaked barleys, Woods and

Snyder apphed their estimated coefficients ol

digestibihty of bariey products, 78 perceixt for

protein and 94 percent for carbohydrate, i hese

products had undergone some reíinenaent and m

chemical composition were much like modern

pearled bariey (185). The energy factors for

pearied bariey, calculated from these coefficients

by use of the customary heat of combustion of

grain products, are 3.55 and 3.95 calories per gram

of protein and carbohydrate, respectively.

For buckwheatflour, 'Jarina,'' and groats, Woods

and Snyder estimated digestibility at 78 percent

for protein and 94 percent for carbohydrate.

Their composition data indicated that the products

were refined forms, the fat, fiber, and ash content

of 1.2, 0.4, and 0.9 percents for the flour being

comparable to composition of modern light buckwheat flour. The above coefläcients of digestibility

applied to the heats of combustion for protein

and carbohydrate in cereal products result in the

following energy factors, 3.55 calories per gram

of protein and 3.95 calories per gram of carbohydrate.

For dark buckwheat flour, we have found no

experunental work on digestibility. The figures

in table 13 have been calculated arbitrarily from

data for light buckwheat flour, assuming that the

ratio of digestibihty of dark to light flour would be

the same as that between whole-wheat flour and

wheat flour of intermediate extraction. Judging

by the ash content, the light buckwheat flour may

have been comparable in degree of refinement to

wheat flour of intermediate extraction and the

dark buckwheat flour to whole-wheat flour. The

coefficients of digestibility thus assumed for dark

buckwheat flour were 74 percent for protein and

90 percent for carbohydrate, and the energy factors

were 3.37 and 3.78 calories per gram, respectively.

For wild rice, data are also to be found in the

summary by Woods and Snyder. They report the

coefficients of digestibility to be 78 percent for

protein and 94 percent for carbohydrate, which

would result in energy factors of 3.55 and 3.95

calories per gram, respectively.

Sorghums and millets, while little used in this

country for human consumption, are important

foods in some parts of the world. In sections of

the Far East, both grains are used extensively,

frequently prepared as a mush or ground into meal

and used in bread.

34
Langworthy and Holmes (93) conducted experiments with the dual purpose of comparing the

digestibilities of the different kinds of sorghums—

kaoUang, feterita, kafir, and milo—and the digestibihty of sorghums in general with that of other

cereals, namely, wheat and corn. The grains were

ground in the same mill and put through a 16-mesh

sieve. By this treatment 5 percent of the bran was

removed from kaoliang, 15 percent from feterita,

19 percent from milo, and 21 percent from kafir.

The differences in the structure of the grains

probably account for the different amounts of the

ground meals that passed through the 16-mesh

sieve, being largest for the softer kaohang which

grmds more readily than the corneous types. The

portions of the wheat and corn kernels removed

was not stated. Each of the sorghums was used as

bread or as mush; the corn and wheat as bread

only.

In one of the kafir bread series the remainder of

the diet consisted of milk, orange, and sugar.

Otherwise, the diets in the bread series had in addition to the bread, applesauce, butter, sugar, and in

most cases, potato. In the series containing mush,

the diet was similar except that potato was

omitted so that essentially all of the protein would

be supplied by the test food.

Whether a sorghum meal was served as bread or

as mush appeared to have had httle influence on its

digestibihty. The carbohydrate was well utilized

in all the sorghums; the average digestibihty for

the kaoliang was 96 percent and the carbohydrate

of the other sorghum meals was as well or better

utilized. Using 96 percent and 4.20 calories per

gram as the heat of combustion, the energy factor

for carbohydrate is 4.03.

The digestibihty of the protein was extremely

variable but considerably lower than that found

for the protein in either wheat meal or cornmeal.

Feterita and kafir, both hard corneous types of

sorghum, showed similar average protein digestibihty, approximately 50 percent. Milo is a somewhat softer type with a larger proportion ofstarchy

endosperm; its average protein digestibility was

about 40 percent. Kaoliang, which is very soft

and has a high proportion of starchy endosperm,

had a very low digestibihty, slightly less than 20

percent.

In an experiment conducted by Abe and others

(^) with Japanese subjects, when kaoliang was the

main food in a mixed diet, the average digestibility

of the protein in the total diet was 77 percent and

the carbohydrate, 99 percent. No estimate was

made for the sorghum alone. For the total diet

the averages in the Langworthy and Holmes

experiments were lower, only 24 percent for protein

in the diets containing kaoliang, and 42 to 64 percent for protein in the other sorghum diets; for

carbohydrate, 96 percent in the diets containing

kaoliang and a range of 96 to 97 percent in the

diets containing the other sorghums. The digestibihty was lower in the experiments of Langworthy and Holmes, particularly for the protein.

We question whether the higher coefficients for

protein indicated by the experiments on Japanese

subjects, possibly accustomed to eating kaoliane;

are applicable for measuring digestibility in persons

with an entirely different dietary pattern. Furthermore, we need to consider the composition of the

other foods used with the sorghums. In many of

the experimental diets of Langworthy, fruit, either

oranges or applesauce, was a major item.

The data available do not provide a good basis

for deriving a satisfactory figure for the apparent

digestibihty of sorghum protein. The variations

observed were extremely wide and the particular

extractions used for the Langworthy experiments

may not be typical of the sorghum meals ordinarily

used for food. A kaoliang meal of 95-percent

extraction may not be t3^pical but is not far short

of whole meal. If a factor for whole-meal sorghum

or nearly whole meal is needed, we suggest as a

tentative factor 0.91 calorie per gram of protein.

This is based on a digestibihty of 20 percent, indicated by the work of Langworthy and Holmes for

kaoliang and the usual heat of combustion for

protein in cereals. We recognize the possibihty

that for persons accustomed to eating kaohang,

this factor may be too low and that it may not

be equally apphcable to sorghums other than

kaoliang.

Millet also was studied by Langworthy and

Holmes (94). The experimental plan was similar

to that used for sorghums. Two millets were

studied, common millet. Setaria itálica, from

which 40 percent of the bran portion was removed,

and proso millet, Panicum miliaceum, from which

29 percent bran was removed by sifting the

meals through a 16-mesh sieve. Both were fed

as bread in a simple mixed diet of potato, orange,

butter, and sugar.

The utilization of carbohydrate in millet was

about like that for the sorghums. The average

digestibility observed for the carbohydrate of each

of the millets was high, 96 percent. The digestibility of protein of both millet meals was variable

but was low, averaging approximately 40 percent

for each kind. For millets as for the sorghums

these samples prepared as described may not be

at all comparable to the millet meals actually

used. We have not attempted to derive calorie

factors for millets but have called attention to

this work since anyone needing data on millets

may be able to adapt the information to their

purpose.

Other grain products include various refined

cereal foods such as breakfast foods prepared

from a mixture of grains and also starches and

flour mixtures. For some a few scattered data

are available but the experimental conditions were

not always such as to make them suitable for use

in obtaining representative coefficients of digestibihty. In lieu of satisfactory data, Atwater's

group factors for cereals were used for these

various products. His factors, 3.87 calories per

gram for protein, 8.37 for fat, and 4.12 for carbo35
hydrate, were predominantly weighted by refined

cereals and therefore should give a fairly close

approximation of the energy yalue of foods that

haye undergone considerable refinement.

Legu mes

The array of data on digestibilitv of foods in

the dry legume group (see appendix table 23)

shows that seyeral studies haye been made of the

more important items. For beans, peas, and

soybeans the digestibility of carbohydrate (determined by difference) was high, ayeraging 96 to 98

percent; for cowpeas it was lower, about 90

percent. These high coefiicients suggest possible

utilization of some of the fiber and pentosans.

Data are inadequate to explain the disappearance

from the gut of much of the complex carboh^^drate

matter. Results of some inyestigations indicate

that some are split by means of bacterial action

into their simpler components and ultimately

into end products that may be discarded by the

body. To what extent intermediate products are

absorbed is an unanswered question.

Both the kinds and amounts of carbohydrates

present are of particular interest in comparing

digestibilities of yarious legumes. Some legumes

are similar in their content of moisture, protein,

fat, ash, and total carbohydrate by difference,

but seyeral experiments indicate that the similarity

does not hold for indiyidual carbohydrates.

Differences in digestibility might be more easily

understood had more information been obtained

on the makeup of the carbohydrate fraction in

the legumes samples used. Data in the literature

indicate that the proportion of the total carboh3''drate (by difference) in the form of the so-called

ayailable carbohydrates, mainly starch, sugar, and

dextrin, is less than one-half for cowpeas, twothirds to more than three-fourths for beans

(kidney, lima, mung) and chickpeas, 85 to 90

percent for lentils and peas. The total amount

of crude fiber plus pentosans yaried for the seyeral

foods, 12 percent for kidney beans, 10 percent for

mung beans, 13 percent for chickpeas, 10 percent

for cowpeas, and 7 percent for lentils. The

undetermined fraction makes up a relatiyely

large portion of the total carbohydrates in beans

and cowpeas but only a small percentage in the

other legumes.

For soybeans as much as two-thirds of the

total carbohydrate is made up of the carbohydrate

fraction usually considered to be of questionable

ayailability. In this fraction a yariety of substances has been found in widely yarying amounts.

These include raSinose, stachyose, pentosans, galactans, arabans, cellulose, lignin, organic acids,

pbytin, and glycosides. In addition to these

some waxes, color principles, tannins, and undetermined hemicelluloses are belieyed to be present.

Bowers (£9) studied the composition of the

complex carbohydrate fraction of defatted soybean meal and the digestibility of separate constituents with a healthy man engaged in I^^^Í^'

ately actiye laboratory work as the subject. I^®

digestion experiment was carried out according to

ciistomary procedures. The methods of analyses

employed were those used by Street and Bailey

(17A) "^ Analyses were made of both the soybean

meal and the feces resulting from a ration of soybean meal porridge, milk, butter, and cane sugar.

Bowers reported the coefficients of digestibility

for the carboh^^drates of cooked soybean meal as

follows: Dextrin and starch, 99 percent; sucrose,

100 percent; raffinose, 100 percent; organic acids,

99 percent; pentosans, 93 percent; galactans, 9b

percent; cellulose, 79 percent; and the remaining

fraction, presumably waxes, color principles, and

possibly undetermined hemicelluloses, 94 percent

by difference. By calculation from the separate

constituents he arriyed at a digestibihty coefficient

of about 94 percent as compared mth 96 percent

which he found independently for total carbohydrate by difference.

Entirely different results for digestibility ol^ the

carbohydrate of uncooked soybeans were obtained

by Adolph and Kao (3) in a series of m yitro

experiments in addition to in yiyo digestibility

experiments with rats. They estimated the ayailability of soybean carbohydrate to be about 40

percent. This figure has been widely used in

assessing soybean carbohydrate {36, 37, 185), but

its application to so^^beans for human use seems

questionable.

In the manufacture of soybean curd and milk,

much of the carbohydrate fraction is remoyed and

the carbohydrates that are left appear to be almost

completely digested. A digestibility coefficient of

98 percent was found for curd in a Japanese

experiment {13If).

Although there may be significant differences

in digestibility of carbohydrates in different legumes as indicated in the yery few studies ayailable, it does not appear wise at the present time

to depart from the group factor, 4.07 calories per

gram, originally used by At^yater for carbohydrate

in legumes.

A large number of studies has provided data

on the apparent digestibility of protein in legumes. Although some yariation may be observed

(see appendix table 23) the data on the whole

support the digestibility and energy factors used

by Atwater. They are suggested here for use

with soybean curd and milk because the data in

the literature are too variable to indicate whether

or not the factors for either protein or carbohydrate

are applicable to these two products.

Nuts

Nuts present problems regardmg digestibility

and composition similar to those of the legume

group. Little has been reported on their digestibility and it is difficult to evaluate the few results

that have been published. The most extensive

work on this food group was reported by Jaffa

36
(75), who determined the digestibihty of diets

composed of fruit and nuts in 28 experiments.

The kinds of nuts studied were almonds, Brazil

nuts, coconuts, pecans, walnuts, and peanuts, a

legume which is used like nuts. In 20 of these

experiments, in which most of the protein was

supplied by nuts, the average digestibility for the

dietary protern was about 75 percent. This value

is lower than usually is found for mixed diets m

which plant foods predominate. Possibly the low

apparent digestibility was the result of the large

quantities of fruit consumed. Unfortunately the

effect of the amount and kind of carbohydrate

from the fruit on the digestibility of the protein

and fat in the nuts could not be determined from

these studies.

The apparent digestibility of the protein of

nuts alone estimated from these 20 experiments

by use of Atwater's figure of 85 percent digestibility for fruit protein, ranged from 54 to 87 percent, averaging 70 percent for nuts as a group.

From the two experiments on peanuts the calculated coefficient of digestibility for protein would

be 81.5 percent. These figures may be too low

since the calculation is dependent on the digestibility of the whole diet. Holmes (70, 72) found a

much higher digestibility, 92 percent, for peanut

protein. He used a simple mixed diet which

included either pressure-cooked peanuts or baking

powder biscuits that had been made with peanut

flour. The peanuts Jaffa used were not described;

presumably they were ordinary roasted peanuts.

In diets in which boiled or roasted chestnuts contributed most of the total protein intake Heupke

and others (67) found that the digestibility of

chestnut protein ranged from 68 to 79 percent.

In view of the variable results for peanuts and

for nuts, there is no good basis for estimating

digestibility of protein in nuts of different kinds

or even as a group. It is preferable to continue

to use, as an interim value for nuts, Atwater's

group coefficient for protein in legumes, 78 percent. The grouping of nuts with legumes is a

common practice and has some basis since there

are many points of similarity in the proximate

composition of these two food groups.

Very little work on the digestibility of carbohydrate material in nuts has been reported. Only

a small fraction of the total carbohydrate intake

was furnished by nuts in Jaffa's series of experiments and the digestibility of carbohydrate in

nuts has not been estimated. Merrill (IW) found

that more than 98 percent of the carbohydrate in

chestnut flour was digested. Heupke and others

(67) found digestibilities ranging froni 96.5 to 99.9

percent in several experiments in which chestnuts

were fed as raw flakes or cooked by boiling or

roasting.

Data reported in the literature on the composition of various nuts indicate that most nuts contain from one-half to two-thirds of their carbohydrate in the form of sugars or starch or both,

up to one-third as crude fiber, pentosans, and

simiilar romplf^x carbohydrate constituents, and a

like amount as undi^tormined matter. Almonds

appear to have a lower proportion of sugar and

starch, averaging around 40 percent, and have

about 25 percent in the form of complex carbohydrates that are of questionable availability.

The nature of the remaining portion is undetermined. The limited data for peanuts are too

variable to estimate the proportions in which the

carbohydrate components are distributed.

As readily can be seen, the information on the

composition and the digestibility of the carbohydrate fraction of nuts is far from complete.

Therefore, it appears best to continue to use the

coefficient, 97 percent, assigned by Atwater to

carbohydrate in legumes and nuts, and his energy

factor, 4.07 calories per gram.

Vesetables

Very few digestibility studies of vegetables had

been made when Atwater proposed for all vegetables as a group the factors 3.11 calories per gram

for protein and 3.99 calories per gram for carbohydrate. Data accumulated since are still limited

but provide some basis for separate factors that

may be applied to smaller groups of vegetables.

Potatoes

Potatoes have been studied more than other

vegetables; results from 10 investigations have

been noted in the literature. In most cases the

composition of the samples used in the digestion

experiments was not reported.

The average digestibility coefficients fouad for

protein by the 10 investigators were from 64 to 85,

averaging 74 percent. These are surprisingly low

values and we consider them tentative estimates.

In several of the experiments the total protein

intake was low. Potatoes contain very httle

nitrogen, only about 0.3 percent, but several

observers have pointed out that subjects have

remained in generally good physical condition for

long periods of time on diets in which potatoes

are practically the sole source of protein.

One such study was reported by Kon and Klein,

1928 (83), who conducted digestion experiments

over a period of 167 days. The two subjects, a

man aged 25 and a woman aged 28, remained in

nitrogen equilibrium and in apparent good health

on a very simple diet mth the daily intake of

nitrogen chiefly from potatoes, averaging only 5.7

grams for the man and 3.8 grams for the woman.

The coefficients of digestibihty of the potato protein were 66 and 62 percent respectively for the

man and woman.

Three subjects in the study by Hmdhede, 1913

(69), also were on simple diets in which potatoes

contributed nearly all the protein over an extended period of time. The diets were planned

to provide the minimum protein intake at calorie

levels just sufficient to maintain nitrogen equihbrium. The digestibility of the potato protem

37
ranged from 71 to 86 percent. For 9 months

one subject, M, had a daily nitrogen intake from

5.8 to 8.4 grams and maintained a schedule of

varying activity, including 3 months of hard labor,

without any apparent Si effects—his excellent

physical condition at the end of the period was

confirmed by four physicians. The apparent

digestibiUty of the potato protein in this case

averaged 84 percent.

In a 10-day experiment Rose and Cooper {H2)

also observed good utilization of potato protein.

The subject, a woman, was able to maintain

nitrogen equilibrium, after the third day, on a

diet of potato, sugar, and agar-agar, which provided a relatively low nitrogen intake, 4.8 grams

per day. The apparent digestibihty was 74

percent.

The detailed analyses reported on the nitrogenous fractions of potatoes by Street, Kenyon

and Watson, 1946 {178), Crook and Watson,

1948 iU), Neuberger and Sänger, 1942 {ISO),

and Headden, 1927 {65), indicated that most of

the nitrogenous matter in potatoes would be

available. The proportions of the different nitrogenous materials vary greatly from sample to

sample, the coagulable fraction (proteins, proteoses,

peptones) ranging from 30 to almost 75 percent

and averaging around 40 percent, and the amino

acid fraction from 30 to 60 percent, averaging

about 50 percent. Of the small remaining fraction, 2 to 6 percent has been determined as

ammonia and nitrate nitrogen.

Of more practical significance than nitrogenous

compounds in estimating energy values for

potatoes are data on digestibility and composition

of the carbohydrate fractions. Results from six

studies reporting digestibility of total carbohydrate ranged from 92 to more than 99 percent,

with an average of 96 percent. These values

appear reasonable in view of the high proportion

of starch, sugar, and dextrin in potatoes. Also

present are lignin, cellulose, pentosans, pectin ,

and other hemicelluloses, but the complex carDOhydrate constitutents of doubtful avaüaDUiiy

appear to make up only about 1 to 3 percent ot

the potato. Sugars have been found m varying

amounts, from less than 1 to as much as 6 V^l^^^-

The quantities of dextrm present are small.

Starch makes up nearly all of the remammg carbo-

^^lî^a^recent compUation the following average

data were obtained: Carbohydrate by àf^^^n^^^^

19.1; starch, 17.1; sugar, 0.3; crude fiber, 0.6;

and undetermined, 1.1 percent. This last fraction

may contain pentosans, pectins, and other carbohydrate constituents not determined as crude

fiber by the Weende method. It has been sho^vn

by Remy {189), Wühams and Olmsted {^87).^^

Weinstock and Benham {186) that this method

fails to measure much of the total fiber.

For potatoes it seems to us best to continue the

use of the heats of combustion which Atwater

assumed for potatoes and other vegetables (see

table 7). As a check on the application ol these

factors to potatoes, we calculated gross heats

from the chemical analyses of three samples

and compared the results of our calculation with

values determined in the bomb calornneter for

the same samples. Data from protein, fat, and

carbohydrate analysis and from the bomb calorimeter were taken from a study of Bryant and

Milner {88). Close agreement between the determined and calculated heats of combustion was

found as shown in table 17. The energy factors

we derived for potatoes were 2.78 calories per

gram for protein and 4.03 calories per gram for

carbohydrate. They were derived by applying

average coefficients of digestibility found in the

literature, 74 percent for protein and 96 percent

for carbohydrate, to the heats of combustion,

5.00—1.25 and 4.2 calories per gram, respectively.

TABLE 17.—Comparison oj determined and calculated gross energy values of potatoes

Composition Heat of combustion per 100

grams

Sample number

Water Protein Fat Carbohydrate

by difference Fiber Ash Bomb calorimeter

Calculated i

from composition

1

Percent

79.5

78.3

81.2

Percent

2.2

2.3

1.9

Percent

0.1

. 1

,3

Percent

17. 4

18.4

15.5

Percent

0.4

Percent

0.8

.9

1. 1

Calories

84. 8

90.0

78.2

Calories

85.0

2

89.7

3

77.4

1 For heat of combustion factors used see table 13, page 25.

Other vegetables

Because published data are lacking on the

digestibility of many vegetables, we have applied

group factors—the energy factors for dried legum.es

to immature shelled beans, peas, and other

legumes, those for fruio to rhubarb and tomatoes,

and those for potatoes to starchy roots and tubers.

For other underground vegetables such as beets,

carrots, onions, parsnips, and radishes we have

applied the energy factor for protein and the

coefficient of digestibility for the carbohydrate

in potatoes, but have made an adjustm.ent in the

heat of com.bustion factor generally used for

38
vegetable carbohydrate to correct for the relatively large proportion of sugar. We assumed

that from two-thirds to three-fourths of the

carbohydrate in most of these underground vegetables is present as sugar, and one-thii^d to onefourth as starch and fiber. On this basis the

weighted heat of combustion would be 4.00

calories per gram and the energy factor calculated

for carbohydrate, 3.84 calories per gram.

Coef&cients of digestibility for protein and

carbohydrate in the few vegetables for which

data have been reported vary widely. We

rounded the median figure for digestibility of

protein, based on 14 experiments, to 65 and have

used it rather than Atwater's figure of 83 percent.

The carbohydrate fractions are an important

source of energy in some vegetables. We have

used a coefficient of 85 percent, the median value

for 13 experiments on a variety of vegetables, in

place of Atwater's figure of 95 percent. For the

remaining vegetables except mushrooms we have

calculated the energy factors by applying digestibilities of 65 percent for protein and 85 percent

of carbohydrate to the heats of combustion used

by Atwater for vegetables. The energy factors

obtained in this way are 2.44 calories per gi^am for

protein and 3.57 calories per gram for carbohydrate.

Mushrooms

Reports in the literature cover various aspects

of the composition of the nitrogenous matter of

mushrooms, but as yet there is no complete picture

of the quantitative distribution of the various

constitutents. From 63 to 72 percent of the total

nitrogen has been termed ^'protein nitrogen" {53,

123, 175). Other known constituents are free

amino acids, amides, purines, and am.m.onia. In

som.e instances appreciable am.ounts of urea have

been determined. Iwanoiï (73) reported that

amino acids are formed autolytically during the

ripening period before spore formation and are in

turn changed into urea. He found that urea was

several times as high at the ripened stage as in

the young immature mushrooms, and that in some

samples the urea nitrogen approxim.ated half of

the total nitrogen. Mendel {116) suggested that

som.e of the nitrogen in m.ushroom.s is bound with

cellulose and that all attempts to separate the

nitrogenous constituent from, the portion that

yields sugar on hydrolysis had failed.

Thus it is apparent that use of the conventional

factor 6.25 to convert nitrogen to protein introduces an error in the value for the nitrogenous

matter, but at present there are insufficient data

to provide a better factor. Hence, we have continued to use the factor 6.25 in calculating total

nitrogenous material in m.ushrooms, although we

realize that the error involved may be of some

significance.

Urea, as well as some of the other nitrogencontaining substances, has a lower heat of combustion than protein, but since we could m.ake no

305774—55 4

accurate estimate for these substances we have

used the heat of combustion of vegetables, 5.00

calories per gram, for nitrogenous m.atter in

mushrooms.

Very little work has been noted on the digestibility of the nitrogenous matter of mushrooms.

Saltet {152), in a 2-day study of a 31-year-old

man, found that his subject digested 69 percent

of the nitrogenous matter when mushroom.s combined with a little butter and seasonings were

fed. A similar result, 72 percent digestibility,

was obtained by Skinner, Peterson, and Steenbock

{161) when mushroom.s were fed to albino rats.

A digestibility coefficient of about 70 percent

seems a reasonable estim.ate and following the

usual procedure of applying it to the heat of combustion, 5.00 calories less 1.25, we derive an

energy factor of 2.62 calories per gram of nitrogenous matter.

The carbohydrate fraction of mushroom.s also

includes a variety of components, not all of which

have been determined quantitatively.

One of the most complete analyses of carbohydrates m mushroom.s reported to date was made

by McConnell and Esselen {109), but 51.8 percent

of the total carbohydrate was still unidentified.

The data from this study expressed as percent of

fresh mushrooms and as percent of total carbohydrate follow:

Carbohydrates in mushrooms

(Agaricus campestris)

Total carbohydrate (by difference) __.

Mannitol

Reducing sugars (as dextrose)

Pentoses, methyl pentoses, hex

uronic acids

Glycogen

Crude hemicellulose

Content

on fresh

basis

Percent

5. 75

.95

.28

. 04

.59

. 91

Proportion

of total

carbohj'^drate

Percent

100. 0

16. 5

4.9

. 7

10.3

15. 8

Other carbohydrate constituents that have been

reported as occurring in mushrooms include cellulose, lignin, trehalose, indican, and amino-hexose.

The published data indicate that not only is the

total carbohydrate fraction complex but also that

some of the components vary in amounts, possibly

depending on variety and other factors such as

drying and storage.

There is no very reliable information from which

an estimate of the digestibihty of the carbohydrates in mushrooms can be obtained. Only one

digestion experiment, reported by Oshima {134),

has been noted in the literature. The subject, a

Japanese army surgeon, was on an experimental

diet consisting of 74 grams of dried mushrooms

(Agaricus sitake) and 40 grams of soy sauce lor

1 day. According to the author a satisfactory

separation of the feces was obtained by the use of

buckwheat flour. The digestibUity found for the

carbohydrate was 84 percent

39
For calculating the energy value of the carbohydrate in mushrooms, Watt and Merrill {185)

used a factor of 1.35 on the assumption that only

mannitol, reducing sugars, and glycogen, which

account for approximately 33 percent of the total

carbohydrate, were available for absorption. The

data from the one digestibility study which has

since been located suggests that such a procedure

may underestimate appreciably the energy value.

Until additional digestibility data on mushrooms

are available, it therefore seems preferable to use

a digestibility coefficient of 85 percent as for most

vegetables and to apply it to a heat of combustion

value which corrects for the presence of appreciable amoim,ts of sugars. Using the composition

data of McConnell and Esselen and assigning

heat of combustion values of 3.75 to mannitol,

reducing sugars as dextrose, and pentoses, 4.19

to glycogen, and 4.20 to the remaining fraction,

the resulting heat of combustion value for total

carbohydrate becomes 4.1 calories per gram. By

applying the coefficient of digestibility, 85 percent,

to this value the energy factor for carbohydrate

in mushrooms is 3.48 calories per gram.

Fruits

The energy from fruits comes largely from

carbohydrate. The energy factor, 3.60 calories

per gram, was applied by Atwater to carbohydrate,

based on the heat of combustion figure of 4.00

calories per gram and digestibility of 90 percent.

In arriving at this heat of combustion value he

took into consideration that the carbohydrates of

fruits are a mixture of sugars, mainly lévulose

and dextrose, but that starch, cellulose, pentosans, and other complex carbohydrates are also

present.

We consider 3.60 a reasonable group factor and

have applied it to most individual fruits, but with

full recognition of the possible inaccuracies

involved. For example, the coefficient of digestibility 90 percent is probably too low for fruit

juices and for sweetened canned or cooked fruit.

The group factor for heat of combustion of

carbohydrate in fruits will not apply equally well

to individual fruits. A compilation of the proximate composition of fresh fruits {38) showed

considerable variation among fruits in the proportions of sugar, starch, acid, and crude fiber

present. There is need for revision and extension

of this compilation to include data available since

its publication, particularly with respect to the

carbohydrate constituents, before further estimates for heats of combustion of carbohydrate in

individual fruits are made.

Attention is called to lemons in particular,

since they have considerable citric acid with a

heat of combustion of only 2.47 calories per gram.

As the value 4.00 would be too high for the heat

of combustion for total carbohydrate (by difference) in lemons and lemon juice, we have used

2.75 derived by the following calculations:

Constituent

Invert sugar

Citric acid (anhydrous) _ _ _ _.

Fiber and unknown constituents

Total carbohydrate

(by difference)

1 gram carbohydrate=2

Grams per

100 grams

1. 5

6. 0

. 2

7.7

75 calories (21.2-^

Calories

per gram

3. 75

2. 47

4. 2

Total

calories

5, 6

14. 8

21. 2

7.7).

Since the fiber fraction of lemon juice is very

low and since both invert sugar and citric acid

may be com.pletely utüized, we took the figure

recommended by Atwater for sugar, 98 percent, as

a reasonable value for apparent digestibility.

The resultant energy factor was 2.70 calories per

gram of carbohydrate. Since lim.es are similar to

lemons in carbohydrate constituents we have

applied the sam.e energy factor for carbohydrate.

For lack of better data for other fruits we have

continued to use the carbohydrate factor, 3.60

calories per gram, derived by Atwater. Likewise,

his factors for protein and fat in aU fruits have

been used.

Miscellaneous foods

Many specific foods have not been studied in

human digestion experiments, as can be seen from

the com.pilation on digestibility coefl&cients (appendix table 23). In many cases when digestibüity data on individual foods were lacking, we

have used a general value for a group of foods for

each food in that group. In other instances when

a food has undergone some treatment to change

its form, the energy factor of the food in its

original form has been applied to the product or

products. These procedures no doubt result in

some errors.

Where the above procedures were not applicable

and when the m.ethods of estimating the energy

factors differed in some respects from the general

procedure usually followed, these deviations will

be explained in turn for the several miscellaneous

foods.

Chocolate and cocoa

Chocolate and cocoa present a variety of

problems in regard fco both chemical composition

and digestibility. Determ.inations of various nitrogen-containing compounds have been made in

a few studies. It appears that from 12 to 23

percent of the total nitrogen present is in the form

of alkaloids, mainly theobromine and c-afîeine,

and 1 to 9 percent as ammonia, and that the

remainder, although not clearly identified, m.ay be

in the form of protein or protein derivatives.

Data reported by Stutzer {177) are the most

complete of the analyses located and seem to be

representative values when compared with several

40
less complete analyses made by other investigators. Stutzer analyzed several kinds of cocoa.

For one product which had not been treated with

alkaline chemic^als (potash, soda, or amm.onia) he

found that 16.6 percent of the total nitrogen was

from alkaloids (mainly theobromine), 1.4 percent

from, ammonia, 6.3 perceut from amides, and

75.8 percent from other nitrogenous matter.

Using these data, we have calculated the gross

energy per gram of total nitrogen in cocoa as

follows:

Compound

Nitrogen per

gram

Conversion

factor

Amount

of compound

Heat of

combustion

Gross

energy

equivalent

Protein _ _

Om.

0. 758

. 166

. 014

.063

6. 25

3. 22

1. 22

5. 35

Gm.

4. 74

. 53

.02

. 34

Cal./gm.

5.80

5. 22

Cal.

27. 5

Alkaloids as theobromine _ . _ __ 2. 8

Ammonia _ _ _

Amides as asparagin. 3. 45 1. 2

Total 1. 001 5. 63 31. 5

Gross energy equivalent of 1 gm. nitrogenous

matter = 5.60 calories.

Digestibility of the nitrogenous portion of

cocoa was studied by some of the early German

scientists. In these studies there ^^as no atcempt

to distinguish between the protein and* nonprotein fractions of the nitiogenous matter. In

some of the studies cocoa supplied all of the

nitrogen, and in som.e the diet included other

protein foods in addition to the cocoa. Experiments conducted by Weigm.ann and by Lebbin,

reported through König {84, pp. 244-245), showed

the following results: For three kinds of cocoa fed

in amounts of 188-304 grams per day along with

sugar and water, Lebbin found protein digestibility coefíicients of 41.1, 45.2, and 41.6 percent;

for a diet of cocoa and beer or wine, Weigm.ann

found a digestibility of 41.5 percent after correction for metabolic nitrogen. The apparent digestibility was calculated to be 12.7 percent. For

two Idnds of cocoa, Neumann {1SÎ) reported that

Beddies found digestion coefläcients of 55.3 and

54.1 percent. In these latter experim.ents, 150

grams of cocoa were consum.ed daily but no

information was given on the remainder of the

diet.

Several studies have been reported in which

cocoa was eaten in combination with other proteincontaining foods. There is some indication that

digestibility of the cocoa may be affected by the

level of cocoa and its proportion of total dietary

nitrogen, the combination of foods used with the

cocoa and possibly its preparation—whether raw

or cooked. Forster {66) found for a diet of milk

alone that the protein digestibility was 93 percent

as compared with 93.2 and 92.4 percent for diets

of milk and cocoa, with the latter taken in amounts

of 20 grams (2 to 3 cups of beverage) and 60 ^ams

(8 cups), respectively. Schlesinger {153) found a

digestibiUty of about 86.5 percent for protein in

a mixed diet consisting of milk, meat, refined

cereal, and fat, whereas when 60 grams of cocoa

were eaten in addition the digestibiUty of the

protein of the total diet was slightly lowered and

was about 84 percent. Beddies, as reported by

Neumann {131), also found the digestibility of

protein to be about 84 percent for a mLxed diet

which included 50 grams of cocoa. Cohn {If.1)

obseiwed a lower digestibility coefficient, 75.5 percent. His diet differed from those of vSchlesinger

and Beddies mainly in that larger amounts of

cocoa, 100 to 130 grams, were used and milk was

not included.

Neumann has reported two studies on cocoa {131,

132). In one series of investigations he determined digestibility of diets made up of sausage,

brie cheese, rye bread, lard, and sugar, in which

cocoa replaced equivalent amounts of protein and

fat of the diet. The digestibihty of the protein of

the diet without cocoa averaged 82 percent. Wlien

35 grams of cocoa were included in the diet, the

digestibility dropped to 75 percent, decreasing still

further to 57 percent when the daily intake of

cocoa was increased to 100 grams. In the second

series, Neumann found very low digestibiUty of

the cocoa protein, namely, 45 and 25 percent in

two experiments in which 35 grams of cocoa were

eaten with 350 grams of sugar as the only other

food in the diet. Inasmuch as he corrected for

the nitrogen in the digestive juices in getting his

digestibility of cocoa, the apparent digestibility

would be still lower. These results indicate that

the digestibiUty of the nitrogen portion of cocoa

is considerably lower than that of the other foods

in the mixed diets studied. Unfortunately the

digestibility of cocoa cannot be calculated in the

several studies on mLxed diets and there is no

means of determining whether the utilization of

cocoa as a ñavoring ingredient in the diet as it is

normally consumed is better than when it is used

alone or with sugar only.

On the basis of studies in which cocoa was the

chief source of nitrogen, we have used a digestibility of 42 percent, although this may be too low

for ordinary appUcation. When the 42 percent

figure is applied to 5.6, the heat of combustion

derived as shown above, less 1.25, an energy

factor of 1.83 calories per gram of nitrogencontaining material results. Information on the

utilization of the nonprotein nitrogen is needed

before a more accurate factor can be developed.

The carbohydrates of chocolate and cocoa present problems similar to those for the nitrogenous

matter. Some unpublished data* for chocolate

Uquor show the following complex composition:

28.4 percent total carbohydrate by difference; 8.0

percent starch; 2.8 percent fiber; 3.5 percent

pentosans; 2 to 3 percent gums and hemiceUulose;

4 Winkler, W. 0. Unpublished data. Food and Drug

Administration, U. S. Department of Health, Education,

and Welfare [n. d.].

41
9.5 percent products such as tannins and cocoa

red; 0.5 percent sugars, mainly glucose; 0.6 percent

organic acids; and the remainder, undeterm-ined

matter. These data were used in estimating the

heat of combustion for the total carbohydrate (by

difference). The value obtained was 4.16 calories

per gram, using as heats of combustion 3.75

calories per gram for sugar, 2.45 for organic acid,

and 4.20 for the remaining constituents and weighting them according to the percentage composition

above.

Digestibility data for the carbohydrate of cocoa

are even Jess conclusive than those reported for

protein. In the few experiments located in which

cocoa was fed in mixed diets, the digestibility of

carbohydrate coidd not be calculated for the total

diet or for the cocoa. The experiments of Lebbin

on diets of cocoa and sugar reported by König

{8/i) indicated that probably less than a third of

the tota] carbohydrate of cocoa is available to the

body. Here, as in the case of protein, this estim.ate may be lower than actually found when

cocoa is used in a mixed diet. As a tentative

coeiïicient, until satisfactory data can be obtained

on the digestibility of cocoa carbohydrate, we are

using 32 percent. This was indicated both by

the work of Lebbin and by the carbohydrate

composition data above for chocolate liquor,

assuming the starch, sugar, and organic acids to be

almost completely digested and the remaining

constituents to be undigested. The energy factor

calculated from this coefficient and the heat of

combustion factor 4.16 is 1.33 calories per gram.

Yeast

The utilization of yeast '^protein'^ has been a

matter of great interest. A number of studies

have been reported on the digestibility of the

nitrogenous matter in yeast, but in only a few of

these were human subjects used. Kuen and

Puringer {87) compared its digestibility in dried

and fresh compressed yeasts, presumably baker's

yeast, fed in a mixed diet that furnished 10 to 11

grams of nitrogen and 2,460 to 2,840 calories

daily. The estimated digestibility was 90 percent

for the nitrogenous matter of dried yeast but only

63 percent for the fresh compressed yeast.

Dirr {50) reported experiments in which either

dried yeast or animal sources of protein were fed

in alternate periods of 7 days each. The daily

nitrogen intake in each case was 10.4 grams from

the test food with additional 3.4 to 5.6 grams

from plant foods. The calorie intake for the four

subjects ranged from about 2,000 to 2,800. The

digestibility of the nitrogen of the total diet

averaged 87.5 percent in the period in which

nitrogen was supplied largely from animal sources,

and 83.4 percent in the period in which yeast

predominated in the diet. These results indicate

that the nitrogenous matter of the yeast was

almost as completely absorbed as animal protein.

DÛT and Soden {61) referred to the yeast as wood

sugar dried yeast and estimated from analyses

that 67 percent of the total N was amino N and

7.5 to 16 percent was ammonia N.

Funk, Lyle, and McCaskey {59) reported experiments in which a dried anaerobic yeast

preparation was eaten in a diet consisting largely

of vegetables and fruits. The daily nitrogen

intake, mainly from yeast, was 5.9 grams and the

digestibility of the nitrogenous matter was estimated to range from about 60 to 80 percent,

averaging about 70 percent. Results of Murhn

and others {125) indicated that the apparent

digestibihty of nitrogen in brewer's yeast was

about 57 percent. The daily nitrogen intakes

were very low, averaging 3.7 grams daily.

The data indicate that the average apparent

digestibility of the nitrogenous matter of dried

3reast is probably in the range of 70 to 90 percent

when the level of intake is fairly adequate. In

deriving an energy factor, we have estimated the

coefficient of digestibihty as 80 percent.

According to an analysis of yeast reported by

Frey {58) the nitrogenous matter is composed of

60 percent monoamino acids, 20 percent diamino

acids, 12 percent purines and p3?TÍmidine bases,

and 8 percent ammonia. These data indicate

that the heat of combustion is lower than if the

nitrógeno as matter of yeast were all protein.

Therefore, for yeast protein we used the heat of

combustion 5.00 calories per gram that we applied

to vegetables. The digestibility coefficient, 80

percent, applied to 3.75 (5.00 less 1.25) results in

an energy factor of 3.00 calories per gram.

Very httle research has been noted on the composition of the different specific carbohydrate

constituents in yeast and none at all on their

digestibilit}?-. Frey {58) has reported that 81.5

percent of the total carbohydrate is glycogen and

18.5 percent, such substances as cellulose and

gums. On the basis of these data we used 80

percent as a tentative coefficient of digestibility

on the assumption that the glycogen is digestible

and the other carbohydrates may not be. This

coefficient applied to the heat of combustion 4.20

calories per gram, assumed for total carbohydrate

(by difference), resulted in the energy factor 3.35

calories per gram.

Food mixtures

To keep pace with marketing practices as well

as buying habits, successive editions of food composition tables contain a growing proportion of

items that are food mixtures. Included are a

wide assortment of baked goods, meat and cereal

mixtures, salad dressings, and others that are

combinations of ingredients. Because the many

food mixtures vary so much in the kinds and proportions of ingredients used, information on their

digestibility from experiments can scarcely be

expected. If the weights of ingredients are

known, calorie factors per unit weight of total

protein, fat, and carbohydrate in the finished

product may be calculated. For products in

42
which the proportioDs of ingredients are fairly

standard, the calorie factors once worked out may

be applied directly to data on the amounts of

protein, fat, and carbohydrate in the product.

To calculate the calories for any given weight

of an item from its recipe, the weight of the finished

product must be knowD in addition to the weights

of the ingredients. Calculations indicating the

derivation of energy factors and of calories per

100 grams of baking powder biscuits made with

skim milk, item 98 in Agriculture Handbook No.

8 (185), are shown in the sample calculation below.

Sample calculation of energy factors forfood mixtures (baking powder biscuits)

Weight

Om.

336

55

244

16

2

Protein Fat Carbohydrate (by difference)

Kind of data

Weight

Specific

energy

factor

Energy

value Weight

Specific

energy

factor

Energy

value Weight

Specific

energy

factor

Energy

value

Ingredients:

Wheat flour, patent.

Fat-_ -_

Gm.

36.3

Cal.lgm.

4.05

Cal.

147. 02

Gm.

3.0

55. 0

. 2

Cal.lgm.

8.37

8. 84

8. 79

Cal.

25. 11

486. 20

1. 76

Gm.

255. 0

Cal.lgm.

4. 12

CaL

1, 050. 60

Milk, skim

Baking powder.

8. 5 4. 27 36. 20 12.4 3. 87 47.99

Salt- _ - -

Total ._ 653 44. 8 183. 32 58. 2 513. 07 267.4 1, 098. 59

Weighted factor (per

gram) _ _ 4.09 8. 82 4. 11

Baked yield:

Total _ . 549

100

44. 8

8.2

58.2

10.6

267.4

100-gram portion. _ 4.09 33. 54 8. 82 93.49 52. 2 4. 11 214. 54

The factors 4.09, 8.82, and 4.11 calories per

gram were applied to the protein, fat, and carbohydrate values of the baked biscuits, with the

resultant calorie value 342 calories per 100 grams.

Similar calculations were made for the other units

of weight given for this item in tables 2 and 3 of

Handbook No. 8.

PART IV. APPLICATION OF CALORIE FACTORS

The physiological fuel value of a food resulting

from applying the factors summarized in table 13

to the amounts of protein, fat, and carbohydrate

present is considered to be a measure of its available energy. Attention is again called to the

interpretation of this term to connote that portion

available to the body as a source of energy. The

difference between total or gross calories of a food

and available calories is the caloric value of the

organic matter in the urine and feces. Whether

this fecal organic matter is entirely of metabolic

origin plus bacterial residues and desquamated

tissue, or whether it usually or only under some

circumstances includes undigested protein, fat, or

carbohydrate residues also, is a question of very

great importance in dealing with such problems

as determining man's use of various foods as

sources of specific nutrients. ^Vhen this question

is resolved the information should be helpful

also in devising methods for estimating energy

values of specific nutrients in food when fed in

various combinations and at different levels;

present methods are actually not completely satisfactory for estimating available energy from the

different nutrients. As a result of changes in

method, however, no big changes in actual total

available calorie values for the foods are anticipated.

In using data on apparent digestibility for

developing the energy factors shown in table 13,

the assumption is made that the amount and

character of the fecal matter (protein, fat, and

carbohydrate) present is dependent on the food;

a low apparent digestibility could result from

greater excretion of metabohc products caused by

that food, from incomplete digestion, or from a

combination of several causes. Whatever the

contributing factors are, the assumption is that

the apparent digestibilities of the energy-yielding

components of that food would not vary widely for

a subject on a reasonably adequate intake of the

nutrient. If the total intake of a nutrient in a

diet is very low, the relative proportion in the

feces is too' high for satisfactory measurement by

this procedure. More information is needed on

the effect of level of the foods on utilization of

nutrients. Most studies in the Uterature at

present are on rather extreme diets, for example,

either very high or rather low levels of protein,

and moderately high and very high levels of the

test food. Data are needed also for intermediate

levels, those which are more realistic in terms of

common food practices.

43
Comparison of calculated and determined

available calories for diets

The end results obtained by use of the current

factors (table 13) for estimating available energy

of diets have been compared with results obtained

by direct bomb calorimeter determinations. An

experiment conducted by Snyder {164) with a

subject on a very simple diet of whole-wheat

bread and milk serves to illustrate the details of

calculation (table 18). Snyder's protein figures

based on the factor 6.25 for converting nitrogen

to protein were recomputed with the factors 5.83

and 6.38 for the bread and milk, respectively; the

necessary adjustments were made in the figures

for carbohydrate by difference. The gross calorie

values for food and feces were from bomb calorimeter determinations and were found to be 4,143

for the food and 418 for the feces. For comparison

we also calculated the gross calories of the food.

The heats of combustion, 5.80, 9.30 and 4.20

calories, were applied to the protein, fat, and

carbohydrate (by difference), respectively, of

whole-wheat bread; and 5.65, 9.25, and 3.95 to

these nutrients in the milk. The calculated gross

calories for bread (2,407) were a httle higher than

the determined (2,353), and the calculated gross

calories for milk (1,737), a little lower than the

determined (1,790), but for the total diet the

calculated gross calories were in excellent agreement with the determined.

TABLE 18.—Summary oj steps Jor checking available energy values calculated by factorsfrom table IS

Weight of

material Protein Fat Carbohydrate Gross energy

Available energy Deviation of

calculated

Type of data

Determined

by bomb

Calculated

by u# of

factors 1

available

energy from

determined

value

Food consumed in 2-day period:

Bread made from wholewheat flour

Grams

1, 020. 8

2, 500. 0

Grams

73. 9

75.2

Grams

12.9

87.5

Grams

442. 4

127.2

Calories

2,353

1,790

Calories Calories

2, 045. 5

1, 582. 5

Percent

Milk

Total 149. 1 100.4 569.6 4, 143 2 3, 561 3,628 -fl. 9

Excreta :

Feces (water-free) 97.0 17. 6 10.2 52. 1 418

Urine 3 164

Total __ __ __. 582

1 Energy factors from table 13 applied to the protein,

fat, and carbohydrate: 3.59, 8.37, and 3.78, respectively,

for bread and 4.27, 8.79, and 3.87 for milk.

2 Gross energy of food minus total energy of excreta

(4,143-582).

The urinary calories were 181 when determined

by bomb, but 164 when estimated according to

Atwater's procedure by multiplying the grams of

digested protein 131.5 by 1.25 calories. The wide

range in the calorie to nitrogen ratio of the urine

(p. 16) indicates that there is less satisfactory

agreement between the usual calculation of urinary energy loss and direct determinations. In

fact, with the difficulties of drying and burning

urine, it may be that much of the discrepancy is

in the bomb determination. We have considered

it advisable when estimating available energy

data to use the calorie-nitrogen factor based on a

large num.ber of samples rather than a bomb

determination of the particular sample of urine.

The calorie-N ratio for this individual was 6.9,

which is lower than the average but within the

range found for a large number of studies, table 8,

p. 12. This individual was also in negative balance, excreting 26.4 grams of nitrogen in the 2-day

period during which he absorbed only 21.8 grams.

However, the errors involved are insignificant

when considered in terms of total available energy.

3 7.9 X difference between amount nitrogen in food and

in feces.

NOTE.—Data used in this procedure taken from experiment No. 171, Studies of Bread and Breadmaking (164).

The available energy of the diet determined

from gross energy values of food, feces, and urine

was 3,561 calories. When the average wheat and

milk energy factors for protein, fat, and carbohydrate shown in table 13 were applied to the

nutrient intake in this experiment, the figure

obiained for available calories was 3,628, differing

from the determined figure by 1.9 percent. As

pointed out earlier, the factor for fat in whole

wheat may be too high since it is based on digestibility of 90 percent. If the average digestibility

is nearer two-thirds, as indicated in a number of

experiments, the energy factor would be approximately 5.95; the figure for available calories, 3,597,

would then be in even better agreement with the

determined value, differing by only 1.0 percent.

We have checked the results obtained by applymg factors from table 13 to data in 108 digestion

expenments which provided information on composition of the foods in the diet and data on bomb

calonmeter determinations of the food and feces.

Although the experimental data needed for using

every factor in table 13 were not provided by these

44
experiments, most of the factors could be tested

in this way. The available energy calculated by

applying the appropriate energy factors to the

composition data for the various foods was in excellent agreement with the comparable values for the

diets obtained by direct determinations of foods

and feces and calculated urinary calories. The

differences between the determined and calculated values, not taking direction into account,

ranged from 0 to 5 percent and averaged 2 percent.

The calculated values were in some cases higher

and in some lower than the determined values.

These positive and negative deviations were noted

even in experiments in which the sam.e type of

diet and the same subject were used and suggest

that the differences may be in part the result of

experimental error.

The digestion experiments that were used to

make this comparison include many types of diets:

Ordinary mixed diets with foods of animal and

plant origin; mixed diets containing large amounts

of legum.es; diets of fruits and nuts; very simple

diets such as combinations of meat and bread,

eggs, milk, and bread, whole-wheat bread and

milk, bread m.ade of lower extractions of wheat

flour and milk, oatmeal and milk, or crackers and

milk; other simple diets containing large amounts

of rice, dry peas, vegetables, or fruit; and a few

diets of single foods. The proportions of protein,

fat, and carbohydrate in the food intake as well

as the level of protein intake varied widely, the

latter ranging from 14 to 184 grams daily. We

have grouped som.e of the diets and summarized

the diñerences we foimd between determined and

calculated values as follows:

In 14 diets of fruits and nuts the absolute

deviation (that is, not taking signs into account)

ranged from. 0 to 5 percent, averaging 2 percent.

In 16 diets containing large quantities of diy

beans, peas, or cowpeas the absolute deviation

ranged from. 1 to 5 percent, averaging 2 percent.

In 7 diets containing a large proportion of

rice or oatm.eal the absolute values deviated by

only 1 percent in all cases.

In 6 diets of whole-wheat bread and m.ük the

absolute deviations ranged from 2 to 4 percent,

averaging 2.5 percent.

In 11 diets containing a large proportion of

cabbage, potatoes, beets, green corn, or applesauce, the absolute deviations ranged from 0 to

3 percent, averaging 1 percent.

In 6 ordinary mixed diets the absolute deviations ranged from 0 to 3 percent, averaging 1.5

percent.

In 36 simple diets in which lower extractions

of wheat flours were fed as bread or crackers

the absolute deviations ranged from 0 to 5 percent, averaging 2.5 percent.

Several general observations appear reasonable

in view of these data:

1. That the energy factorsjn table 13 give an

accurate estimation of thej^available energy

when applied in various diets containing foods

of both animal and plant sources either as mixed

diets or simple diets of two or more foods.

2. That the factors are equally suitable when

applied to foods in diets in which various plant

foods are predominant, as in fruit and nut diets,

diets in which large amounts of beans or peas

are eaten, and diets in which large proportions

of the calories are supplied by rice, wheat, or

vegetables.

3. That the factors applied to the several diets

of single foods give results in good agreement

with the determined values for available energy.

This indicates that the factors are applicable to

foods used alone in the diet, but further confirmation with additional data is needed.

4. That the level of protein fed apparently

does not affect the extent of agreement obtained

in estimating available energy by use of the

factors. This was indicated particularly in the

group of experiments in which fruit and nuts

made up a large part of the diet and the daily

protein intake ranged from as low as 14 grams

to a maximum of 85 grams; there was no evidence of difference in the percentage deviations

between calculated and determined values at

the different levels of protein intake.

The energy values of the 108 diets were calculated also by appl}âng the general factors, 4, 9, 4.

There were larger differences between the direct

determinations and calculated values than were

observed when the values were calculated by use

of the factors from table 13. The largest differences were noted in those diets in which foods of

plant origin predominated.

Data have been summarized in table 19 from a

few of the experiments selected to represent different t3^pes of diets. The data illustrate the extent

of agreement between available calories directly

determined and those calculated by use of the

factors from table 13 and by use of the general

factors 4, 9, 4. Although there is good agreement

between the determined and the calculated available energy values as illustrated by the data in

table 19, examination of the data show that for

some kinds of diets similar agreement for the

available energy value of specific nutrients does

not necessarily follow. For example, in the case

of experiment 388 which represents a diet low in

protein and fat, the apparent digestibility of the

total protein from the diet was 45 percent, with an

estimated 25 calories available from proteiQ instead of the 46 which would be calculated by the

factors from table 13. Likewise, an estimated

146 calories would be available from fat if the

calculation were based on the apparent digestibility for the total fat in the diet, instead of the

201 obtained by use of the factors with the individual foods. For carbohydrate, however, the

data from this experiment would indicate approximately 861 available calories instead of the 823

obtained by appUcation of the factors from table

13 to the carbohydrate of the items in the diet.

45
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eg ov '(=! ft

00 00 '^

•^ -2 2

CQ 0) ft .

Ö o t^iOCO

'Y^ o - •

^H • r-H ' 1 o

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03 CQ gj ^

ft-^ CO ¿'fM

-:> g ñ bOft

^âa?g g¿ bDbD:3^

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ci

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5

bo bD^ l-^oo

PO dco^

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a a ö^ a

bObOg o ^

OO-QT, G

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bObßXi goö

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co"

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^ a d S

TT '=* a ft-^ -g

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^c^2gog^?5cq ^a

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. M^ ^g-2.s ^co a aco-^r

¿«DMXOC- .bC bO . O

bot5 bo^: ftbbr3oo^9?"tí

O^MOoa;cA«c^c^p,g

46
+ + H-

(M

+

CO

+ + + I

I I +

(M VO (M

I +

Ci 00

»o ai

xt^

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o C5 CO •^ o 1—1

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1>

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Ö (D 0) (33 .

bOd) Ü tiûo

co^i-o^

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to rt

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a CD >^ o o d

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bßo O

-P '^

•^^ o Ci

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. ^ CD J^

a.sa

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m m

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bD <D

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rd l>> <S

te d ^ T3 bC

g bß^ O d

bß - "

'-' bß bß bß bßA

X2 TÍ^ d '-' d

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^ Ö ^ -CD g g^

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CD bß Oí

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'tí^tí.d^""'S-d^c^c3to bßT-H bßbßd)'-* bßd P^'—' cj'CO

1> (M

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47
In this experiment these discrepancies in the

values for available calories from the different

nutrients are large, as is to be expected in view of

the items in the diet and the very low level of

protein and fat. This example is useful, therefore,

to point out that although the calorie factors in

table 13 are satisfactory for calculating total

available calories in a diet of widely different

composition and character from the ordinary

mixed diet, under some conditions there may be

considerable error in calculating available calories

from specific nutrients. Caution should be used

also in applying general digestibility coefficients to

such diets with a view to obtaining data on available nutrients.

General factors and more specific factors

for calculating calories in individual foods

When the factors shown in table 13 are applied

to individual foods and the resulting calories

compared with calories obtained by use of the

general factors 4, 9, 4, very large differences are

observed for some foods. A fist of foods representative of different groups has been assembled

below in tabular form to illustrate this difference:

Food

Animal foods:

Beef

Salmon, canned

Eggs

Mük

Fats:

Butter

Vegetable fats and oils.

Cereals:

Cornmeal, whole ground

(unbolted)

Cornmeal, degermed

Oatmeal

Rice, brown

Rice, white or milled-_.

Wheat flour, whole

wheat

Wheat flour, patent

Legumes:

Beans, dry seeds

Peas, dry seeds

Vegetables:

Beans, snap

Cabbage

Carrots

Potatoes

Turnips

Fruits:

Apples, raw

Lemons, raw

Peaches, canned

Sugar:

Cane or beet

Energy value per 100

grams edible portion

derived by use of—

Specific

factors

General factors 4, 9, 4

(a) (b)

Calories

273

143

162

68

Calories

268

138

158

69

716

884

733

900

355

363

390

360

362

367

356

396

356

351

333

364

355

355

338

339

346

349

35

24

42

83

32

42

29

45

85

35

58

32

68

64

44

75

385 398

Eatio

col, b

col. a

Percent

98

97

98

101

102

102

103

98

102

99

97

107

102

103

120

120

107

102

109

110

138

110

103

The significance of some of the differences

illustrated above becomes more apparent when

related to emergency feeding problems. For

example, the general factors overestimate the

energy value of whole wheat by 22 calories per

100 grams, and a ton therefore would supply some

200,000 fewer calories than calculated. To supply

the higher number of calories, estunated, however,

2,132 pounds instead of 2,000 would be needed.

Application of general factors to

national food supplies

Although general factors 4, 9, 4 may not be suitable for estimating available energy values for individual foods, the question arises as to whether they

may be suitable for calculating calories of presentday food supplies. Food consumption patterns have

changed over the years (182), There have been

major shifts in consumption of foods within groups

and between groups. As a result there has been

some shift in the proportions of protein, fat, and

carbohydrate supplied by the different foods

within a group, and also a shift in the proportions

of these nutrients from the various food groups in

the national food supply.

We have grouped the foods into a few large

categories and have calculated average coefficients

of digestibility and calorie factors for the protein,

fat, and carbohydrate of each of these groups.

For this we weighted data selected from table 13

by the amount of the nutrient each food in the

group supplied. These food group averages are

shown in table 20. The average or general calorie

factors for the total food supply also weighted

by current distribution data on nutrients were

found to be 4.00, 8.92, and 3.97 calories per gram

as shoA\ai in table 20. These factors if rounded to

simple whole numbers are the same as the general

factors that have been used nearly 50 years. No

large error is introduced in the calculation of

national per capita figures per day if general

factors rounded to whole numbers are used instead

of the imrounded 4.00, 8.92, and 3.97. The net

result of applying these rounded factors to the

amounts of protein, fat, and carbohydrate of the

food suppl}^ would be to overestimate the total

available calories from about one-half to less than

1 percent. On a 3,000-calorie diet this would be

less than 30 calories.

General factors such as these provide a quick

means of calculatmg the physiological fuel value

from composition data of the total food supply

in this country. They may be used with family

or institutional diets also if the pattern is comparable in the types and proportions of food to those

used in this country. However, for limited or

unusual diets such as are found in some areas or

for food supphes of totally different composition,

these general factors might not be suitable. Data

in table 21 illustrate the differences that may

result from applying the rounded general factors

48
TABLE 20.- -Factors for digestibility, heats oj combustion, and physiological fuel values of nutrients in food

groups as used in present-day mixed diets ^

Protein Fat Carbohydrate

Classes of food materials Proportion of

total in

mixed

diet

Apparent

digestibility

Heat of

combustion less

1.25 2

Physiological

fuel

value

Proportion of

total in

mixed

diet

Apparent

digestibility

Heat of

combustion

Physiological

fuel

value

Proportion of

total in

mixed

diet

Apparent

digestibility

Heat of

combustion

Physiological

fuel

value

Meats, fish, poultry

Percent

31

7

25

Percent

97

97

97

Caî.jgm.

4.40

4.50

4.40

Cal.jgm.

4. 27

4. 36

4. 27

Percent

36

4

18

19

Percent

95

95

95

95

CaJ.lgm.

9. 50

9.50

9. 25

9. 40

Cal.lgm.

9.02

9. 02

8. 79

8.93

Percent Percent Cal.lgm. Cal.lgm.

Eggs

Dairy products 8 98 ~3~9b~ "3."87 Separated fats

Total food of animal origin 63 97 4.41 4.28 77 95 9. 42 8.95 8 98 3.95 3. 87

Cereals, _ _ _ 23

6

6

2

86

78

70

85

4. 55

4. 45

3,75

3.95

3.91

3. 47

2. 62

3.36

2

3

1

1

90

90

90

90

9. 30

9.30

9.30

9. 30

8. 37

8.37

8. 37

8.37

40

3

9

8

32

98

97

93

90

98

4. 20

4. 20

4. 19

4. 00

3. 95

4. 12

4. 07

3. 90

3.60

3.87

Legumes and nuts __ __

Vegetables

Fruits, ________

Sugars and sirups

Separated fats and oils 16 95 9. 30 8. 84

Total food of plant origin 37 82 4.37 3. 58 23 94 9.30 8. 74 92 97 4. 10 3. 98

Total food 100 91 4. 40 4. 00 100 95 9. 39 8. 92 100 97 4.09 3.97

1 Based on United States of America food consumption data, 1949 (182).

4, 9, 4 and the specific factors for individual foods

or food groups to different kinds of diets. Diet

A may be considered comparable to that used

currently in this country. It has fairly large

quantities of meat, milk, fats, and sugar, and

relatively small quantities of cereals; the greater

proportion of the cereals are refined products.

Diet B, on the other hand, follows the dietary

pattern of some of the Eastern European countries

and has very high proportions of unrefined cereals

and potatoes and relatively small amounts of meat,

fat, eggs, and sugar.

Results of applying the general and specific

factors ia this example show that for Diet A

either set of factors would be satisfactory. No

significant error is to be expected from applying

general factors in this case because the proportions

of the different types of food are the same as

2 Heat of combustion corrected for incompletely oxidized

products in the urine.

those used in developing the general factors.

In the case of Diet B, which is also a mixed diet

but one in which the proportions of different t3rpes

of food are very difi'erent, calories calculated by

the use of the general and specific factors are not

in as good agreement.

General factors may therefore be used for estimating the energy value of average family diets

or of the national food supply of this country

from the total quantity of protein, fat, and carbohydrate. The more specific factors should be

used for most other calculations, such as those for

experimental and therapeutic diets, individual

foods, food supplies of a totally different character

from that of this country, and particularly for

areas of the world where the food supplies consist

largely of unrefined cereals and vegetables.

49
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50
CONCLUSIONS

It is recognized that some of the physiological

fuel factors for food groups and individual foods

developed as shown in this publication and

summarized in table 13 are based on a limited

amount of data and that factors for food groups

may not always be equally suitable for individual

foods within the group. Also revisions are anticipated as more complete information becomes

available on the various constituents in the

nitrogenous matter, fat and carbohydrate oí

food, and on their heats of combustion and

digestibilities. Moreover it is realized that there

are problems mth direct bearing on the digestibilit}^ of protein, fat, and carbohydrate that have

not been resolved satisfactorily at this time.

Although all of the calorie factors may not be

entirely suitable as a result of the various limitations existent in the basic data, nevertheless when

they were applied to the nutrients in foods fed

alone or m various combinations, the estimated

total available energy of the food was always

in excellent agreement with the value determined

by use of the bomb calorimeter.

In view of the agreement noted and until more

basic information becomes available, the modification of the method of Atwater and Bryant as proposed in the present publication for estimating

the available (or physiological) energy value of

foods seems the most satisfactory procedure to

use; the calorie factors presented in table 13

are recommended for calculating the total available

energy value of foods until there is basis for further

revision or refinement of the factors.

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(1)

(2)

(3)

(4)

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57
APPENDIX. TABULAR SUMMARY OF EXPERIMENTS ON DIGESTIBILITY OF FOODS OF PLANT ORIGIN BY HUMAN SUBJECTS

Apparent Digestibility and Available Energy

Scope of compilation.—The compilation of human digestion experiments given in table 23 presents data on the

apparent digestibility of protein, fat, carbohydrate, and

energy, and in some cases the availability of the total

energv of various foods of plant origin. It covers research

in this field since 1875. Data published in languages other

than Enghsh may not have been covered completely but the

greater portion 'is believed to have been reviewed. The

reports included in the compilation may be identified by

the numbers in the last column of the table, which refer

to Literature Cited, page 51.

Coefficients of apparent digestibihty of fat are shown m

the table, but in many cases are not considered to be reliable. With the exception of a few kinds of plant foods,

such as nuts, the fat content is too low to contribute more

than a small part of the total fat intake. Thus, in calculating the digestibihty of the fat of the test food even a

small error in the assumptions made for digestibility of

fat of the remainder of the diet may result in a relatively

large error in the estimated digestibility of the test food.

Too much importance should not be given to the reported figures for gain or loss of body nitrogen in studies

in which the experimental periods were short and in

studies in which no preliminary period on the experimental

diet was indicated. If the period on the experimental

diet had been sufiSciently long the subjects might have

reached nitrogen equilibrium.

This compilation includes studies in which the apparent

digestibility of the test food was reported or could be calculated from data given by the author. A wide variety of

experimental conditions are represented, some of which

were too extreme for derivation of coefiicients of digestibility for general use as represented in table 13. However,

they are useful in considering the effects that various conditions of dietary intake and experimental procedures may

have upon the digestibility of foods and for this reason

are included in the compilation.

Order of foods.—^The order in which the food groups

appear in the table follows that used by Atwater and

Bryant in their report of 1899 (17) and by the Food and

Agriculture Organization ad hoc Committee in its report

of May 1947 (65). This order seemed desirable in that

the first two groups, "Grain, Grain Products" and ''Legumes and Nuts," are both important sources of calories,

and it is on foods in these two groups the greater portion of

the research on digestibility of foods of plant origin has

been done.

The food items within each group have been arranged

alphabetically except where some other arrangement is

believed to be more useful to the reader. For example, the

wheat items are in the order of their relation to the original

grain, starting with the items most similar to the whole

grain in composition and form. Thus, the whole-grain

flours appear first, followed by intermediate extractions,

80-percent extraction and lower extractions.

The common plant names in table 23 are followed by

their scientific names to aid the user in identification of

items. Occasionally the scientific names were given by

the authors (items 23, 24, 88-91, 316-320). Otherwise we

have used the ones preferred in Standardized Plant Names

{79) with a few exceptions where other names were recommended by the Horticultural Crops Research Branch of

the Agricultural Research Service, U. S. Department of

Agriculture.

Apparent digestibility.—The coefficients of digestibility

reported in the table represent apparent digestibility In

calculating the apparent digestibility no attempt has been

made to distinguish between metabolic products and undigested food in the feces. Using protein as an example,

these coefficients are calculated as follows:

Protein in- Protein (from

take from— test food) in Coefficient of apparent

test food ÎË^Ë^_^ XI00=digestibility of protein

Protein intake from test food of test food as percent.

Where there were no data on the basal diet and the diets

used were relatively simple, the fecal protein for the diet

exclusive of the test food was calculated from the coefficient of digestibilitv of the various items m the diet. i;or

example, in a very^simple diet of bread and milk in which

bread was the test food, it was commonly assumed that

milk protein would have a digestibility of 97 percent.

Then the fecal protein from milk would be 3 percent oí the

milk protein intake (100-97) and that from bread would

be the difference between the total fecal protein and the

milk fecal protein. . When the authors determined the digestibihty ot a mixed

diet during a preliminary period and then substituted the

test food for a specified proportion of the mixed diet, it was

assumed that reducing the intake of the basal diet did not

change its digestibility. If the test food replaced 15 percent of the basal diet, the fecal protein in the test period

due to the basal diet was considered to be 85 percent of the

fecal protein found experimentally for the basal period.

The resulting value was subtracted from the total fecal

protein in the test period to obtain the protein from the

test food in the feces.

The coefficients of digestibility of fat, carbohydrate, and

of energy have been calculated in the same way.

The proportion of gross energy available to the body was

reported in a limited number of studies. To obtain this

value the energy lost in the urine, as well as the energy

value of the feces, was deducted from the gross energy of

the food intake. The absorbed fat and carbohydrate were

considered to be completely oxidized, and the unoxidized

organic matter of the urine was assumed to be mainly

nitrogenous products. The energy loss in the urine was

assumed to average 1.25 calories per gram of absorbed

protein. On these assumptions the available energy of the

test food was calculated as follows:

Gross energy of test food—fecal energy from test

food—energy löst in urine (digestible protein from test

foodX 1.25) = available energy from test food.

Available energyX100^Percent of gross energy available

Gross energy to the body.

Table 22 shows in detail the results of calculations for

estimating the coefficients of digestibility of protein, fat,

carbohydrate, and energy, and the proportion of energy

actually available to the body. This experiment was taken

from one of the early reports of Snyder (164). -*

Adaptations of published data.—All the studies in which

original basic data were reported by the authors have been

recalculated prior to inclusion in table 23. Differences,

when found, between the results as originally reported and

the recalculated figures were of three types:

58
1. Whereas in most studies investigators assumed digestibility coefficients for the basal foods close to or the

same as those shown in table 13, in occasional studies

they applied other coefficients of digestibility. As a result, the original figures for digestibihty and proportion

of energy available from the test food were in some cases

considerably different from results we obtained by applying the usual coefficients to the basal diets. If our recalculated figures differed from the reported results by more

than 1 percent, they were entered in table 23, and attention called to this change by a footnote.

2. In some studies the authors did not report apparent

digestibihty or available energy but reported the basic

data needed for making such calculations. For these

cases we have calculated the values entered in table 23

as noted in a footnote.

3. In still other experiments when we used the basic

data and assumptions reported by the authors in calculation, we obtained a different result. Our recalculated

values have been entered in brackets in talkie 23-

TABLE 22.—Use of digestibility data to determine coefficients of apparent digestibility and available energy

Sample

No.

Weight of

material

Protein

(Nx6.25) Fat Carbohydrate Ash Heat of

combustion

70

Food consumed:

Bread (made from graham flour) _ _ _

Grams

908. 3

3, 250. 0

Grams

70. 5

95.9

Grams

11. 5

113.8

Grams

389. 0

167.4

Grams

8.6

25.7

Calories

2 093

69 Milk 2 327

Total ___

'

166. 4 125.3 556.4 34.3 4, 420

71 Feces (water free) _ _ 90. 0 16.3

2.9

9. 4

5.7

49. 6

3.3

14. 7 392

Estimated feces from food other than bread 83

Estimated feces from bread _ _ 13.4 3. 7 46. 3 309

Total amount digested 150. 1

57. 1

115. 9

7. 8

506. 8

342. 7

19.6 4,028

Estimated digestible nutrients in bread _ 1,784

Coefficients of digestibility of total food _

Percent

90.2

81.0

Percent

92.5

67. 8

Percent

91. 1

88. 1

Percent

57. 1

Percent

91. 1

Estimated coefficients of digestibility of bread _ 85.2

Proportion of energy actually available to body:

In total food 86.9

In bread alone ___ ___ 81. 8

NOTE.—This table appears as table 18 in U. S. Department of Agriculture Bui. 101 {164)-

Terms and symbols used.-—^References to "authors" in

either the footnotes or descriptive columns in table 23

apply to the authors of the specific digestibility reports and

not to the compilers of table 23.

The proportion of protein, fat, carbohydrate, and energy

supplied by the test food in the diet has been shown in the

table wherever suitable information on composition and

amounts of food were reported. In some cases composition data given were not complete and we have used

figures from Agriculture Handbook No. 8 (185) to supply

missing composition data and have entered the results in

parentheses in table 23.

Parentheses were used also in the descriptive columns

for added explanatory phrases as interpreted from the

authors' description. To illustrate, for item 9 the term

"hominy" was not used in the text of the article but since

there was little doubt as to the identity of the product this

interpretation of the test food was noted in parenthesis in

addition to the author's description of the product.

Quotation marks have been used with certain food items

to indicate that they were quoted directly from the article.

This was done whenever a term might have different connotations. For example, entire-wheat flour, as used in

studies reported in the early part of the 20th century, was

a flour of intermediate extraction having part of the bran

removed. Today this term applies to a whole-grain

product.

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See general remarks, item 42, and plan of 1st report. "2 Daily N-balance for

H was -1.3 gm.; for M, +1.1 gm.; for S, —0.3 gm.

See general remarks, item 42, and plan of 2nd re- port.K. 13 of the 15 sub- jects were in positive N-balance, averaging +1.4

gm. S and

H in 2nd experimentsho"wed negative N-balances of -0.4 and

-1.5 gm. per day, respec- tively.

See general remarks, item 42, and plan of 1st report.12 Daily N-balance for

H was

-0.2; for M, +1.3; for S,

. —0.7 gm. See general remarks, item 42, and plan of 2d report.i* The 5 subjects in this ex- periment were in positive N-balance, averaging +1.1

gm. per day. See generalremarks, item 42, and plan of 2d report. " The 6 subjects in this ex- periment were in positive N-balance, averagmg +1.4

I gm. per day. See general remarks, item 42, and plan of 2d report." The 4 subjects in this ex- periment were in positive N-balance, averagmg +0.4

I gm. per day.

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Boiled rice eaten in mixed diet.ii

Average daily intake: 84 gm. pro- tein, 2,610 calories.

Boiled rice eaten in mixed diet.^^

Average daily intake: 88 gm. pro- tein, 2,510 calories.

Boiled rice eaten in mixed diet."

Average daily intake: 79 gm. pro- tein, 2,230 calories.

Boiled rice eaten in mixed diet.i^

Average daily intake: 81 gm. pro- tein, 2,310 calories.

Boiled rice eaten in mixed diet."

Average daily intake: 76 gm. pro- tein, 2,600 calories.

Boiled rice eaten in mixed diet. 13

Average daily intake: 81 gm. pro- tein, 2,660 calories.

Boiled rice eaten in mixed diet.13

Average daily intake: 82 gm. pro- tein, 2,660 calories.

Boiled rice eaten in mixed diet.ia

Average daily intake: 78 gm. pro- tein, 2,360 calories.

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.Subject, 1 man. Experi- mental period, 2 days.

Subjects, men. Experimen- tal period, 2 days. More digestive disturbance from peas cooked in hard

water.

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Subjects, men. For e.xperi-

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99
Composition and Heat of Combustion of Foods

Composition and heat of combustion data are recorded

in table 24 for test foods used in digestion experiments

described in table 23 whenever such data were reported.

Composition data are useful in identification of a food item

and in interpretation of experimental results in digestion

experiments in which that item is used.

For ease in using the data the test foods with composition

reported carry the same item number in both tables 23

and 24. The composition data are recorded as reported

by the authors and the factors they used for converting

nitrogen to protein are noted in footnotes. More complete

description of the food items is given in table 23.

TABLE 24.—Composition and heat of combustion ojfood items used in experiments on human digestibility

(table 23)

Test food, description Protein Fat

Carbohydrate

Total (by

difference) Fiber

Ash

Heat of

combustion per

gram

Ref. No.

GRAINS, GRAIN PRODUCTS

Barley Products {Hordeum vulgäre) :

1 Barley, flaked

2 Barley, germinated, flaked

Buckwheat Products {Fagopyrum esculentum) :

5 Flour

Corn, Corn Products {Zea mays):

8 Hominy

9 (Hominy) hulled using alkali,

steamed

maize imadded

Meal

Meal, granulated:

a

b

Meal, waxy variety of

ported from China

Meal, white

Corn endosperm, toasted,

sugar and salt

Oats, Oat Products {Avena sativa) :

25 Rolled oats

Rolled oats

Rolled oats

Rolled oats, quick-cooking

Meal, granulated or pinhead

Meal

Meal, rolled

Meal

Meal:

a. Coarsely ground pinhead

b. Medium ground

Average

Rice Products {Oryza sativa) :

Polished or white

Undescribed

Rye, Rye Products {Sécale céréale) :

54 Flour No. 18

Flour III, a comercial flour

Flour No. 17

Flour No. 4

Flour No. 16

Flour No. 15

Flour II

11

14

15

16

20

27

28

32

33

34

35

36

38

Rice,

49

50

55

56

57

58

59

60

Percent

9.79

10. 18

11. 34

10.96

0

0

0

9.52

11. 79

7.77

10. 54

9. 88

7.49

8.66

11. 02

7.90

7. 36

7.51

7.35

7.45

8. 12

15.

15.

15.

(12. 92)

13.50

9.25

11.40

10. 96

9.30

10. 06

11.00

11.26

Percent

18.87

1 10. 62

2 8. 13

*9. 44

2 9. 96

2 8. 92

5 10. 05

1 6. 87

*8. 50

*8. 69

*8. 88

*10. 63

2 7. 38

2 14. 69

2 15. 69

3 15. 10

2 16. 13

1 12. 43

1 13. 17

1 12. 21

1 13. 25

3 11. 70

3 8. 60

10. 10

*8. 55

4 8. 27

*15. 48

*14. 92

*15. 78

*15. 67

*16. 14

*15. 38

*15. 33

Percent

1.24

1.21

1.22

.67

5. 30

5.00

5. 65

. 65

. 98

1. 92

4.24

6. 17

1. 68

6.96

7.23

6. 14

6. 31

7. 53

7. 27

7.28

7.21

7. 78

7.50

.31

. 3

2.01

1.89

1. 89

1.78

1.76

1.71

1.89

Percent

79.22

76.89

77. 69

78. 61

76. 77

77. 96

76. 67

82.67

78. 25

81. 12

75.04

71. 52

80.38

67. 89

64. 23

Percent

0. 77

1.27

.37

1. 51

.69

.78

.46

.46

.40

1.67

1. 83

68. 53

72.23

70. 23

71. 37

69.57

64.47

67. 15

65. 86

77. 89

77.26

71.08

69. 82

69.47

71.38

70. 30

70. 22

69. 84

.90

1. 94

1. 04

1. 15

1. 12

.85

.93

.89

Percent

0. 88

1. 10

1.62

.32

7. 97

8. 12

7.63

.29

.48

.50

1.3

1. 80

3.07

1. 80

1. 83

' N X 5.7.

2 N X 6.25.

3 N X 5.83.

1. 84

1. 52

1.72

1. 70

1.78

1. 62

1.47

1. 54

.33

.67

2. 18

1.97

1.90

1.87

1. 74

1.69

1. 68

Calories

3.986

4. 440

4. 164

4. 625

3. 823

4.023

3. 869

4. 560

3.854

62

62

134

120

121

62

120

G7

97

126

164

126

88

129

62

62

62

62

102

167

144

141

141

141

141

141

141

141

^ N X 5.95.

*N-conversion factor not reported.

TOO
TABLE 24.—Composition and heat of combustion offood items used in experiments on human digestibility

{table ^5)—Continued

Test food, description Water Protein Fat

Carbohydrate

Total (by

difference) Fiber

Heat of

combustion per

gram

Ref. No.

GRAINS, GRAIN PRODUCTS—Con.

Rye, Rye Products {Sécale céréale)—Con.

61 Flour, whole grain, most external

layer removed

(Whole-grain Swiss rye before processing) Ö

Flour, flours No. 6 and No. 18

blended in equal amounts

Flour, finely ground flour commonly used in Wurzburg

Flour, flours 0 and III blended in

equal amounts

Flour, made by Steinmetz process

from rye of Silesia

(Whole-grain rye of Silesia before

processing) 7

Flour, made by **old process" of

milling from Swiss rye

Flour No. 13

Flour No. 12

Flour, 75 percent yield

Flour I

Flour No. 10

Flour No. 2

Flour No. 7

Flour No. 6

Flour, 62 percent yield

Flour 0

Flour No. 1

Branny portion, fraction of 67-85

percent ^

Branny portion, fraction of 67-95

percent ^

Branny portion, fraction of 85-95

percent ^

Wheat, Wheat Products (Triticum aestivum):

Flours, whole grain and nearly whole

grain:

92 Graham, 100-percent extraction,

Scotch Fife, hard spring

(Whole grain before processing)»

Graham, 100-percent extraction,

Scotch Fife

(Whole grain before processing)io__-

Graham, 100-percent extraction

Graham, milled from Oregon white

winter wheat

(Whole grain before processing) ^i—

Graham, milled from hard winter

Weissenburg wheat from Oklahoma

(Whole grain before processing)'2__

Graham, milled from Michigan soft

winter wheat

(Whole grain before processing) ^3-

Graham

64

65

66

71

72

73

74

75

77-

78

79

80

81

82

83

84

85

86

87

93

94

96

97

98

Percent

13.0

(13.0)

9. 16

11.6

11.32

(13.0)

13.0

(13.0)

11. 19

11. 56

14.73

11.84

11.76

12. 14

11. 87

9.07

(13.0)

11. 25

12.49

14. 8

14.8

14. 2

8. 61

8.50

13.21

10.41

10.51

8. 15

8.99

7.73

8. 65

11.23

10.25

11.82

Percent

3 (8. 40)

3 (8.28)

*12. 42

3 9.33

*10. 76

3 (9. 62)

3 (9.21)

M7. 11)

*11. 85

*11. 42

3 9.79

*10. 22

*10. 81

*9. 80

*9. 31

*9. 35

3 (8. 69)

*6. 59

*5. 36

*20. 2

*18. 5

*16. 2

1 12. 65

1 12. 65

1 14. 21

1 15. 50

2 14. 00

1 8. 18

1 8. 32

1 15. 33

1 15. 33

1 12. 24

1 12. 34

*10. 63

Percent

1.30

1.38

1. 14

1. 69

1.00

. 97

.73

. 78

.59

102

1 N X 5.7.

2 N X 6.25.

' N X 5.83.

5 Reported as cellulose.

' Items No. 61 and 72 were prepared from this sample.

' Items No. 71 and 82 were prepared from this sample.

8 The rye was milled to 67 percent and the branny fraction obtamed from the remaining portion. Refers to

original grain with 5 percent loss from cleaning.

^ Items No. 92, 112, 135, 146, and 147 were prepared

. 59

3.3

3.4

3.6

2.44

2.36

2.01

2.28

2.52

1. 68

1.83

1. 79

1. 83

1.41

1. 35

1. 71

Percent Percent

' (2. 03)

« (1.65)

75. 72

75.34

74.78

74.37

« (1.57)

5 (2. 71)

5 (. 66)

76. 13

75. 69

76. 60

77. 42

80. 37

80.85

81. 17

58.9

59.6

61. 1

74. 58

74.69

68.56

69. 88

70.97

80. 27

79. 10

73. 83

72.87

73.27

74. 23

74. 12

(-67)

2.9

4.6

7.5

Percent

1. 52

(1-61)

1.40

1.24

1. 20

(1.18)

(1.84)

(1.10)

1.04

.96

.86

.81

. 77

. 73

. 62

.62

(.55)

.43

.39

2.8

3.7

4. 9

72

80

2.25

2.01

1. 93

2.00

1. 72

1. 76

1. 32

1.32

1.85

1.83

1.72

Calories

148

140

3.971

4.023

4.004

3. 990

4.008

4. 178

4. 110

3.906

4.000

from this sample. Also from this wheat were prepared

items No. 94, 114, and 137 appearing in another report.

10 Items No. 93, 113, and 136 were prepared from tnis

^^^ R^ems No. 96 and 115 were prepared from this sample

12 Items No. 97, 116, 139, 173 and 174 were prepared

from this sample. . , ...

13 Items No. 98, 118, and 141 were prepared from this

sample.

*N-conversion factor not reported.

101
TABLE 24.—Composition and heat of combustion ojfood items used in experiments on human digestibility

{table ;^5)—Continued

Test food, description Water Protein Fat

Carbohydrate

Total (by

diflerence) Fiber

Heat of

combustion per

gram

Ref. No.

GRAINS, GRAIN PRODUCTS—Con.

Wheat, Wheat Products {Triticum aestivum)—Continued

Flours, whole grain and nearly whole

grain—Continued

106 Whole meal, milled from Canadian Percent

wheat 15.

107 Whole meal, milled from English

wheat 15.

108 Whole meal 13. 50

109 100 percent of wheat kernel 14. 38

Flours, intermediate extractions:

112 '^Entire wheat" 10. 81

113 ''Entire wheat" 13. 51

114 "Entire wheat" milled from hard

spring wheat, Scotch Fife 10. 99

115 "Entire wheat" milled from Oregon

white winter wheat 8. 66

116 "Entire wheat" milled from hard

winter Weissenburg wheat from

Oklahoma 7. 46

117 ' 'Entire wheat" milled from Indiana

soft winter wheat 9. 60

(Whole grain before processing) __ i« 8. 09

118 "Entire wheat" milled from Michigan soft winter wheat 11.01

123 90-percent extraction, milled from

English wheat 15.

124 90-percent extraction, milled from

Manitoba wheat 15.

131 80-percent extraction, milled from

English wheat 15.

132 80-percent extraction, milled from

Manitoba wheat 15.

Flours, lower extractions:

135 Standard patent, milled from hard

spring wheat, Scotch Fife 10. 54

136 Straight patent, milled from hard

spring wheat, Scotch Fife 12. 38

137 Straight patent, milled from hard

spring wheat, Scotch Fife 11. 55

138 Standard patent, about 70 percent

yield, milled from Oregon white

winter wheat 8. 94

139 Standard patent, about 70 percent

yield, milled from hard winter

Weissenburg wheat from Oklahoma 9. 93

140 Standard patent, milled from Indiana soft winter wheat 10.30

141 Standard patent, contained less

than 72 percent wheat kernel

milled from Michigan soft winter

wheat 10. 97

145 70 percent of wheat kernel, milled

from a mixture of Girka and

Minnesota wheats 15. 02

146 1st patent, milled from hard spring

wheat, Scotch Fife 10. 55

147 2d patent 10^49

148 Patent, milled from hard spring

wheat, Scotch Fife 12. 36 *12. 44

1 N X 5.7.

2 N X 6.25.

» N X 5.83.

" Available carbohydrate 55.20 percent; undetermined

matter (pentosans, etc.) 9.78 percent.

Percevt

3 15. 40

3 8. 52

3 11. 66

2 10. 92

1 12. 26

1 13. 72

2 13. 00

17.52

1 15. 16

1 12. 80

1 13. 16

1 12. 01

Í 8.32

1 13. 51

18. 15

1 13. 05

1 11. 99

1 13. 60

2 12. 75

16.90

1 13. 74

1 12. 30

1 10. 92

2 11. 57

1 11. 08

1 11. 14

Percent

2.23

1.83

1.82

1.81

2.24

1.69

2.28

1.67

1.64

1.54

1.52

1.53

1.61

1.30

1.43

1.25

.92

.93

.50

.81

1.15

1.20

1.62

Percent

(14)

(15)

71.49

71.69

73.67

70. 10

72.51

81. 08

74.52

74.40

75.38

74. 17

75.36

72.04

73. 67

82.47

74. 89

75. 94

77. 15

72. 19

76.85

76.75

73. 07

Percent

2.36

2.02

1. 15

1.15

. 17

.24

Percent

1.53

1.20

1.02

1.22

1.07

1. 22

1.66

1.85

1.28

0.50

.68

.60

.44

. 52

.53

.46

.41

.37

.42

.51

Calories

4.032

3.877

3. 944

3.900

4. 159

4.020

4.090

3. 860

4.050

3.861

3.889

3.880

4. 040

4. 010

3.799

4.032

4.006

104

104

28

146

164

166

195

168

168

166

166

106

106

106

106

164

166

195

168

168

166

166

146

164

164

170

16 Available carbohydrate 63.60 percent; undetermined

matter (pentosans, etc.) 9.02 percent.

*Vi^^"^^ ^^: 11^ and 140 were prepared from this sample.

'^JN-conversion factor not reported.

102
TABLE 24.—Composition and heat of combustion ojjood items used in experiments on human diaestibility

{table ^5)—Continued

Test food, description Water Protein Fat

Carbohydrate

Total (by

diflerence) Fiber

Heat of

combustion per

gram

Ref. No.

GRAINS, GRAIN PRODUCTS—Con.

Wheat, Wheat Products—Con.

Flours, lower extractions—Con.

149 Patent, baker's grade, milled from

Scotch Fife

151 Patent

163 30 percent of wheat kernel, milled

from a mixture of Odessa, California and English wheats

Flours, other:

164 Semolina, milled from durum

wheat, Kubanka

(Whole grain before processing) i7_.

165 Semolina, ^^ milled from durum

wheat, Kubanka

(Whole grain before processing) io_

Flour mixtures:

173 "Bran flour," a mixture of 86 percent straight grade and 14 percent

very finel}'- ground bran from hard

Weissenburg wheat

174 "Germ flour," a mixture of 93 percent straight grade flour and 7

percent ñnely ground germ milled

from hard winter Weissenburg

wheat

Wheat breakfast foods:

Whole grain and partially reñned:

175 Flaked

179 Meal

180 Meal

182 Meal

184 Puffed, whole grain

185 Rolled

187 Rolled

189 Shredded

190 Shredded

Reñned:

192 Endosperm, granulated

193 Endosperm, granulated

194 Endosperm, farina

195 Endosperm, farina

196 Endosperm, farina

Wheat breakfast food mixtures:

198 Wheat and barley malt

199 Wheat, whole grain and barley

malt, ''Force"

200 Wheat, whole grain and barley

malt mixture, "Force"

201 Wheat and barley malt, "Malta

Vita"

202 Branny portion, fraction of 73-95

percent 21

203 Branny portion, fraction of 82-95

percent 21

LEGUMES AND NUTS

Beans, dry (Phaseolus vulgaris) :

210 Common white, navy beans.

211 Common white, navy beans.

Percent

8. 01

11.07

14. 63

10. 77

8.76

7. 57

10. 48

9. 69

9. 63

2. 72

7. 06

8. 08

8. 08

4.90

9. 19

11. 35

6. 20

5. 62

9. 35

12. 68

10. 55

10. 58

11. 37

6. 47

10. 86

7. 37

11. 32

14. 7

14. 3

11. 21

11. 19

Percent

*15. 50

*12. 75

2 8.91

2 11. 64

2 12. 37

2 11. 57

2 12. 45

13. 96

1 14. 87

2 11. 69

2 11. 25

2 16. 32

2 15. 16

2 15. 06

1 9. 81

2 11. 14

1 10. 60

1 9. 97

Í 10. 54

2 11. 81

1 9. 70

1 9. 18

*13. 03

1 11. 63

1 9. 81

1 12. 20

*15. 7

*15. 0

2 18.

2 20.

1 N X 5.7.

2 N X 6.25.

" Item No. 164 was prepared from this sample.

^8 Composition for macaroni, dry; used here for flour as

Snyder found by previous analyses that flour and uncooked

macaroni made from it have practically same composition.

Percent

2.22

.90

.96

1. 27

2.07

0. 89

2. 48

1.48

1. 66

1. 49

1. 89

1. 47

1. 35

1. 94

2. 27

2. 12

1. 37

1. 35

. 85

2. 40

1. 36

1. 10

. 77

.77

1. 65

2. 13

1. 52

4. 2

4.4

1. 63

1. 58

Percent

73. 52

74. 84

75. 18

75. 56

74. 92

79. 06

72. 92

73.62

72. 97

79. 13

78. 45

72. 68

74. 08

76. 70

77. 22

73. 85

80. 06

81. 27

78. 76

72. 46

77. 97

78. 60

74. 27

78. 96

74. 77

78. 29

72. 03

61. 1

61. 3

64.89

62. 54

Percent

14

2. 83

1. 88

20 2. 10

20 2. 07

2. 34

1. 07

2. 42

. 27

, 44

36

, 28

1. 85

7.6

10. 3

Percent

.75

. 44

.32

. 76

1.88

0. 91

1. 67

1. 25

. 87

4. 97

1. 35

1. 45

1. 33

1.40

1. 51

1. 54

1. 77

1.79

. 50

. 65

. 42

. 54

.56

2. 17

2. 86

2. 40

2. 93

4. 3

5. 0

4. 02

4.00

Calories

4. le

3. 876

3.962

4.020

3. 877

4. 061

3. 822

3. 841

3. 885

3. 922

170

97

146

168

168

168

168

129

129

129

126

129

62

168

127 and

155

129

129

126

62

62

97

62

9

30

30

184

184

10 Items No. 165 and 187 were prepared from this sample.

20 Estimated by authors of article. u^^^r.^

21 The wheat was milled to 73 percent and the branny

fraction obtained from the remaining portion.^ Keters TíO

original grain with a 5 percent loss from cleanmg.

*N-conversion factor not reported.

103
TABLE 24.—Composition and heat of combustion offood items used in experiments on human digestibility

(table ^5)—Continued

Test food, description Water Protein Fat

Carbohydrate

Total (by

difference) Fiber

Ash

Heat of

combustion per

gram

Ref. No.

222

223

224

225

226

227

228

LEGUMES AND NUTS—Continued

Beans» dry—Continued

212 Common white, navy beans, skins

removed

213 Common white, navy beans

216 Kidney beans

Cowpeas, dry (Vigna sinensis) :

221 Clay

Clay

Lady

Lady

Lady

Whippoorwill

Whippoorwill

Whippoorwill

Peas, dry (Pisum sativum) :

235 Peas

236 Peas

237 Peas

238 Peas, split

Soybeans, Soybean Products (Glycine max) :

248 Soybean flour, about 6.5 percent fat.

249 Soybean flour, about 3.3 percent fat.

252 Soybean curd (Tofu)

Ground nuts or peanuts (Arachis hyvooaea) :

259 Peanuts :._.._.

261 Peanut flour, partially defatted

Tree nuts:

264 Almonds (Prunus amygdalus)

Brazil nuts (Bertholletia excelsa)

Brazil nuts

Chestnuts, fresh (Castanea sativa)..

Chestnut flour (Castanea dentata) _ .

(Kernel before processing) 22

Coconuts (Cocos nucífera)

Pecans (Carya illinoensis)

Walnuts (Juglans regia) (presumably Persian or English)

265

266

267

268

269

270

271

VEGETABLES

Percent

13. 32

12. 82

n. 25

13. 37

10. 77

11. 32

10. 27

10. 05

8.08

12. 84

8. 36

13. 0

13. 2

11. 53

12. 61

4. 17

6.5

87. 80

4.88

4 44

4 42

4 33

5. 28

6.36

44. 89

19. 17

4 30

3.97

272 Beans, snap (Phaseolus vulgaris) _ _ _ 92. 44

274 Beets (Beta vulgaris) 82. 6

275 Beets 85, 4

276 Cabbage (Brassica olerácea var.

çapitata) __ 94 7

277 Cabbage """ 94 g

278 Cabbage ~~~ 94.4

280 Cabbage, savoy "'__ 89! 4

281 ,Carrots (Daucus carota) "_ 90 53

282 Carrots ~ 86." 3

283 Collards (Brassica olerácea var.

acephala) 88. 44

284 Corn, green (Zea maî/s) ~_" 76^0

286 Potatoes, white (Solanum tuberosum) _ 79 5

287 Potatoes, white '__'_ 78.3

288 Potatoes, white ~ ¿l! 2

291 Potatoes, white "_~ 73*4

292 Potatoes, white ~ 74.33

293 Potatoes, white ~ 80 16

294 Potatoes, white 23 " 75^ j

295 Potatoes, white 77.66

296 Potatoes, white 74. 80

2 N X 6.25.

22 Item No. 268 was prepared from this sample.

Percent

*23. 75

*22. 06

2 25. 38

2 23. 19

2 21. 94

2 25. 50

2 22. 38

2 23. 75

2 23. 00

2 19. 94

2 21. 44

2 2L 2

2 2L 2

2 22. 81

2 23. 44

2 49. 31

244. 1

2 5. 83

2 32. 64

*58. 98

2 17. 28

2 19. 78

2 18. 00

2 5.4

2 6.38

2 3.85

2 5.25

2 15. 67

2 24 58

2 1

*1. 9

*2. 2

16

*. 9

*1. 0

* 9

22! 1

2.86

2 1. 6

2 3. 13

*4. 9

*2. 2

*2. 3

*1. 9

*2. 3

*2. 38

2 1.49

2 2.0

2 2. 04

2 1.20

Percent

1. 71

1. 41

1.45

1. 78

1. 73

1.75

1.75

1. 35

1. 48

1. 70

1. 2

1. 2

1. 51

. 71

6. 50

3.3

4 41

47.33

9. 69

54. 30

63.31

66.07

1. 4

3. 32

2. 10

51. 00

71. 52

62. 92

. 13

. 3

. 2

.3

.3

. 1

. 6

.33

.2

. 54

1. 4

. 1

. 1

.3

.03

.07

Percent

59. 50

58. 38

58. 49

61. 79

57. 88

62. 15

60. 94

63. 64

62. 17

64. 74

6L 9

61.8

61. 71

61. 68

34.22

40. 4

1. 25

12. 59

23. 05

21. 22

8.96

8.00

38.3

81. 54

47.75

23.44

6.96

6. 62

5.44

13. 8

10.8

3.3

3.0

3.7

6.6

7. 40

11.0

5.53

17. 3

17.4

18.4

15.5

23.3

22. 32

17.35

21. 8

19.47

23. 19

Percent

5. 10

5.9

. 11

1. 98

2. 54

2. 58

2.96

4. 22

13. 77

3. 17

1.87

1. 1

1. 12

L51

.5

. 4

,28

39

Percent

3.38

3.58

3.50

3.72

3. 57

3.45

3, 51

3.93

3. 57

3.76

2. 7

2.6

2. 44

1. 56

5. 80

5. 7

.71

2. 56

3.84

2. 78

3. 62

2.65

2. 40

1.41

1. 14

1.55

1.91

.83

1. 4

1.4

.8

. 9

. 9

1.3

.88

.9

2.36

.4

.8

.9

1. 1

1.0

.94

.93

1.0

. 84

.81

Calories

3. 915

3. 913

023

922

997

071

908

4. 040

3. 716

3.480

3. 040

3. 129

3. 125

3.397

3.958

2.372

2. 712

3.551

3.318

.673

.599

.214

. 210

. 203

1. 112

.848

. 900

.782

1.014

^^Z^i^^''^§^ ^^ ^^''•'^^ samples weighted by their con- sumption during period of digestion experiment.

'*^N-conversion factor not reported.

104
TABLE 24.—Composition and heat of combustion offood items used in experiments on human digestibility

(table ;g5)—Continued

Test food, description Water Protein Fat

Carbohydrate

Total (by

difference) Fiber

Ash

Heat of

combustion per

gram

Ref. No.

VEGETABLES—Continued

297 Potatoes, white

298 Potatoes, white

299 Potatoes, white

300 Potatoes, white

301 Potatoes, white 23

302 Potatoes, white 23

311 Pumpkin (Cucúrbita pepo)

312 Sweetpotatoes, partially dried

{Ipomoea batatas)

FRUITS

322 Bananas {Musa paradisiaca var.

sapientum)

323 Grapes {Vitis spp.),"^^ mixture of

Tokay, Muscat and Cornichon..

MISCELLANEOUS

330 Yeast, dried

331 Yeast, fresh compressed

Percent

73.66

76.23

77.66

74. 80

74.3

79.4

84.01

4.96

77. 15

86.8

6.82

69. 92

Percent

2 1.95

2 2. 11

2 2. 04

2 1.20

2 1.8

2 1. 4

2 1. 94

2 1.71

2 1.60

2.7

25 47. 25

2 15. 20

Percent

. 19

.76

.24

.2

Percent

23.38

20.70

19.47

23. 19

23.

18.

13.23

90.27

20.20

12.0

Percent

1.30

7.65

.52

.5

Percent

1.01

.96

.84

.81

.8

.9

. 63

2.30

.81

.3

Calories

4.478

1.443

69

69

69

69

69

69

134

134

75

75

87

87

2 N X 6.25.

23 Average of several samples weighted by their consumption during period of digestion experiment.

24 Weighted average of 3 kinds of European type grapes

used in the digestion experiment.

26 N X 6.25. Authors also reported 7.05 percent

protein N and 0.51 percent purine N.

105

U. S. GOVERNMENT PRINTING OFFICE: 1955