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

All fruit juice (except lemon, lime} unsweetened _
Lemons, limes. _ .

Protein Fat Carbohydrate
Koctor Rector Factor
F . 0 tobe
Food or food group Gait | Heat OC) apoiied | COM | eat of | applied | C2°M | stent of | applied
Atgestl | tion Tess | geytag | dinesti- |CUREAN) FO iD, | Gipesti. | CombUS] to In
ility | 1.257 | Rested | “pitty peated | “Sility nutel-
ena ents ents
a @}@ !]@]o] ola |] |] o | a
Eggs. Meat products, Milk products: Pot, |Cabjgm.|Caligm.| Pa. |Col.jgm,|Caljom.| Pot. | Cat.Jgm.| Cal.iom.
Eges -| 97} 450} 436] 95 | 9. 02 | 9813.75 | 3.
Gelatin. 97 | 4.02 | 3.90] 95} 9.50 | 9.02 |._.._-f----_|.-2 2.
Glycogen._-_-.-.__-_------- 2. 2222-22-22 2222 fe ee ---|------|------|--- = - -
Meat, fish -| 97 | 4401427) 95] 9
Milk, milk pr 97) 440/427] 95) 9.
Fats, separated:
utter... -| 97/440) 427) 95/9.
Other animal fats --.-|---.--|---.-- 95 | 9.
Margarine, vegctable_ 4.40) 427) 95] 9.
Other vegetable fats and oils. p=. fee 95 | 9,
Fruits:
‘All (except lemons, limes). __-_ 9.
9.
o2

Lemon juice, lime juice, unsweetene:

Grain products:
Barley, pearled__._
Buckwheat flour, dark —
Buckwheat flour, light.
Cornmeal, whole ground
Cornmeal, degermed.
Dextrin. -
Macaroni, spagi
Oatmeal, rolled oats
Rice, brown...
Rice, white or polishe
Rye flour, dark __
Rye flour, whole grain
Rye flour, medium _
Rye flour, light. _
Sorghum (kaoliang), whole or nearly w
Sorghum (feierita, kafir, milo),

extraction...
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

Legumes; Nu
Mature dry beans, cowpeas, peas, other legumes;
nuts
Immature lima beans, co

ywpeas, peas, other legumes.
Soybeans, dry; soy flour, flakes, grite___.....-
Sugars:
Cane or beet augar (sucross)
Glucose -
Vegetables:

Potatoes and starchy roots
Other underground erops 5
Other vegetables

Chocolate, cocca
Vinegar.
Yeast___

9.30 | & 37 941 4.20 3.95

9.30 | 8.37 90 | 4.20 3.78

9.30 | 8 37 94 | 4.20 3. 95

9.30 | 8.37 96 | 4, 20 4,03

9. 30 | 8 37 99 | 4.20 4.16

- 98} 4.11 4.03

4, a 98 | 4. 20 4.12
4. 9. 98 | 4.20 4.12
4. a 98 , 4.20 4.12
4, 9. 99 [ 4 20 416
4, 9. 90 | 4.20 3.78
4. 9 92 | 4 20 3. 86
4 9 95 | 4.20} 3.99
4, 9. 97 | 4.20 4.07
4. 9, 96 | 4.20 4.03
4, 42, 28 90 | 9.30 | & 4O7 1 4.20 | 14.07
4. 3. 59 90 |. 9. 30 | 8. 90 | 4,20 3.78
4.55 | 3.78 90 | 9.30 | 8 94 | 4.20 3.95.
4,55 | 4.05 90 | 9.30 | 8 98 | 4.20 4,12
4.55 | 3.59 90 | 9,30 | 8.37 90 | 4.20 3. 78
4.55 | 1.82 90 ; 9.30 | 8.37 56 | 4.20 2. 35
4.55 | 3. 87 90 | 9.30 | 8 37 98 | 4. 20 4.12
4.55 | 3.55 90 | 9. 30 | 8.37 94 | 4.20 3.95
4,45 | 3,47 90 | 9.30 | 8. 37 97 | 4.20, 4,07
4.45 | 3.47 90 | 9. 30 | 8. 37 97 | 4.20 4.07
4.45 | 3.47 90 | 9. 30 . OF 97 | 4.20 4.07
98 | 3.95 3. 87

98 | 3.75) 3.68

3.75} 2.62] 90/930]/837] 985/410! 3.48
3.75 | 2.78 90 | 9.30 | 8.37 96 | 4.20 4.03
3.75 | 2.78 90 | 9.30 | 8 37 96 | 4.00 3. 84
3.75 | 2.44 90 | 9. 30 | 8. 37 85 | 4.20 3.57

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.

1 The correction, 1.25 calories, has been subtracted from
the heat of combustion. This gives values applicable to

‘ams of digested protein and identical with Atwater's

factors per gram of available protein.

? Carbohydrate factor, 3.87 for brain, heart, kidney,
liver; 4.11 for tongue and shellfish.

Revision made since 1955.

5 Vegetables such sa beets, carrots, onions, parsnips,
radishes.

* Coefficient of digestibility, 98 percent; heat of com-
bustion, 7.07 calories per grain’; factor to apply to ingeated
alcohot, 6,93 calories per gram.

b-)




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A complete outline of the work, and the pro-
cedures 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 four experiments
of the series as “respiration experiments;” for
these, analyses were made of food intake, drink,
feces, urine, and respiratory products. No deter-
minations were made of the heat given off from
the body nor of the heat equivalents of external
work in these experiments. He called the remain-
ing experiments, Nos. 5 through 55, “metabolism.
experiments.”’ They included measurements
of energy in addition to the data obtained in the
respiration experiments. Each metabolism experi-
ment 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, Di-
gestibility data were available from the second
art ag well as from the first part of each metabo-
ism experiment.

The respiration calorimeter, described in detail
in U. S. Department of Agriculture Bul. 63 (21),
was especially designed for this series of experi-
ments. 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. Durin;
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
“light” 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. Nitro-
gen 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 10 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 b
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 calorim-
eter. 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 possi-
bility that a lag in nitrogen excretion by the
subjects would introduce some error in urin
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 in the urine the perspiration
losses should be recognized. However, as nitrogen
losses have been shown by 25 work experiments of
Atwater and Benedict (76) to be small, averaging
only 0.29 gram per day, and as data on compa-
rable energy loss in the perspiration were lacking,
the data in table 8 apply to urinary losses only.

The series was planned 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 experi-
ments the amounts of protein, fat, and carbo-
hydrate, and the gross calories found by deter-
mination were reported. The amounts of other
nutrients present in the diet have been calculated
from tables of nutrient composition; these cal-
culated values are shown in table 10.

In the annual report for 1899 (17) Atwater con-
tinued 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 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 deriva-
tion 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


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A very wide range, 57.6-97.9 percent, in the
coefficients of digestibility for fat in catmeal was
calculated from data in 16 experiments published
by Harcourt and Fulmer (62). These investi-
gators 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 oat-
meal from data supplied we assumed the digesti-
bility 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 ma:
be some error in our calculated results for oatmeal.

The data from Harcourt and Fulmer suggest that
there may be some relation between digestibilit:
and fiber content of oatmeal. When the oatmeals
of both 1.94 percent fiber and lower fiber contents
were fed to the same subjects under similar experi-
mental conditions, the fat digestibility of the oat-
meal 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 motil-
ity 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-extracteble matter) in the feces? The ex-
perimental evidence is inadequate either to inter-
pret 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 Sugi-
moto, Higuchi, Momyeda, Tonaka, Yasuda, and
others and was published by Saiki in 1926 (178).
The subjects, all Japanese men, ate rice as part of
amixed 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 stil] 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 Jayer and germ.

(c) 70% polished rice—Rice which still re-
tained about 30 percent of the outer layer and
germ,

(d) Polished rice (white rice)—Rice which
had been polished 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-polished rice,
nearly 6 percent in making the 70-percent polished
rice, and approximately 7 to 8 percent in making
the fully polished white rice. According to one
article in the series no polishing powder was used.
Judging from other information given, the unpol-
ished rico may have been the same as brown rice;
the half-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-per-
cent 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-pelished, 74 percent for 70-percent polished,
and 73 percent for polished white rice.

Rubner (144) and also Snyder (/67) 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 de-
scribed in either study. The number of experi-
ments 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, respec-
tively, 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 earbo-
hydrate.

The energy factors we suggest for the protein
and carbohydrate of brown 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 un-
accustomed to eating large quantities of rye
bread experienced pain in the digestive tract and
in some cases diarrhea, a factor that might vitiate
the digestibility figures. But on the basis of the
data reported there was no clear-cut means of

33


TasLe 24.—Composition and heat of combustion of food items used in experiments on human digestibility
(table 28)—Continued

Carbobydrate Heat of
‘Test food, description Water | Protein Fat Ash | Gumbus” | Ref. No,
FRAO, | Fiber erm
LEGUMES AND NUTS—Continued
Beane, dry—Continued
212° Common white, navy beans, akina | Percent Pereent
removed. -| 13.32 165
213 Common white, navy beans_ 7] 12 82 3 38 137
cat Kidney beans. 2) 11 25 358 184
wpeas, dry (Vigna sinensis)
221 Cle: 18. 37 & 50 184
10.77 3.72 184
iL 32 3.57 184
10. 27 3.45 184
10. 05 351 184
8 08 3 93 184
12. 84 357 184
8 36 376 184
13.0 27 145
13.2 2.6 145,
1b 53 244 140
12. 6 1 56 113
Seybeans, Soybean
748 Soybean flour, about 6.5 percent fai.| 4.17 5.80 29
249 Soybean flour, about 3.3 percent fat| 6.5 57 29
252 Soybean curd (Tofu) _- {] 87. 80 271 134
Ground nuts or peanuts (Arachie hypogaea):
259 Peanuts | 4.88 2.56 15
261 Peanut flour, partially defatted -_ ~~ 44 3% 84 |... 64
Tree nuts:

264 Almonds (Prunus amygdalus).. 2.78 | 3.129 5
265 Brazil nuts (Bertholletia excelsa) 3.62) 3.125 TS
266 Brazil nuts 265 | 3.397 75
267 Chestnuta, fresh (Castanea sativa) a7
268 Chestnut flour (Castanea dentaia) 2.40 | 3.958 120

(Kernel before processing)” _ 1.41 | 2372
269 Coconuts (Cocos nucifera) 114 | 2.712 75
270 Pecans (Carya illinoensis) £55 | 3551 75
271 Walnuts (Juglane regia) (presum-
ably Persian or English)... ...-—- 97 191 | 3318 7
VEGETABLES
272 Beans, snap (Phaseolus eulgaris) 92.44] 41.16 13 83 [----2-.- 57
274 Beets (Beta vulgaris) 82.6 | #19 :3 14 673 33
275 Beets 85.4 | 2.2 :2 14 599 33
-| 94.7 *9 3 8 33
277 | 948 | *1L0 3 9 33
278 =| 944 #9 = 9 33
280 Cabbage, savoy “] 804 | 221 :6 13 144
281 Carrots (Daucus carota) -| 90.53} 2.86 133 188 134
282 Carrots. | 863 | #716 12 19 144
283 Collards (Brassica oleracea var.
acephala) -| 88.44] 13.13 54) 5.53] 151] 2.36 |--.-...- 4
284 Corn, green (Zea mays) |) 76.0 | #49 4 | 173 :5 14 | Lie 33
286 Potatoes, white (Solanum tubero-
sum). -| 79.5 | 42.2 wl 4 8 33
287 Potatoes, white “| 73.38 | #23 M1 4 ‘9 33
288 Potatoes, white | 82 | #19 23 5 V1 33
291 Potatoes, white 7] 734 | 423 |. . 3 10 144
292 Potatoes, white. 2) 74.33 | #2. 38 . : 42
293 Potatoes, white “| 80.16 | # 1.49 134
294 Potatoes, white # “| 75.4 | 220 69
295 Potatoes, white -| 77.66 | 2.04 69
296 Potatoes, white “| 74.80] 1020. 69
2N X 6.25, 3 Average of several samples weighted by their con-
* Item No. 268 was prepared from this eample. sumption during period of digestion experiment.

*N-conversion factor not reported,

104


less complete analyses made by other investi-
gators. Stutzer analyzed. several kinds af cocoa.
For one product which had not been treated with
alkaline chemicals (potash, soda, or ammonia) he
found that 16.6 percent of the total nitrogen was
from alkaloids (mainly theobromine), 1.4 percent
from ammonia, 6.3 percent 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:

sssount| eat or

1
Nitro- | Conver-

Compound tt; eorn, | enerey

TM | ferobe" | Scr ja [tin equiva,

. ! om, | Gm. |Cal.jom.| Cat.
Protein_.._._....-- (0. 758 | 6.25 | 4.74] 5.80) 27.5

Alkaloids as theo- i |

bromine -|-166 | 322) .53) 522; 28
Ammonia Tord | 122 | loz | ooo howe
Amides as asparagin.| 1063 | 5.35 | ° 34) 345 |" 1.2

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 was no atcempt
to distinguish between the protein and §'non-
protein fractions of the nitrogenous matter. In
some of the studies cocoa supplied all of the
nitrogen, and in some the diet included other
protein foods in addition to the cocoa. Experi-
ments conducted by Weigmann and by Lebbin,
reported through Konig (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, Lebbm found protein digesti-
bility coefficients of 41.1, 45.2, and 41.6 percent;
for a diet of cocoa and beer or wine, Weigmann
found a digestibility of 41.5 percent after correc-
tion for metabolic nitrogen. The apparent di-
gestibility was calculated to be 12.7 percent. For
tao kinds of cocoa, Neumann (131) reported that
Beddies found digestion coefficients of 55.3 and
54.1 percent. In these latter experiments, 150
grams of cocoa were consumed daily but no
information was given on the remainder of the

let.

Several studies have been reported in which
cocoa was eaten in combination with other protein-
containing 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 (56) 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 grams

(8 cups), respectively. Schlesinger (143) found a
digestibility 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 digestibility 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 mixed diet
which included 50 grams of cocoa. Cohn (41)
observed a lower digestibility coefficient, 75.5 per-
cent. His dict differed from those of Schlesinger
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,
182). In one series of investigations he deter-
mined 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 digestibility of the protein of
the diet without cocoa averaged 82 percent. When
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 digestibility 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 correeted 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 digestibility 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 mixed diets and there is no
means of determining whether the utilization of
cocoa as a flavoring 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 digestibil-
ity of 42 percent, although this may be too low
for ordinary application. 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 nitrogen-
containing 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 pre-
sent problems similar to those for the nitrogenous
matter, Some unpublished data* for chocolate
liquor 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 hemicellulose;

4Winkier, W. 0. Unpublished data, Food and Drug
Administration, U. §. Department of Heaith, Education.
and Welfare [n! d.J.

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65


(76), who determined the digestibility of diets
composed of fruit and nuts mm 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 protein was about 75 percent. This value
is lower than usually is found for mixed diets in
which plant foods predominate. Possibly the low
apparent digestibility was the result of the large
quantities of fruit consumed. Unfortunaicly 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 digesti-
bility for fruit protein, ranged from 54 to 87 per-
cent, averaging 70 percent for nuts as a group.
From the two experiments on peanuts the calcu-
Jated coefficient of digestibility for protein would
be 81.5 percent. These figures may be too low
since the calculation is dependent on the digesti-
bility of the whole diet. Holmes (70, 72) found a
much higher digestibility, 92 percent, for peanut
protem. 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 con-
tributed most of the total protein intake Heupke
and others (67) found that the digestibility of
chestnut protein ranged from 68 to 79 percent.

Jn 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 @ group. It is preferable to continue
to use, as an interim value for nuts, Atwater’s
group coefficient for protein in legumes, 78 per-
cent. 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 grou

Very little work on the digestibility of carbo-
hydrate material in nuts has been reported. Only
a small fraction of the total carbohydrate intake
was furnished by nuts in Jaffa’s series of experi-
ments and the digestibility of carbohydrate in
nuts has not been estimated. Merrill (720) found
that more than 98 percent of the carbohydrate in
chestnut flour was digested. Heupke and others
(67) found digestibilities ranging from 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 composi-
tion of various nuts indicate that most nuts con-
tain from one-half to two-thirds of their carbo-
hydrate in the form of sugars or starch or both,
up to one-third as crude fiber, pentosans, and

similar complex carbohydrate constituents, and @
like amount as undetermined matter, Almonds
appear to have a lower proportion of sugar and
starch, averaging around 40 percent, and have
about 25 pereont in the form of complex carbo-
hydrates that are of questionable availability.
The nature of the remaining portion is undeter-
mined. 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 carbo-
hydrate 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.

Vegetables

Very fow digestibility studies of vegetables had
been made when Atwater proposed for all vege-
tables as a group the factors 3.11 calories per gram
for protein and 3.99 calories per gram for carbo-
hydrate. 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 found for
protcin by the 10 investigators weve from 64 to 83,
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 little
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
arc practically the sole source of protein.

One such siudy was reported hy 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 equilibriufa and in apparent good health
on a very simple diet with the daily intake of
nitrogen chiefly from potatocs, averaging only 3.7
grams for the man and 3.8 grams for the woman.
The coefficients of digestibility of the potato pro-
tein were 66 and 62 percent respectively for the
man and woman.

Three subjects in the study by Hindhede, 1913
(69), also were on simple diets in which potatoes
contributed nearly all the protein over an ex-
tended period of time, The diets were planned
to provide the minimum protein intake at calorie
levels just sufficient to maintain nitrogen equi-
librium. The digestibility of the potato protein

37


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68


Tas iE 24.—Composition and heat of combustion of food items used in experiments on human digestibility

(able 23)—Continued

Carbohydrate Heat of
1 bus
‘Test food, description. Water Protein. Fat Ash Moe Ref. No.
Fas0,| river an
VEGETABLES—Continued
Percent Percent | Percent | Percent | Calortes
297 Potatoes, white. -| 73. 66 -| 23.38 | 1,01 {- 69
298 Potatoes, white. -| 76,23 -{ 20.70 [- - 96 |. 69
299 Potatoes, white. -| 77. 66 -} 19.47 |. 84 |. 69
300 Potatoes, white. -| 74. 80 -| 23.19 |_ -8l ye 69
301 Potatoes, white . -| 743 -| 23.2 1. -8 FL 69
302 Potatoes, white *. -| 79.4 -{| 184 |. 9 |. 69
311 Pumpkin (Cucurbi -| 84.01 13. 23 1.30 + 63 | 134
312 Sweetpotatoes, partially
(Upomoea bataias) ....-.----.-- 4.96 90. 27 7. 65 2, 30 j---.- 2 134
FRUITS
322 Bananas (Musa paradisiaca var.
sapientum). -----------]| 7715 | 71.60 .24) 20.20 . 52 -81]--.--... 75
323 Grapes (Vitis spp. mixture of*
Tokay, Muscat and Cornichon_.| 86. 8 27 .2 12.0 5 +3 J----.--. 75
MISCELLANEOUS
330 Yeast, dried____..____ - 6. 82 [35 47. 25 aT
331 Yeast, fresh compressed. 69. 92 | 3 15. 20 87
*N X 6.25, * Weighted average of 3 kinds of European type grapes

” Average of several samples weighted ‘by their con-
luring period of digestion experiment,

sumption

uN

x

used in the digestion experiment.

6.25, Authors also reported 7.05 percent

protein N and 0.51 percent purine N,

105

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PART Il.

Definition of 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 para-
graphs 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 suffici
ently accurate to permit the determination of the
undigested residue separately and he did not
compute digestibility.

Availabilty was the term Atwater used to
designate the quantity or proportion of the food
or of the nutrients which could be used for build-
ing 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 bile and other digestive secretions. Inas-
much as these metabolic products are not used
for tissue building or as fuel, they are not avail-
able in the sense in which Atwater employed the
term. He computed the amounts of available
nutrients (protein, fat, carbohydrate) by sub-
tracting 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 follows:

Nin food—N in feces
N in food

X100= 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 urinary 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 incom-
pletely oxidized matter of the urine is assumed to

e of protein origin and the available energy of
protein is the gross energy of the absorbed protein

DIGESTIBILITY AND AVAILABLE ENERGY OF FOODS

(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 followinglproce-
ures:

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 grams X7.9).

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 of
the extent of the differences resulting from these
methods of calculation under other conditions
follows the section on ealoric-nitrogen ratio of the
urine, page 18.

Atwater distinguished between plysical 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
(58, 111, 159). Likewise, in the present publica-
tion physiological fuel value is the term used to
connote energy value of a food obtained by sub-
tracting energy lost in the excreta (feces and urine)
from the total energy value of the food, no con-
sideration being given to the specific functions
served in the body.

ity of fat, carbohydrate,
and protein

Digest

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 nitrog-
enous 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


Energy value of foods

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. O. Atwater and his
associates at the Storrs (Conn.) Agricultural Ex-
periment 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 calorics shown in official food com-
position tables. Properly applied, these general
factors provide a satisfactory measure of available
energy m average diets and food supplies in this
country. Following Atwater’s period little atten-
tion 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 during and
followmg World War JI 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 country. On several occasions he called at-
tentioh to the correct application of the general
calorie factors 4, 9, 4 and pointed out their limita-

PART I.

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, é
and H,0, 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

1 The authors express appreciation to Mildred Adams
for her review of the manuscript and her invaluable sug-
gestions; 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.

tions and misuse when applied to individual foods
and different types of dicts (114, 118)?

The Food and Agriculture Organization, faced
with the urgency of assessing energy values of food
supplies in Vatious countries and population groups,
convened an ad hoc committce 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 knowl-
edge 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 (85) 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 deter-
mining fuel values of food, and the need for revisin;
calorie data for foods to take account of additional
research accumulating since his time. This pub-
lication has been prepared to provide more back-
ground 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. 8. Department of
Agriculture Handbooks No. 8 (188) and No. 34
(100) and in various other sources, in¢luding food
tables published by the Food and Agriculture
Organization (36).

SOURCES OF FOOD ENERGY

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 theso 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

‘italle figures in parentheses refer to Literature Cited,
p. 61.


9.5 percent products such as tannins and cocoa
red; 0.5 percent sugars, mainly glucose; 0.6 percent
organic acids; and the remainder, undetermined
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 weight-
ing 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 could not be calculated for the total
diet or for the cocoa. The experiments of Lebbin
on diets of cocoa and sugar reported by Konig
(84) indicated that probably less than a third of
the total carbohydrate of cocoa is available to the
body. Here, as in the case of protein, this esti-
mate may be Jower than actually found when
cocoa is used in a mixed diet. As a tentative
coefficient, 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 Tiquor,
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
53 percent for the fresh compressed yeast.

Dirr (80) 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 source:
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 com pletely absorbed as animal protein.
Dirr and Soden (61) referred to the yeast as wood
sugar dried yeast and estimated from analyses

42

that 67 percent of the total N was amino N and
7.5 to 16 percent was ammonia N.

Funk, Lyle, and McCaskey (69) reported ex-
periments 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 esti-
mated to range from about 60 to 80 percent,
averaging about 70 percent. Results of Murlin
and others (/25) indicated that the apparent
digestibility 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
yeast 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 digestibility as 80 percent.

According to an analysis of yeast reported by
Frey (68) the nitrogenous matter is composed of
60 percent monoamino acids, 20 percent diamino
acids, 12 percent purines and pyrimidine bases,
and 8 percent ammonia. These data indicate
that the heat of combustion is lower than if the
nitrogenous 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 little research has been noted on the com-
position of the different specific carbohydrate
constituents in_yeast and none at all on their
digestibility. Frey (68) has reported that 81.5
percent of the total carbohydrate is glycogen and
18.5 percent, such substances as cellulose and
gume. 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

difference), resulted in the energy factor 3.35
ajories per gram.

Food mixtures

To keep pace with marketing practices as well
as buying habits, successive editions of food com-
position 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 pro-
portions 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


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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 sass:
Slightly revised February 1973

For sale by the Superintendent af Documents, V5. Governmant Printing Ofice, Washington, D.C. 20402
Price $1.25 domestic postpaid or $1.00 GPO Bookstore. Stock Number 0100-02770


CONTENTS

Introduction..-_.-.-.-----

Part I. Sources of food energy.

Part IL. Digestibility and available energy of foods.

Fat__
Determination of fat conten
Heat of combustion

Carbobydrate.
Determination of carbohydrate content
Heat of combustion.

Protein...
Determination of protein content.
Heat of combustion.

Determined versus eal:
of foods__ -

Other sources of energy
Organic acids.
Alcohol -

Definition of terms.

Digestibility of fat, carbol
Availability of energy from digested nutrients
Carbohydrate.
Protein.
Aleohol. -

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

3. Factors for caleulating protein

different carbohydrates and assumed factors
for carbohydrates of different groups of food
materials... ._

content of food

4, Average determined heats of combustion of

proteids and nonprotcids and calculated heat
of combustion of protein

5. Comparison of calculated heats of combustion

8, Summary of data showing calorie.

with results of direct determinations__

6, Frosh truits classified as to organie acid content.

. Factors for heats of combustion and fuel values
of nutrients in different groups of food mate-
rials and in mixed die

trogen ratio
of urine based on early studies of energy
metabolism and digestibility...

9. Daily food intake in the experiments from which

10. Daily nutrient intake in the expe

i

Atwater originally obtained the calori

nitrogen ratio of 7.9 for urine_

imi
which Atwater originally obtained the calorie-
nitrogen ratio of 7.9 for urin

Comparison of data for available energy
obtained by direct determination only and
in part by calculation... ._..-.-----22----

19

Part III, Derivation of current calorie factors ____

Part IV. Application of e:
Comparison of calculated and determin

Genera] factors and more

Physiological fuel values of plant products_

Physiological fuel values of foods of animal

roducts of wheat... ----.---
Produets of grain other than wheat
Legumes.

Miscellaneous food:

able calories for diets_. .

specific factors for
calculating calories in individual foods. -. ..

Application of general factors to national food

Conclusions.
Literature cited
Appendix

Apparent digestibility and available enersy_
Composition and heat of combustion of foods.

Text Tables

12.

13.
14.

18.
16.
17.

18.
19.

20.

21.

supplies

Tabular summary of experiments on
digestibility of foods of piant origin by human
subjects.

Effects on energy metabolism of replacing
portions of dictary carbohydrate and fat by
aleghol__ woes

Data used for caleulating energy values of foods
or food groups by the Atwater system __

Energy values of wheat flours calculated by use
of specific energy factors for protein, fat, and
carbohydrate —

Apparent. digestibility and physiological fuel
value of wheat flours_

Coefficients of apparent “Gigestihility 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 - --

Factors for digestibility, heats of combustion,
and physiological fuel values of nutrients in
food groups as used in present-day mixed
diets

Comparison of energy values for different
dietary patterns calculated with specific and
with general calorie factors.

Appendix Tables

22. Use of digestibility data to determine coeffi-

cients of apparent digestibility and available
energ

23, Apparent digestibility and available energy of

foods of plant origin for human subjects...
Grains, grain produets.
Legumes and nuts-

59
60
87

23.

24.

Apparent digestibility, ele —Continued
Vegetables, vegetable products
Fruits...
Miscellancou:

Composition and heat of combustion of food
items used in experiments on human digest-
ibility (table 23)



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 with 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.

igestion 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 @ 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 car-
bohydrate calculated by use of these coefficients 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
alimentary pasies made from flour and water.
Digestibility of those containing eggs or milk might
be somewhat different, but as only small émounts
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 varicty of
baked products including yeast breads, baking
powder biscuits, cakes, cookies, crackers, and
others (48). 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, weight-
ing the energy factors of each ingredient in propor-
tion 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 co-
efficients were in every case very low. The aver-
age apparent digestibility found for the protein of
bran was 40 percent based on 14 digestion experi-
ments, 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 cus-
tomary deduction for urinary nitrogen 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 experi-
mental diets were low in protein the apparent
digestibility as determined may have been too
low. Hence, rather than use these data in esti-
mating 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 mix-
tures, we derived weighted energy factors if we
knew the kind and approximate proportions of
ingredients.

Products of grain other than wheat

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 remain-
ing grains.

Although information on the various cereals is
not strictly compara ble, 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 Water-
man (77) observed significant differences in the
extent to which proteins may be digested in vitro
by pepsin and trypsin. They found that arachin,
cascin, 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 avail-
ability of purified protcins have observed wide
differences in the proportions of a given amino

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12


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76


hydrate, were ‘predominantly weighted by refined
cereals and therefore should give a fairly close
approximation of the energy value of foods that
have undergone considerable refinement.

Legumes

The array of data on digestibility of foods in
the dry legume group (see appendix table 23)
shows that several studies have been made of the
more important items. For beans, peas, and
soybeans the digestibility of carbohydrate (deter
mined by difference) was high, averaging 96 to 98
percent; for cowpeas it was lower, about 90
pereent. These high coefficients 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 carbohydrate
matter. Results of some investigations 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 various legumes. Some legumes
are similar in their content of moisture, protein,
fat, ash, and total carbohydrate by difference,
but several experiments indicate that the similarity
does not hold for individual carbohydrates,
Differences in digestibility might be more easily
understood had more information becn obtained
on the makeup of the carbohydrate fraction in
the legumes samples used. Data in the literature
indicate that ihe proportion of the total carbo-
hydrate (by difference) in the form of the so-called
available carbohydrates, mainly starch, sugar, and
dextrin, is less than one-half’ for cowpeas, two-
thirds 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 varied for the several
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 relatively
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
availability. In this fraction a variety of sub-
stances has been found in widely varying amounts.
These include raffinose, stachyose, pentosans, ga-
lactans, arabans, cellulose, lignin, organic acids,
phytin, and glycosides. In addition to these
some waxes, color principles, tannins, and undeter-
mined hemicelluloses are believed to be present.

Bowers (29) studied the composition of the
complex carbohydrate fraction of defatted soy-
bean meal and the digestibility of separate con-

36

stituents with a healthy man engaged in moder-
ately active laboratory work as the subject. The
digestion experiment was carried out according to
customary procedures. The methods of analyses
employed were those used by Street and Bailey
(74). Analyses were made of both the soybean
meal and the feces resulting from a ration of soy~
bean meal porridge, milk, butter, and cane sugar.
Bowers reported the coefficients of digestibility
for the carbohydrates of cooked soybean meal as
follo: Dextrin and starch, 99 percent; sucrose,
100 percent; raffinose, 100 percent; organic acids,
99 percent; pentosans, 93 percent; galactans, 96
percent; cellulose, 79 percent; and the remaining
fraction, presumably waxes, color principles, and
possibly undetermined hemicelluloses, 94 percent
by difference. By calculation from the separato
constituents he arrived at a digestibility coefficient
of about 94 percent as compared with 96 percent
which he found independently for total carbo-
hydrate by difference.

Entirely different results for digestibility of the
carbohydrate of uncooked soybeans were obtained
by Adolph and Kao (3) in a series of in vitro
experiments in addition to in vivo digestibilit:
experiments with ais. They estimated ihe avail-
ability of soybean carbohydrate to be about 40
percent. This figure has been widely used in
assessing soybean carbohydrate ($6, 87, 185), but
its application to soybeans for human use seems
questionable.

In the manufacture of soybean curd and milk,
much of the carbohydrate fraction is removed 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 (734).

Although there may be significant differences
in digestibility of carbohydrates in different leg-
umes as indicated in the very few studies avail-
able, it does not appear wise at the present. time
to depart from the group factor, 4.07 calories per
gram, originally used by Atwater for carbohydrate
mm legumes.

A large number of studies has provided data
on the apparent digestibility of protein in leg-
umes. Although some variation 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 regarding digestibility
and composition similar to those of the legume
group. Little has been reported on their digest-
ibility 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



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 caJories
from specific nutrients. Caution should be used
also in applying general digestibility coefficients to
such diets with a view to obtaining data on avail-
able nutrients.

General factors and more specific factors
forcalculating 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 list of foods repre-
sentative of different groups has been assembled
below in tabular form to illustrate this difference:

Energy value per 100
grams edible portion
derived by use of—
a) Ration
Food wl.
Specific |General fac-}‘oole
factors | tors 4, 9, 4
fa) (b)

Animal food Catoriea | Calories | Percent
Beef_.- wens - 27 268 9
Salmon, canned. - 143 138 97
BE - 162 158 98
Mi - 68 69 101

Fats:

Butter____-__-_---_--- 716 733, 102
Vegetable fats and oils.. 884 900 102
Cereals:

Cornmeal, whole ground

(unbolted) ------ - 355 367 103
Cornmeal, degermed____ 363 356 98
Oatmeal. - - 390 396 102
Rice, brown_ - 360 356 99
Riee, white or m: ae 362 351 97
Wheat fiour, whol

wheat________ - 333 355 107
Wheat flour, pate - 364 355 98

Legumes:

Beans, dry seeds. 338 346 102
Peas, dry seeds. - 339 349 103
Vegetables:
Beans, snap_ 35 42 120
24 29 120
42 45 107
83 85 102
32 35 109
Apples, raw. - 58 64 110
Lemons, raw_ - 32 44 138
Peaches, canned _ 68 75 110

Sugar:

Cane or beet_____.-.-- 385 398 103

48

The significance of some of the differences
illustrated above becomes more apparent when
related to emergency feeding problems. For
example, the genera] 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, estimated, 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 suit-
able for estimating available energy values for indi-
vidual foods, the question arises as to whether they
may be suitable for calculating calories of present-
day food supplies. Food consumption patterns have
changed over the years (782). There have been
major shifts in consumption of foods within groups
and between groups. As « 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 shown 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 unrounded 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 supply 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 calculating 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 compara-
ble 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 supplies 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


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55


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 antici-
pated as more complete information becomes
available on the various constituents in the
nitrogenous matter, fat and carbohydrate of
food, and on their heats of combustion and
digestibilities. Moreover it is realized that there
are problems with direct bearing on the digesti-
bility of protein, fat, and carbohydrate that have
not been resolved satisfactorily at this time.
Although all of the calorie factors may not he
entirely suitable as a result of the various limita-

tions existent in the basic data, nevertheless when
they were applied to the nutrients in foods fed
alone or in 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 modifica-
tion of the method of Atwater and Bryant as pro-
posed in the present publication for estimating
the available (or physiological) energy value of
foods scems 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.

LITERATURE CITED

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(5) Association of OrriciaL AGRICULTURAL CHEM-
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(6) Arwarer, W. O.
1902, PRINCIPLES OF NUTRITION AND NUTRITIVE
VALUE OF Foon, U. S Dept. Agr.
Farmers’ Bul, 42, 48 pp., illus.

3

Expt.

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OF FOOD MATERIALS. Conn, (Storrs)
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(1902-1903), pp. 123-146.

1904. EXPERIMENTS ON THE DIGESTIBLLITY OF
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(0) ——

1906, PRINCIPLES OF NUTHITION AND NUTRITIVE

VALCE OF Foop. U. 8. Dept. Agr.
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(11) Arwarer, W. 0.
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(20) ——— and Brrann, A. P.

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illus.

51


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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 excel-
lent 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 calcu-
lated 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 valucs.
These positive and negative deviations were noted
even in experiments in which the same 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 legumes; 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 made 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 prams daily. We
have grouped some of the diets and summarized
the difterences wo found 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 dry
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 oatmeal the absolute values deviated by
only 1 percent: in all cases.

Tn 6 diets of whole-wheat bread and milk the
absolute deviations ranged from 2 to 4 percent,
averaging 2.5 percent.

In 11 diets containing a large proportion of
cabbage, potatocs, beets, green corn, or apple-
sauce, the absolute deviations ranged from 0 to
3 percent, averaging 1 percent.

th 6 ordinary mixed diets the absolute devia-
tions 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 10 5 per-
cent, averaging 2.5 percent.

Several genera] observations appear reasonable
in view of these data:

1. That the energy factors in table 13 give an
accurate estimation of the available energy

waa OS 4

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 Jarge 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 te
foods used alone in the diet, but further confir-
mation 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 Jarge 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 evi-
dence 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 calcu-
lated also by applying 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 differ-
ences 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 differ-
ent types 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 avail-
able 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
protem and fat, the apparent digestibility of the
total protein from the diet was 45 percent, with an
estimated 25 calories available from protein in-
stead 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 digesti-
bility for the total fat in the diet, instead of the
201 obtained by use of the factors with the indi-
vidue] foods. For carbohydrate, however, ihe
data from this experiment would indicate approx-
imately 861 available calories instead of the 823
obtained by application of the factors from table
13 to the carbohydrate of the items in the diet.

45


PREFACE

The kilocalorie, which has been defined as the amount of heat energy needed to raise the temperature
of a kilogram of water 1° C., is the unit that bas been used traditionally for expressing the energy value
of foods. Recently the International Bureau of Weights and Measures has recommended that the joule,
a unit of energy applicable to electrical, work, and chemical energy, be adopted as the preferred unit for
all forms of energy. The joule is derived from basic units in the International System of Units (SI) and
is defined as a measure of force (newtons) times distance (metres).

In the interest of uniform nomenclature, some nutritionists have proposed that the kilojoule replace
the kilocalorie. The conversion factor for expressing kcalories as kjoules, as reeommended by the Com-
mittee on Nomenelature of the International Union of Nutritional Sciences, is 1 kealorie equals 4.184
kjoules, based on the kealorie determined at 14.5° to 15.5° C.

Use of kjoules in place of kcalories as the unit of measure for energy in no way invalidates the
principles underlying the Atwater system for determining energy value of foods and the energy needs
and energy expenditures of man.

The Atwater system is based on the demonstrated principle that the oxygen used, the carbon dioxide
formed, and the energy evolved in oxidizing foods are the same whether this oxidation takes place in the
body of man or in a bomb calorimeter. Furthermore, Atwater has clearly shown that by applying appro-
priate factors, which allow for metabolic losses, to the contents of protein, fat, and carbohydrate in a
food, the physiologically available energy value of that food can be calculated with outstanding accurac
The results obtained by this procedure are in excellent agreement with data from measurements made
by bomb calorimetry on food and metabolic products. Results of studies by Atwater and others could
be expressed cither in kcalories or in kjoules.



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CALORIMETER AND EXPERIMENTS ON
THE CONSERVATION OF ENERGY IN THE
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Coun, H.
1895. UERER CACAO ALS NAHRUNGSMITTRE.
Ztsehr. f, Physiol, Chem. 20: 1-27.
Consranriwinr, A.
87. UBER DIE AUSNUTZUNG DES WEIZENKLE
BERS IM DARMKANALE TND UBER DIE
VERWENDUNG DESSELBEN ZUR ERNAH-
RUNG DES MENSCHEN. Ztechr. f. Biol.
23: (433-455.
Crameron, K. W., Inwiv, M.
Nuttson, H.R.
1951. THE APPARENT DIGESTIBILITY OF ESSEN-
‘ALLY SIMILAR DIETS BY RATS, GUINEA
PIGS, SHEEP, SWINE AND RY HUMAN
supiects. Jour. Nutr. 43: 541-550,
Groox, E. M., and Watson, D. J.
1048. STUDIES ON THE STORAGE OF POTATOES.
I. CHANGES 1N COMPOSITION DURING
STORAGE IN champs. Jour. Agr. Sci.
[London] 38: 440-457, illus,
Danies, L. J.
1951. THE NUTRITIVE yALUE oF aLconoL. N.Y
State Jour. Med. 51: 1283-1284.
Dustxacuan, H. 8. R., Du, 8. §, and Scpranman-
van, V.
1948. PROTEIN VALUE OF SOVA-RE
MAN FEEDING EXPERIME!
Jour. Med, Res. 36: 145-

- and SuBRARMANYAN, V.

1, Luovn, L. B., and

MILK: HU~
« Indian
48.

1949. INFANT FERDING EXPERIMENTS WITH SOYA-
BEAN mick, Indian Jour. Med. Res.
37: 77-83.
Devet, Wi. J.
923. DIGESTIBILITY OF BARED GOODS MADP
FROM PateNT FLOUR. Jour, Home
Econ. 15: [699}-701.
1924. THE DIGESTIBILITY OF TEPARY BEANS.
Jour, Agr. Res. 29: 205-208.
Dinr, K.
1942. DRER DEN WERT DER WUCHSHEFEN FUR
DIE MEN: LICHE ERNAHRUNG. m1.
MITTEILUNG: UBER DIE  BIOLOGISCHE

RRAUCHRARKEIT DER BERGIN-TOLZZUC-
KERTROCKENHEFE, Biochem. Ztschr.
312: [233]-251.

and Soven, O.

1941, UBER DEN WERT DER WUCHSHEFEN FUR
DIE MENSCHLICHE ERNAHRUNG. I. MIT-
TEIGUNG: AMINOSAUREGEHALT VON ER-
GIN-HOMZUCKERTWOCKENHRFE. —Bio-
chem. Ztschr. 309: [829]-336,

Enismann, F.

1901, DIB BROTSURROGATE IN HUNGERSZEITEN
UND IHRE AUSNUTZUNG TM _MENSCR-
LICHEN VERDAUDUNGSKANAL. Ztschr. f.
Biol. 42: [672]-709,



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

(table 23)—Continued
Carbohydrate Het of
‘Pest food, description Water | Protein Fat smo ash | mbUS | Ret, No.
‘otal (by
differenes) | Fiber ram

GRAINS, GRAIN PRODUCTS—Con.

Wheat, Wheat Products (Triticum aesti-
um)—Continued
Flours, whole grain and nearly whole
grain—Continued
108 “Whole meal, milled from Canadian | recen | racn | Poeent | Poeent | Poont | Pore | Colo

wn 1 [F158 40/ 223] C4 2.36 |-- 104
107 Whole meal, milled trom Bn
wheat. 15, 58.52] 1.88 104
108 Whole m 13.50 #1168] 1. 82 28
108 100 percent o} 14.38/710.92] 1.81 146
Flours, intermediate extractions:
112 “Entire wheat”. -| 10.81] 112.26] 2,24 164
113 “Entire wheat” 13.61 /11372) 1.69 168
114 “Entire wheat"? mil
epring wheat, Scotch Fife. 10.99 | 713.00) 2.28 195
115 “Entire wheat” milled from Oregon
white winter wheat. .__-_--_-- 8.66) 1752] 1é67 168
116 “Entire wheat” milled from hard
winter Weissenburg wheat from
Okiahoma._ 7.46) 215.16]; 1.64] 74.52 | ----8S 122] 4.159 168
117 “Entire wheat” milled from Indiana
soft winter wheat...-.... 960]11280] 1.54 1.66] 4,020 166
ole grain before processing) goog] '13 16] 1.52 185 | 4090
118 “Entire wheat” milled from Michi-
gan soft winter wheat... ... wor] !izo01} 163] 74.17 |--- 1.28] 3 860 166
123 90-percent extraction, milled from
inglish wheat _ 15, es - 1 ne es 106
124 90-percent extraction, milled from
Manitoba wheat. > i 106
181 80-percent extraction, milled from
inglish wheat, 15 Les ne 106
132 80-percent extraction, milled from
Manitoba wheat. -| 15 106
Fours, lower extractions:
Standard patent, milled from hard
apring wheat, Scotch Fife____._. 10.54]? 11.99) 1.61] 75.36 |----2-- 0.50 4.050 164
136 Straight patent, milled from hard
spring wheat, Scoteh Fife......_ 12.38 {'13.60| 1.30] 72.04 |... 68 | 3,861 166
137 Straight patent, milled from hard
spring wheat, Scotch Fife__ iss }*1275] 1.43] 78.67 |... 60 | 3.889 195

138 Standard patent, about 70 percent
yield, milled from Oregon white
winter wheat__...-.-.------_- 894] 1690) 1.25] 82.47 }.---2 2 -44| 3.880 168
139 Standard patent, about 70 percent
Yield, milled from hard winter
eissenburg wheat from Okla-

homa___
140 Standard patent, milled from In
ana soft winter wheat_.
141 Standard patent, contained less
than 72 percent wheat kernel
milled from Michigan soft winter
wheat
145 70 percent of wheat kernel, milled”
from a mixture of Girka and
Minnesota wheats
146 Ist patent, milled from hard spring

9.93 | 113.74 . 92 74, 89 |--- 2. . 52 4.040 168
1A. 30 |? 12. 30 -93 | 75.94 J... .53 | 4,010 166

10. 97 | 1 10, 92 +60) 77.15 |-------- 46} 3.799 166

15. 02 | 211,57 81 72,19 |---| Al |-.-----.) 146

wheat, Scotch Fife. 10.55 | 1108} 115] 76. 37 | 4.032 164
uy 10.49 11114] 120] 76 242 | 4.006 164
wheat, Beoteh Bite + 12,36 | 412,44] 162] 73.07 |..-2.--- 251 [eee 170
IN X 5.7. 1 Available carbohydrate 63.60 percent; undetermined
tN XK 6.25. matter (pentosans, etc.) 9,02 percent.
EN X 5.83. ¥ Ttems No, 117'and 140 were prepared from this sample.
¥ Available carbohydrate 55.20 percent; undetermined *N-conversion factor not reported.

matter (pentosans, ete,} 9.78 percent.

102


Physiological Fuel Values of Foods of Animal Origin

For determining physiological fuel values of
foods of animal origin, Atwater’s factors for the dif-
ferent 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 publication (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 dif-
ferent 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 inter-
mediate 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 digesti-
bility, 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 unknown 50
ears ago. For them we have used the digesti-
bility 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 carbo-
hydrate carried over from the milk in which the
fats were blended or churned. The caloric 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 digesti-
bility 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 ap-
proximations 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
believe it covers the bulk of early and recent
research in which apparent digestibility of the test
food was measured. Articles in which apparent
digestibility was not reported or could not be
calouluted for the test food alone were not in-
cluded. By this criterion, digestion experiments
such as those of F. Erismann (52) were excluded.
His coefficient of digestibility for protein has been
quoted by various authors in early publications as
applying to peas, but a translation of the original


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Langworthy and Holmes (93) conducted experi-
ments with the dual purpose of comparing the
digestibilities of the different kinds of sorghums—
kaoliang, feterita, kafir, and milo—and the digesti-
bility 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 kacliang which
grinds more readily than the corneous types. The
portions of the wheat and corn kernels removed
was not stated. Hach of the sorghums was used as
bread or as mush; the corn and wheat as bread
only.

In onc of the kafir bread scries the remainder of
the diet consisted of milk, orange, and sugar.
Otherwise, the diets in the bread series had in addi-
tion to the bread, applesauce, butter, sugar, and in
most cases, potato. In the scries 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 mea] was served as bread or
as mush appeared to have had little influence on its
digestibility. The carbohydrate was well utilized
in all the sorghums; the average digestibility 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.

Tho digestibility 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 digesti-
bility, approximately 50 percent. Milo is a some-
what softer type with a larger proportion of starchy
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 digestibility, slightly less than 20
percent.

In an experiment conducted by Abe and others
(7) 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 per-
cent 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 digesti-
bility was lower in the experiments of Lang-
worthy 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 kaoliang,
are applicable for measuring digestibility in persons
with an entirely different dictary pattern. Further-
more, 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 # satisfactory figure for the apparent
digestibility 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 typical but is not far short
of whole meal. Ifa 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 digestibility of 20 percent, indi-
cated by the work of Langworthy and Holmes for
kaoliang and the usual heat of combustion for
protein in cereals. We recognize the possibility
that for persons accustomed to eating kaoliang,
this factor may be too low. Better digestibility of
protein and carbohydrate was indicated for the
more refined meals of the other sorghums.

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 italica, 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 digest-
ibility 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 algo 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 digest-
ibility. 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 carbo-

35


1. Whereas in most studies investigators assumed di-
gestibility 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 re-
sult, the original figures for digestibility and proportion
of energy available from the test food were in some cases
considerably different from results we obtained by apply-
ing the usual coefficients to the basal diets. If our recal-
culated figures differed from the reported results by more
than i percent, they were entered in table 28, and atten-
tion called to this change by a footnote,

2. In some studies the authors did not report apparent
digestibility 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 cafcula-
tion, we obtained a different result. Our recalculated
values have been entered in brackets in table 23.

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

Weight of | Pr ie Hest of
ee mata] Ru | om | Gite, | am [tthe
Food consumed: Grams Grams Calories
70 Bread (made frorn graham flour) - 389. 0 8.6 a
69 Milk. 167.4 25,7 2, 327
556, 4 34,3 4,420
71 | Feces (water free} 49.6 7 392
Estimated feces from food other than bread. 3.3 |-. - 83
Estimated feces from bread. 3
Total arnount digested. 1. 8
Estimated digestible nutrients 7
Coefficients of digestibility of total food__..
Estimated coefficients of digestibility of bread_
Proportion of energy actually available to body:
Tn total foo:
In bread alone

Nore.—This table appears as table 18 in U. S. Department of Agriculture Bul. 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 composi-
tion 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 deseription of the product.

Quotation marks have been used with cortain food items
to indicate that they were quoted directly from the article.
Thia was done whenever a term might have different con-
notations. For example, entire-wheat flour, as used in
atudies 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.

59


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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 pro-
tein 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 fiours, more than 50 trials on wheats
of intermediate extractions, and over 100 digesti-
bility trials on straight and patent flours. The
average coefficients of digestibility are shown in
teble 14. The variation in digestibility found for
the protein of wheat was much greater than that
for the carbohydrate, and there was greater varia-
tion 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 spring
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 aro inadequate to determine to what extent
variation observed in digestibility within groups
and between groups may have been due to differ-
ences in protein and carbohydrate content of the

ours.

Tanie 14.—Energy values of wheat fours calculated by use of specific energy factors for protein, fat, and

carbohydrate
: Bneeey 0° | ava

| * | Available
Energy value | 20s to be f food
‘Type of four Composition) Soemelent ct | “otasuihle. | Spbtisd 9 ne sete

nutrients .

(col 3X col.) * eel

a) @) @) a) (8) 6)

Fxsentially whole wheat 7 (97-100 percent extraction) : Percent Percent Cal.igm. catigm, | cat s200 gm

Protein (NX 5.83)_ 12.6 ee 3.50 45.
Pat_...---- La, +90 9. 30, 8.37 15.9
Carbohydrate 71.8 90 4, 20 3.78 271. 4
|. - 332, 5
Intermediate extraction (85-98 percent) :? ' ~~
Protein (N X 5.7) 12.0 83 3.78 45,4
Fat. ~ 18 +90 8 37 15.1
Carbohydrate by difference. 73.0 94 3.95 288, 4
Total. 348. 9

Patent? (70-74 percent extraction): |

Protein (N X 5.7) 1.7 80 4, 55 4.05 47.4
Fat 13 +90 9. 30 8.37 10, 9
Carbohydrate by 74.5 98 4, 20 4.12 306.9

Total... -.--- 22-22-22 2 --- 2 ee ee een nef ee eee] eee oon eee ee eee ee 365.

1 Composition data are calculated to a 12-percent
moisture basis.

2The 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
fiour.

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 coefficients of di-
gestibility obtained by using data from all of
the samples of that type. The coefficients based
on all the samples within the group, therefore,
were used for deriving the energy factors shown
in table 14, It is unhkely that the heat of com-
bustion value would be the same for the nitroge-

41.25 calories have been deducted from the heat of
combustion of | gram of protein to correct for loss of
incompletely metabolized products in the urine.

+ Assumed coefficient of apparent digestibility for fat in
plant products, 90 percent, used because actual data for
wheat were unsatisfactory,

nous portion of flours of different extraction rates.
The relative proportions of protein and norprotein
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
was 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.


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some foods cause greater loss than others (104,
106, 147). Other workers, including Murlin and
coworkers (40, 127, 128) and Bricker, Mitchell, and
Kinsman (31), 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 appre-
ciably 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 re-
ported 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 caleula-
tion 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 digesti-
bility of a food was studied. In some experi-
ments 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 co-
efficients 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 ox-
periments 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

coups and for nutrients in a mixed diet were as
follows:

Carl
Food group Protein | Fat arboby:
Percent, Percent Percent
Animal foods - 9 95

Coreals__— - 85 90 98
Legumes, drie : 78 90 97
Sugars and starches... __.._|.-----.-|-.------ 98
Vegetables. 83 90 95
Fruits____ 85 90 90
Vegetable 84 90 97
‘Total food +. 92 95 97

| Weighted by consumption statistics based on a survey
of 185 dietaries,

When these coefficients 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 determina-
tion, 93.3; for fat the calculated value was 94.5
and that found by determination, 95.0; for carbo-
hydrate 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 (/93) 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 similer conclusion was reached by Bryant ($2)
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 per-
cent. As its heat of combustion was about


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84


Protein

Determination of 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
perticular food. The factors used are based on
the nitrogen content of the predominating pro-
tein present in various foods. As a great many
commonly occurring proteins contain approximate-
ly 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 justi-
fy their derivation. Table 3 lists these factors
along with others obtained from him through
personal communication.

TaBie 3.—Faelors for caleulating protein from
nitrogen content of food *

Food Factor ! Food

Plant origin—Con.

6,25 Legumes—Con.
5.55 Beans—Con,
6. 25 |} Soybeans_..| 5.71
6. 38 |} Velvetbeans.| 6. 25
Plant origin: H Peanuts......| 5. 46
Grains and cereals: | Nuts:
Barley... 33 || Almond:
Corn (maize) 25 Brazil. -

Millet 83 Butternuts_
83 Cashew.

95 Chestnuts.

Coconuts_ -
25 Hazelnuts... _
Hickory.
83 Pecans _

31 Pine nuts

REA THAI
we
g

Embryo_- 80 Pistachio-
Endosperm._| 5. 70 Wainuts_ !
Legumes: Seeds: H
Beans: Cantaloup.._.
‘Adzuki 6.25 Cottonseed.”
Castor_ 5. 30 Flaxseed. .____
Jack - 6.25
Lima 6.25 .
Mung- 6.25 :
Navy 6 :

25 | Sunflower _

1 Adapted from table 5 of U. 8, Department of Agricul-
ture Circular 183, revised edition, February 1941 (76) and
from unpublished data obtained by personal communica-
tion 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 repre-
sent crude protein, since as a rule the figures are

4

derived by applying the appropriate factor to the
total nitrogen present. This procedure involves
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
protem may be other nitrogenous compounds,
such as nitrates, nitrites, purine bases, choline,
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 non-
protein 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 (168), 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 nitrogen-
containing 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
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 non-
protein 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 com-
bustion 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 com-
bustion of “proteids’” and “nonproteids” and.
calculated heat of combustion of “protein.” At-
water used the term “proteid” to.cover the true
proteins, and the term “protein” to cover both the



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(156) Suzrwan, H.C.
1902, THE METAROLISM OF NITROGEN, SULPHUR,
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teu. U.&. Dept. Agr. Off. Expt. Stas.
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1911. cHEMIgTRY OF FOOD AND NUTRITION. 355
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(159) ———

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Rubner and Thomas (f81) found the urinary
ealorie-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 then Rubner had observed in earlier
experiments except for the infant on mother’s
mille Sherman (156) reported a series of metab-
olism experiments on very simple diets of crackers
and milk and in some cases butter. In one series
periods of restricted and liberal intakes were alter-
nated. Experimental periods of 3 to 5 days fol-
lowed consecutively, two series for 12 days each
and a third series for 20 days, to provide a better
basis for following and interpreting 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
publications (25, 26). When body material is
metabolized the calorie-nitrogen ratio appears to
be even more variable than- that found for differ-
ent 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 (111, 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 cal-
eulated 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 (64), Rubner (149), Rubner and
Heubner (150), and Tangl (180).

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 avail-
able energy of the whole diet is calculated by the
three procedures outlined on page 8 have been
brought together in table 11. “Phe experiments
selected represent the more extreme conditions on
record as follows: (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
(8) 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 smal] compared with the gross energy

18

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 mitrogen 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 Pods 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, & 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 organie matter than
in the ratio of calories to nitrogen. He found
closer relationship when be related the energy to
carbon but in view of the difficulty in determing
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 urine
and far more readily determined. Benedict’s
suggestion for making the urinary energy deduc-
tion on this basis rather than using the more
variable calorie-nitrogen ratio in estimating avail-
able 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 mainte-
nance 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 onorey 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


TaBLe 20.--Factors for digestibility, heats of combustion, and physiological fuel values of nutrients in food
groups as used in present-day mized diets '

Protein Fat Carbohydrate
prope] 1 |
Classes of food materiais: j Bropoy- Appar- | Heat of | Physio- Fropor- ‘Appar Physio- Propor- Appar Physio-
Von of Heat of ) aysic- | ‘tion of Foat of tion of | AP Heat of | BY:
Haale | fly, Heoetme| Maen | tim | ta. | coma] Mee! | Sota] 4888, | can] Hated
nixed | ity “tase | value | mised | “Soity vatoe | a? | HG value
Percent ' Percent | Catigm., Catlgm.| Percent} Percent | Catjgm.| Cal fom.| Percent | Percent! Calfom.: Cal.jom.
Meats, fish, poultry 31-97 | 4.40] 427 | 36) 95] 9.50 | 9.02 i
Eggs... 7; 97/450) 436! “4| 95) 9.50 | 9.02
Dairy products = 95| 07/440! 4.271 18] 95] 9.25) 8 79
Separated fats. a 9.40 | 8.93222) |
Total food of animal origin...) 68 | 97 942;895| 8| 98(/395| 387
Cereals | 23] 86 930/837] 40) 98/420] 412
Legumes and nuts = $| 3 9.30|837| 3] 97|420| 4.07
‘ogetables. 6 930/837| 9] 93|419| 390
Fruits } 2] 85 9301837} 8] 90} 400} 3.60
Sugars and sirups. ioc aan 32 98) 3.95] 287
Separated fats and oils 9. & 84 |...---|---.]-----_]------
Total food of plant origin... 37| 82/4.37]/3.58| 23] 94|/930|/874| 92] 97/4 10| 3.98
Total food. .---.------ yoo | 91) 440/400] 100; 95[9.39: 892] 100) 07| 409] 3.97

1 Based on United States of America food consump-
tion date, 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 in 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

? 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 types
of food are very different, calories calculated by
the use of the general and specific factors are not
in as good agreement.

General factors may therefore be used for esti-
mating the energy value of average family diets
or of the national food supply of this country
from the total quantity of protein, fat, and carbo-
hydrate. The more specific factors should be
used for most other calculations, such as those for
experimental and therapeutic diets, individual
foods, food supplies of « 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


(53)

(65)

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53


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and Meremt, 1. H.
1904. ENTIRE WHEAT FLOUR. Maine Agr. Expt.
Sta. Bul. 103, pp. [61]-76, illus.

and Measnt, L. H.

1904, STUDIES ON THE DIGESTIBILITY AND
NUTRITIVE VALUE OF BREAD AT THE

MAINE AGRICULTURAL EXPERIMENT STA-

Tron 18w-ia. U, S. Dept. Agr. Off.

Expt. Stas. Bul. 143, 77 pp.

and Swrper, H.
1906, CEREAL BREAKFast Foops. U. S. Dept.
‘Agr. Farmers’ Bul. 249, 36 pp.

57


carbohydrate might not be applicable to foods.

He also reco; nized {het methods of determining
how much of resent in a food were not
entirely Srtanaey umerable 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 ‘hewling were pointed out in 1890
by an ad hoc Committee of the Association of
Official icultural 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 ex-
traction 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 (triglycerides) thus
extracted are other materials “haying simi similar
solubility, such as the sterols, and chlorophyll and
some other pigments. Special precautions are
necessary to insure complete extraction; carbo-
hydrate-containing foods, particularly those high
in starch, present additional problems (61, 66, 108,
184).

A method based on acid hydrolysis before ex-
traction 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 shown to
contain appreciable amounts of fat not extracted
without preliminary hydrolysis (78).

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 assump-
tion that all the fatty acids are present as
triglycerides.

The determination of fat in foods is fraught
with complications. Particular care is necessary
to avoid oxidation of fat during sample prepara-
tion and 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 cther extract
from a food depends on the particular fatiy acids
making up the triglycerides and on the compo-
nents and proportions of the other ether-extract-
able materials present. The triglyecrides of beef,

2

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 triglycorides 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 deter-
mination of the fat content of the food. The
table containing the data which Atwater as-
sembled 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 Leen reproduced here as table 1.

TasLe 1.—Average determined heats of combustion
of fats and oils as assumed factors for fat of dif-
Serent groups of food materials

Heat of combustion per gram
Kind of material nn en
Determinea | ASfumed or
Catories Catories
9.50
Beef “ether extract?’ 9 24
Mutton fat...... 9.51
Mutton, “ 9. 32
Pork fat...-..-.- 9. 50
. 9.13
ard._ 9.59
Cottolene 9. 82
Butterfat 927
Wheat o' 9. 36
Wheat, “et 9. 07
Rye oil. 9. 32
Rye, “ether extract” 9. 20
faize oil.----—~ 9 28
Oats, “ether extract” 8.93
Barley, “ether extract” 9.07
Nut oil (except cocoanut) 949
Olive oil__ 9.47
Cocoanut oil 9. 07
Fat of meat, fish, eggs, ete. -
Fat of dairy products 7
Fat of cereals. _-._ :
Fat of vegetables a1 7

Nors.—This table appears as table 7 in The Avail-
ability 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 called
“total carbohydrate” or “carbohydrate by dif-
ference,” a practice used by Atwater in his food
tables and continued in this country. In addition
to the true carbohydrates, this “difference” frac-
tion may include such compounds as organic acids.


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73


article shows that the coefficient. was applied to
bread made of 50 percent pea meal and 50 percent
rye flour 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 applicable to the
particular item or small food group have been used.

Products of wheat

By far the largest proportion of the digestion
experiments reported have been concerned with
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
Rubner 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, par-
ticularly in the United States and in Great Britain.
For 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 stand-
point of proximate constituents the straight
Ppstent 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 so-
called entire-wheat flour 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 mill-
ing yields from several commercial millings of
cleaned wheat reported in 1941 by Sherwood and
others (160) have been included here as follows:

Percent

Milled fraction:
Patent flour___
First clear flour.

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 closa to 91
percent. Usually the entire-wheat flour was re-
ferred to as being of about 85-percent extraction.
Probably this was a little low. Woods and Mer-
till (194) stated that 100 pounds of cleaned No. 1
wheat would make 85 to 88 pounds of entire-wheat
flour, that the large mills 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.

Snyder (164, 166, 168) also conducted experi-
ments 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 Merrill. The flours of
longer extraction were obtained by removal of
part of the coarser bran 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 infor-
mation it is likely that entire-whoat flour repre-
sented about 90 percent of the cleaned wheat.
The latter figure was also arrived at independently
by a milling expert in the United States Depart-
ment of Agriculture who estimated the probable
extractions of a number of samples of entirce-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 milling
and composition of the flours suggests that the
entire-wheat flour was not always of uniform
extraction, we bave considered it preferable in
this publication to assume that the data applied to
flours within the range of 85- to 93-percent extrac-
tion, Likewise for flours designated as standard
patent we have assumed that the data applied to
flours of 70- to 74-percent extraction. With re-

27


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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 (66) 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
eountry indicate thet the average per capita
consumption (man, woman, and child) of alcoholic
beverages in recent years is approximately equiv-
alent 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 physio-
logical 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 published by Carpenter (85), Mitchell and
on (122), Keys (80), and Klatzkin and others
82).

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-intestinal tract, 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 Emited usefulness in
body metabolism than energy from proteins, fats,
and carbohydrates. In view of the fact that in
estimating calorie values, differences in avail-
ability and efficiency of use of the energy from
these common sources are not considered, it does

20

not seem necessary at present to discount the
energy value of alcohol, particularly since there
is considerable evidence that when intake is
moderate a large part may be available for mus-
cular work and that all of it may serve as a source
of body heat.

The calorie factor for alcohol used in the tables
in this publication is the one proposed by Atwater
and Benedict (15), 6.9 calories per gram, based
on respiration calorimeter studies in which they
found that 98 percent of the heat of combustion
{7.07 calories per gram) was utilized by the human
body in its combined needs for energy in muscular
work, building tissue, and maintaining body
temperature.

Altogether Atwater and Benedict conducted 26
experiments, each lasting 2 to 4 days, in which
they compared the metabolism of man on diets
with and without alcohol. These experiments
were part of the long series already referred to
and are included in table 8.

In three groups of the first experiments, the
periods with end without aleohol 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
sampling for analysis of ventilating 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 other ex-
perunents in the entire series. One difference
was that in this group of experiments all periods
were more than 1 day.

A preliminary digestion experiment of 3 or 4
days preceded each metabolism experiment. Each
subject was on the experimental diet he was to
have in 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 metabo-
lism experiment in the calorimeter. The amounts,
heats of combustion, and composition of food,
feces, and urine were determined.

During the metabolism experiment these deter-
minations were continued and in addition deter-
minations 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


vegetable carbohydrate to correct for the rela-
tively large proportion of sugar. We assumed
that from two-thirds to three-fourths of the
carbohydrate in most of these underground vege-
tables is present as sugar, and one-third to one-
fourth 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.

Coofficients 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 diges-
tibilities of 65 percent for proteim 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 gram for
protein and 3.57 calories per gram for carbo-
hydrate.

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 becn termed “protein nitrogen” (63,
128, 178). Other known constituents are free
amino acids, amides, purines, and ammonia. In
some instances appreciable amounts of urea have
been determined. Iwanoff (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 somo
samples the urea nitrogen approximated half of
the total nitrogen. Mendel (116) suggested that
some of the nitrogen in mushrooms 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 intro-
duces an error in the value for the nitrogenous
matter, but at present there are insufficient data
to provide a better factor. Hence, we have con-
tinued to use the factor 6.25 in calculating total
nitrogenous material in mushrooms, although we
realize that the error involved may be of some
significance.

Urea, as well as some of the other nitrogen-
containing substances, has a lower heat of com-
bustion than protein, but since we could make no

accurate estimate for these substances we have
used the heat of combustion of vegetables, 5.00
calories per gram, for nitrogenous matter in
mushrooms.

Very little work has been noted on the digesti-
bility 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 mushrooms com-
bined with a little butter and seasonings were
fed. A similar result, 72 percent digestibility,
was obtained by Skinner, Peterson, and Steenbock
(161) when mushrooms were fed to albino rats.

A digestibility coefficient of about 70 percent
seems a reasonable estimate and following the
usual procedure of applying it to the heat of com-
bustion, 5.00 calories less 1.25, we derive an
energy factor of 2.62 calories per gram of nitrog-
enous matter.

The carbohydrate fraction of mushrooms also
includes a variety of components, not all of which
have been determined quantitatively.

One of the most complete analyses of carbohy-
drates in mushrooms reported to date was made
by McConnell and Esselen (/09}, 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 carbohy-
drate follow:

Carbohydrates in mushrooms
(Agartews campestris}

Total carbohydrate (by difference)
Mannitol. ..---..-.---.---
Reducing sugars (as dextrose) -
Pentoses, methy! pentose

uronic acids.
Glycogen. -
Crude hemicellulose

Other carbohydrate constituents that have been
reported as occurring in mushrooms include cellu-
lose, 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 digestibility of the carbohy-
drates 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 for
1 day. According to the author a satisfactory
separation of the feces was obtained by the use of
buckwheat flour. The digestibility found for the
carbohydrate was 84 percent

39


APPENDIX. TABULAR SUMMARY OF EXPERIMENTS ON DIGESTI-
BILITY OF FOODS OF PLANT ORIGIN BY HUMAN SUBJECTS

Apparent Digestibility and Available Energy

Scope of compilation.—The compilation of human di-
gestion experiments given in table 23 presente data on the
apparent digestibility of protein, fat, carbohydrate, and
energy, and in some cases the availability of the ‘total
energy of various foods of plant origin. It covere research
in this field since 1875. Data published in languages other
than English may not have been covered completely but the
greater portion is believed to have been reviewed. The
Teports 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 digestibility of fat are shown in
the table. but in many cases are not considered to be reli-
able. With the exception of a few kinds of plant foods,
such 4s nuts, the fat content is too low to contribute more
than a small part of the total fat intake. Thus, in caleu-
lating the digestibility of the fat of the test food even &
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.

‘oo much importance should not be given to the re-
ported 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 sufficiently 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 cal-
culated from data given by the author. A wide variety of
experimental conditions are represented, some of which
were too extreme for derivation of coefficients of digesti-
bility for general use as represented in table 13. However,
they are useful in considering the effects that various con”
ditions 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 tho 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 (56). This order seemed desirable in that
the first two groups, “Grain, Grain Products” and “Le-
gumes 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 similer 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 piant 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 recom-
mended by the Horticultural Crops Research Branch of
the Agricultural Research Service, U. S. Department of
Agriculture.

58

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 un-
digested food in the feces. Using protein as an example,
these coefficients are calculated as follows:

Protein in- Protein (from
take from— test food) in

test food feces Joeflicien z
SE X 100 = 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 eoefi-
cient of digestibility of the various items in the diet. For
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 digestibility of 97 percent.
Then the fecal protein from milk would be 3 percent of 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 digestibility of a mixed
diet during 4 preliminary period and then substituted the
test food for a specified proportion of the mixed dict, it was
assumed that reducing the intake of the basal diet did not
change its digestibility. If the test food replaced 15 per-
cent 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 waa
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:

Coefficient of apparent

Gross energy of test food—fecal energy from test
food—energy lost in urine (digestible protein from test
food X 1,25) available energy from test food.

Available energy X100__ Percent of gross energy available
Gross energy to the body.

Table 22 shows in detail the results of culculations 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 beet
recalculated prior to inclusion in table 23. Differences,
when found, between the results as originally reported and
the recaleulaled figures were of three types:


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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 (764) 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 calo-
rimeter 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 caiculated gross
calories for bread (2,407) were a little 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 agree-
ment with the determined.

Tasir 18.—Summary of steps for checking available energy values calculated by factors from table 18

Available energy Deviation of
Weight of Carbo. | ee
‘Type of data rasteriat Protein Fat bydrate | GrSSenerey | 5, ined | Caleulated | energy from
yetermined - “py'use of | determined
i y ' faetors 1 value
Food consumed in 2-day period:
Bread made from whol Grams Grams Grams Grams Calories Calories Calories Percent
1, 020. 8 73.9 12.9 442.4 A 2, 045. 5 |_
2, 500. 0 75, 2 87,5 127.2 1, 790 1, 582.5 |_
penne eee eee ee lenee eee j_149.1! 100.4] 569.6 | 4,148 | 43,561 | 3, 628
Feees (water-free)_ | 97.0] 176] 102] 621 418 |.
ine. .o.----.---------.|----------|-- Jee e ee e|e--e- eee 1164 |.
wae e eee! 582 |.--.-.- ._|.----2..--].---- 8

1 Energy factors from table 13 applied to the protein,
fat, and carbohydrate: 3.59, 8.37, and 3.78, respeetively,
for bread and 4.27, 8.79, and 3.87 for milk,

2Gross 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 urin-
ary 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 number 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 bal-
ance, 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.

a4

37.9 x difference betweon amount nitrogen in food and
in feces,

Norz.—Data used in this procedure taken from experi
ment 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 carbo-
hydrate shown in table 13 were applied to the
nutrient intake in this experiment, tho 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 digesti-
bility of 90 percent. If the average digestibility
is nearer two-thirds, as indicated in a number of
experiments, the energy factor would be approxi-
mately 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 apply-
ing factors from table 13 to data in 108 digestion
experiments which provided information on com-
position of the foods in the diet and data on bomb
calorimeter determinations of the food and feces.
Although the experimental data needed for using
every factor in table 13 were not provided by these


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acid in the feces of experimental animals, depend-
ing on the protein fed. These and other studies
indicate that among the various kinds of cereals
considerable difference in digestibility and physi-
ological fuel values may be expected. Digestibil-
ity 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.—Coefiicienis of apparent digestibility for

grain products
Protein Carbohydrate
Grain Product Goeth | espert- | C2, | expert
feestious] Monts /eeishity| ments
Cornmeal, whole- Percent | Number | Percent | Number
ground, bolted. ._ 60 3 96 3
Cornmeal, degermed. 76 21 99 21
Oatmeal, rolled oats 76 48 98 24
75 22 98 22
g4) 119 99 119
65 0) 90 0}
67| Q) 92) @)
a qy 95 10]
75 Q) 97 iO}
wheat, 97 — 100
percent extrac-
tion. 79 72 90 72
Intermediate, 85 |
93 percent ex-
tion... ---.---- 83 53 94 83
Straight or patent,
70~ 74 percent
extraction_ 89] 104 98 i 104

| Coefficients of digestibility for rve flours were derived
less directly as explained on pages 33 and 34. Therefore,
the number of experiments is not indicated.

Commeal

The cornmeals studied have been of two gencral
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, re-
moved. In addition to the meals, digestibility
data for several other corn products have been re-
ported. 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

32

was close to 60 percent for protein and 96 percent
for carbohydrate. These values are in line with
digestibility coefficients for ficld corn (pressure-
cooked) and for hulled corn (hominy), which,
judging by their fat and fiber content, must have
contained the germ and more of the branny por-
tion 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 carbo-
hydrate. Hence, the energy factors suggested
and used here for protein and carbohydrate in
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 digestibility of frozen raw
cornmeal and toasted corn breakfast products also
were found in the literature 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 energy fectors 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 digesti-
bility cocfficient of 90 percent for fat in plant
foods introduces very little error in the total
energy value of the food. Oatmeal, which con-
tains 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 digestibility of fat in oatmeal has been reported
or could be calculated from data given, the results
have varied from 56 pereent to complete digesti-
bility. These values were largely from two studies,
@ recent report by McCance and Glaser, 1948
(162), 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 pereent. 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 sirup. The daily
intake of 34 to 67 grams of fat was supplied
entirely by the oatmeal.


TaBLe 6.—Fresh fruits classified as to organic acid content |

a peroent and over 21to3 poreent l

1 to 2 pereont

0.5 to 1 percent Less than 0.5 percent

Lemons (C). Cranberries (C). Apricots (M). Apples {suramer) (M). | Apples (fall) (M).
Limes (C). Currents, red, black, | Carissa or natal plums | Blackberries (C). Apples (winter) (M).
Tamarind (T). and white (C). ). | Blueberries (C). Bananas (M)
Gooseherries (C). Cherries, sour (M). _{ Cherries, sweet (M). | Cherimoya (C).
Grandillas, purple, or | Grapefruit, all (C): | Crab apples (M). Feljoa (C),
passion fruit (C). Groundcherries (in Grapes, pulp or juice, |) Figs (C).
i cluding poha and Jujubes (C).

Loquats (C).

i Nectarines (M),

Oranges (C)

prunes (M).

black (C).

oranges (C).

i cape-gooseberry) (C).
i Kumquats (C),
! Loganberries (C).

Plums, excluding

Pomegranates (C).
' Raspberries, red and

Strawberries (C).
Tangerines, other
Mandarin type

American type, all
T) Limes, sweet (C)

Grapes, European type, | Muskmelons (C).

all (T) Papayas (C).
Guavas (C). Pears, all (C).
Mamey or Mammee | Persimmons, Japanese

apple (C). or Kaki (M)

Mangos (C).
Mulberries, black,
white, and red’(M)

Peaches, all (M).
Pineapples (C).
Plantains (M).
Prunes (M).

Quinces (M).

Persimmons, native
M

(M).
Prickly pears (Mo.
Roseapples (C).
Sapodilla or sapota (C).
Sapote or Marmalade
plum (G).
Sugar apples or
sweetsop (C).
Watermelons (M).

| Total free agli expraseed in terms of the predominating acid as malic (M), anbydrous citric (C), or tartarlo (7) In the edible portion of frutt,

3 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. Cab-
bage, for example, contains only a fraction of a
percent of acid as malic and citric, while sauer-
kraut has around 1.5 percent lactic acid. Simi-
larly 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 gram-
molecular weight data are as follow:

Calories
per gram
3. 488

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, gooseberrie = 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 some
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 un-
certainty.


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46


Foods of animal origin, except the milk prod-
ucts, contain little carbohydrate. Foods of plant,
origin, on the other hand, have a variety of car-
bohydrates. The principal ones are starch, sug-
ars, and cellulose, but appreciable amounts of
pentosans, dextrins, gums, and other carbohy-
drates also may be present. It has been general~
ly assumed that the starches, at least when
cooked, and the monosaccharides and disaccha-
rides are well used by the body. Much less is
known about the utilization of cellulose, pento-
sans, and other of the more complex carbohy-
drates.

“Carbohydrate by difference” has been shown
to be generally satisfactory for estimating energy
values of foods (17). However, for certain pur-
poses, 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, “carbo-
hydrate by difference” minus fiber, may be calcu-
lated. As the digestibility of fiber may be ver
low, nitrogen-free extract is considered a muucl
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
oe vegetables into different carbohydrate groups
2, 87).

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 ex-
pressed as glucose based on analyses in which cop-
per was used. For routine determinations, this
procedure is not entirely satisfactory since the ex-
tent 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 noncarbohy-
drate reducing substances. Improvements have
been made in procedures involving the use of cop~
per 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,
chromatographic separation, and differential spec-
trographic analysis give promise of quantitative
determinations for specific carbohydrates.

Heat of combustion

Atwater assumed that 97 percent of the carbo-
hydrate 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 corn-
starch 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 com-
bustion of pentosans as compared with polyhexoses
might offset the lower heat value of the sugars.

n 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 carbo-

ydrate in fruits.

The main carbohydrate of animal source is milk
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 ap-
preciable 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 gen-
eral dietary calculations.

The table prepared by Atwater summarizing
data on heats of combustion to apply to carbohy-
drate is reproduced here as table 2.

TABLE 2.—Average determined heats of combustion
of different carbohydrates and assumed factors for
carbohydrates of different groups of food materials

‘Heat of combustion per gram

‘Kind of material

Determinea | Assumed or
Catories Calories
Pentoses ? _| 3.72 to 4. 38 |.
Dextrose : 375 |.
Levulose- 2 3.76 |
Cane sugar. : 3.96 |_
Milk sugar : 3.86 |:
Cellulose. 2 420 |-
Starch - : 4.20 |
Dextrin - 4.11 |.
Glycogen. 7 4.19 |.

Carbohydrates of animal
foods, meats, dairy prod-

ucts, ete - 3.90
Carbohydrates of cereals : 420
Carbohydrates of legume: - 4. 20
Sugars. : 3.95

-| 4 20
- 4, 20
: 4,00

1 Apparently includes not only the simple pentoses but
also the pentosans.

Norz.—This table appears as table 8 in The Availability
and Fuel Value of Food Materials (17).

3


For calculating the energy value of the carbo-
hydrate in mushrooms, Watt and Merrill (184)
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 appre-
ciable amounts 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
er 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 levulose
and dextrose, but that starch, cellulose, pento-
sans, 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 digesti-
bility 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 proxi-
mate composition of fresh fruits (88) showed
considerable variation among fruits in the pro-
portions 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 esti-
mates for heats of combustion of carbohydrate in
individual fruits are made.

Attention is called to Jemons 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 differ-
ence) in lemons and lemon juice, we have used
2.75 derived by the following calculations:

40

j 1
‘Grams per’ Calories j Total

Constituent 100 grams ) per gram | calories

Invert sugar.
Citrie acid (anhydrous) - —_-_
Fiber and unknown consti

uents___-_.-------------

Total carbohydrate
(by difference)...

1 gram carbohydrate = 2.75 calories (21.2+7.7).

Since the fiber fraction of lemon juice is very
low and since both invert sugar and citric acid
may be completely utilized, 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 limes are similar to
lemons in carbohydrate constituents we have
applied the same 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 all fruits have
been used.

Miscellaneous foods

Many specific foods have not been studied in
human digestion experiments, as can be seen from
the compilation on digestibility coefficients (ap-
pendix table 23). In many cases when digest-
ibility 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 methods 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 to both chemical composition
and digestibility... Determinations of various ni-
trogen-containing compounds have been made in
a few studies. It appears that from 12 to 23
percent of the totel nitrogen present is in the form
of alkaloids, mainly theobromine and caffeine,
and 1 to 9 percent as ammonia, and that the
remainder, although not clearly identified, may 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


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14


origin as well as a variety of plant products. He
found that when he applied the heats of com-
bustion he had worked out for protein (actually
nitrogenous compounds), fat (as ether extract),
and carbohydrate (usually determined by differ-
ence) to amounts present, the results were in good
agreement with those obtained by bomb calo-
rimeter, 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 deter-
minations now available, we would expect even
better agreement between the gross heats obtained
by calculation and the determined values.

Taste 5.—Comparison of calculated heats of
combustion with results of direct determinations

Number] Average heat of | Caleu-

combustion per | lated

att, | gramofwater- | resuits

Kind of food material vyses in substance | {0 per.
vrizo| Bey | Gar | HE

eter. | Cateu- | “deter.

average | mined | lated | mined

Calories | Caloriea| Percent

Beef __ 55 | 6507 | 6619 | 101.
Beef, canner 7 | 6197 | 6268 | 101.
Mutton. 10 | 7146 | 7316 | 102,
Pork_ 10 | 7835 | 7944 | 101.
Poultry. 5 | 6310 | 6508 | 103.

‘ish _ 3 | 6317 | 6427 | 101.
Eges_ 10 | 7103 | 7160 | 100.
Butter 20 | 8832 | 8918 | 101.
Milk_.. 37 | 5437 | 5413 99,
Breakfasi 3 | 4367 | 4360 | 99.

Bread, crackers, ete-

Corn (maize) meal an
preparations

Rye preparation:

Batley preparati

36 | 4536 | 4513 | 99.
4580 | 4624 | 101.

Vegetables, fresh_
Vegetables, cooked
Vegetables, canned.
Fruits, fresh__
Fruits, canned

Average 276 sample:

2

8

&

<

-

S

BS

&
ORONMPAYSAWDNDWHS AMMOOWH RAL

Note.—This table appears as table 9 in The Availability
and Fuel Value of Food Materials (77). Figures for heat
of combustion are in terms of small or gram calories rather
than large or kilogram calories customarily used for foods.

6

We have compiled ss calorie data for a
number of samples of wheat and flours produced
in this country. For 15 samples of wheat or whole-
wheat flour reported in the literature (164, 166,
168, 188, 194, 195) differences between determined
and calculated gross heats varied from 0.1 percent
to 1.9 percent and averaged only 0.6 percent. For
16 additional samples of wheat flour of varying
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 circum-
stances 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, salicylic, aconi-
tic, 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 suffi-
ciently abundant that they should be determined
separately for estimations of ener values of
those foods, inasmuch as they are distinctly differ-
ent chemically from carbohydrates and their heats
of combustion are lower than for carbohydrates
generally. Total free acid is commonly deter-
mined 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 con-
sidered 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 predomi-
nate 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 smal] amounts in fruits. Of the
fruits listed in table 6 only 7 have been re-
ported 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 pro-
portion 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 tho acid con-
stituents of vegetables, but the amounts in most
vegetables tend to be less than 0.5 percent.



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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 e little larger and sometimes a
little smaller than the change calculated for the
comparable experiments when no alcohol was
included. In these six ups 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

PART lil. DERIVATION OF

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 aceumu-
lated to make possible some revisions and addi-
tions. 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 extrac-
tion and had compiled preliminary material for

otatees. 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
Teviewed are given in appendix tables 23 and 24.
The resulting summary of data on human digesti-
bility 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 col-
umn F in table 10 of his report cited above and
eproduced as table 7, page 10. The figures in
olumns 4, 7, and 10 are the specific factors to be
epplied to the grams of protein (nitrogenous
material), fat (usually ether extract}, and carbo-
hydrate (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 Nico-

corresponding nonalcohol experiment. Under the
conditions of these experiments Atwater and
Benedict concluded that alcohol must have been
used by the body about as efficiently as the nutri-
ents 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 in the body
for internal or muscular work.

CURRENT CALORIE FACTORS

laysen ($0) compared utilization of protein and
dry matter in wheat and rye brans by man with
utilization by sheep and swine. Later Crampton
and others (43) compared man’s use of several
grain products with that of rats, sheep, and swine.
However, there is insufficient 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 economi-
cally and more rapidly.

‘he energy factors shown in table 13 do not
rest on equally reliable 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 previ-
ous dietary pattern. Lack of uniformity was
observed in the experimental procedures used,
including lengths of the preliminary and experi-
mental periods, choice of marker, and the relative

‘oportion of the diet furnished by the test food.
The foods tested, expecially in the early digestion
experiments, were not always adequately de-
scribed, nor was the chemical composition of the
sample always reported. In some cases, reason-
able 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.


9.5 calories per gram, its fuel value was 9.0 calories
per gram (93.5 .95=9.02),

Carbohydrate

The fuel value of carbohydrates was determined
in like manner. For example, cereal carbohy-
hydrate 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 (4.2 .98=4.12).

Protein

For protein (nitrogenous products), in addition
to the use of the coefficient 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

every gram of nitrogen present in the urine there
was sufficient unoxidized matter to yield 7.9 calo-
ries, the equivalent of approximately 1.25 calories
(7.9+-6.25) per gram of available (absorbed) pro-
tein. The Beat of combustion of a gram of ab-
sorbed 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 thia
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 con-
tent of the urine on the assumption that the sub-
jects were in N-equilibrium and that all of the
nonmetabolized part of the available N was re-
covered in the urine. The fuel value, 4.40 calo-
ries, would then be applied to each gram of protein
available as a source of fuel.

TABLE 7.--Factors for heats of combustion and fuel values of nutrients in different groups of food materials
and in mized diet

Fuel value
tiated by | Hoot of com- | Poet | Totst energy
Kind of food material each group | bustion per | nutrient | Per sama Per
ber 100 grams | gram Actually | nutrients sralchie Per gram total nutrients
ao
A B c -@10 ay F F revised ?
Protein
Grams Cateries Calories Calories Calories
- 43.0 5. 65 0. 97 5. 50 40 4.25 4, 27
- 60 5.75 -97 5. 60 4 50 4. 35 4.37
- 12.0 5.65 +97 4, 50 4.40 4,25 4, 27
- 61.0 5.65 .97 5. 50 4. 40 4,25 4, 27
- 310 5. 80 . 85 495 4. 55, 3.70 3.87
Legumes_ - 20 5.70 +78 4.45 4,45 3.20 3.47
Vegetables_. + 55 5, 00 - 83 415 3.75 2. 90 3.11
Fruits___. - 5 5. 20 85 4. 40 3.95 3.15 3. 36
Vegetable food - 39.0 5.65 - 85 4. 80 4 40 3.55 3.74
Total food__ 100.0 5.65 - 92 6. 20 440 4.00 4.05
Meat and eggs. - 60.0 9. 50 ~ 95 9. 00 9. 50 9. 00 9. 03
Dairy products. 32.0 9. 25 -95 8. 80 9, 25 8. 80 8.79
Animal food. 92.0 j 9.40 - 95 8.95 9. 40 8.95 8. 93
Vegetable food. 808 9. 30 - 90 8.35 9. 30 8.35 8. 37
Total food... _ - 100. 0 | 9. 40 95 8. 90 | 9. 40 8. 90 8. 93
Carbohydrates

Animal food_ wee! 5.0 3. 90 - 98 | 3. 80 3. 90 3. 80. 3. 82
Cereals. 55. 0 420; 98 4:10 4. 20 4,10 411
Legumes_ 10 4. 20 97 4.05 4, 20 405 4,07
Vegetables. 13.0 4, 20 95 4,00 420 4.00 3. 99
Fruits. - 5.0 400 . 90 3.60 4. 00. 3, 60 3. 60
Sugars. - 240 | 3.95 - 98 3.85. 3.95 3.85 3.87
Vegetable food. 95.0 4.15 97 | 4.00 415 400 4,03
Total food___ 100. 0 4.15 97 4.00 | 4.15 4. 00 4.03

1 Values for fats and carbohydrates, same as corre-
sponding values in eolumn B. Values for protein, same as
corresponding values in column B minus 1.25.

Values for fats and carbohydrates, same as corre-
sponding values in eolumn D. Values for protein, same as
eerresponding values in column D minus [.25.

‘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.)

Nore.—This table appoars as table 10 in The Availa-
bility and Fuel Value of Food Materials (17) with the
exception of column F, revised. ‘The figures in this colomn
appear in tabular form in Investigations on the Nutrition
of Man in the United States (98, p. 18).


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 (1/04). 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

er gram (5.65 less 1.3 calories per gram for urinary
loss) used by Sherman (£48) for the heat of com-
bustion of protein in a mixed diet, The apparent
digestibility of protein was found from the experi-
ment 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 apply-
ing 58 percent, tho coefficient of apparent digesti-
bikt 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 proteip 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 5.5
ealories 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 con-
sidered usefully available. Provision is made for
excluding urinary calories, a loss resulting from
incomplete combustion of protein, but no deduc-
tion is made for nitrogenous matter in feces; like-
wise no deduction is made for fecal fat. The
fraction of the carbohydrate measured is con-
sidered 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 ener;
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, 104, 108),
because, as he has pointed out (107), it is unusual
to make allowance for losses in the feces in pre-
senting data on composition of foods.

Physiological calories based on data from the
experiment with the two whole-wheat flours 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 loss
(4.35 Xgrams of protein content in the wheat).

Data reported by McCance and Walsham from
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 | English
Gross ealories: Caloricr | Catories
Food (bomb). 372 35
Feeal (bomb)_ =45 40
Urinary loss (
woeceee -7 -8
Available calories (from above)__.----| 310 302
Available calories celeulated by
MeCance procedure_...--.-------- 209 304
Physiological calories calculated by
MeCance procedure_._....-.-.----| 320 320

Additional calculations of energy values.—
Using data from the same study, McCance and
Walsham also made two additional energy cal-
culations—one attributed to the Atwater pro-
cedure 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 pro-
tein, fat, and carbohydrate of a mixed diet to a
specific food is not a procedure used by Atwater.
Moreover, McCance and Walsham de not report
values for ash content and thus it is not possible
to determine the tota] 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 dis-
crepancies 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 calcula-
tion.

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 ag 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 b
difference, and available carbohydrate (starcl
and sugar); (2) bomb calorimeter determinations
of the foods and excreta. Such an evaluation, if


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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 esti-
mating energy values. In column E of their table
they listed the factors to apply to a gram of avail-
able 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 avail-
able in a mixed diet. The factors in column E
were therefore to be applied to absorbed nutrients.

The fuel value factors listed in column F in-
cluded a correction for digestibility 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
policy was followed with respect to the factors
used to estimate energy values of foods (6, 8, 10,
18, 19, 20, 68, 89, 187, 169, 171). For a period of
time the Atwater and Bryant general factors ap-
peared in the literature as 4, 8.9, 4; then a refer-
ence to a further rounding of the factors to 4, 9, 4
was made in the 1910 revision of Farmers’ Bul-~
letin 142 (/1). The 4, 9, 4 factors later came into
widespresd usage in estimating calorie values of
food and not ouly 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 diges-
tion 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 0.97 X 5.65, or 5.48.

Since only 0.97 gram is available from each gram
of ingested protein, only 0.971.25 or 1.21 cal-
ories 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 * of the report and have been included.
as column F revised here in table 7. The cor-
rected 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 advis-
ability of applying 7.9, the calorie-nitrogen ratio
in urine published by Atwater (12, 17), to ener,
calculations for which dietary conditions may be
greatly different. Lusk (107) summarized data
showing that the ratio was affected by the propor-
tion of dietary protein, fat, and carbohydrate,
Other questions have been raised concerning the
effect of negative or positive nitrogen balance,
and of high-feuit 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 Agricultura] Experiment Station report for
that year (12,. p. 167), stated, ‘ .. . the heat of
combustion of the water-free substance of the
urine 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 labora-
tory, in which the heat of combustion of the
water-free substance of the urine was determined.”

At the time this statement was published, resulta
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 nitro-
gen when calculated for these 16 experiments to
average 7.9 calories, or the equivalent of 1.26 cal-
eI per gram of absorbed protein (7.9-+-6.25=
1,26).

The study that included the 55 metabolism
experiments was made at Middletown, Conn.,
during the years 1896-1902 under the auspices
of the U. 8. Department of Agriculture in coopera-
tion with the Storrs (Conpn.) Agricultural Experi-
ment Station and Wesleyan University. The
subjects were normal healthy men of similar
weight, around 65 to 79 kg.

2 A note on one of the marked copies on file in U. S.
Department of Agriculture reads, “A copy showing cor-
rections a made on slips seat to Magnus Levy in leiier'of
July 8, 1904.”


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98


TaBLe 24.—Composition and heat of combustion of food items used in experiments on human digestibility

(table 23)—Continued

Carbohydrate 1 Heat of
‘Test food, description Water | Protein a Asn | Gomes | Ref. No,
Fa O38 | winer gram
GRAINS, GRAIN PRODUCTS—Con.
Wheat, Wheat Products—Con.
Flours, lower extractions—Con,
149 Patent, baker’s grade, milled from | Percent | Percent | Percent | Percent | Percent | Percent
Scotch Fife. -| 8 01 | #15. 50 4 75 170
151 Patent. =} 11.07 | #12. 75 190) 74.84 14 +44 97
163 30 percent. of wheat kernel, milled
from # mixture of Odessa, Cat
fornia and English wheats______ 14.63 | 7891 £96 | 75.18 }.---..-. 282 | ee 146
Flours, other:
164 Semolina, milled from durum
wheat, Kubanka. ---} 10.77 |? 11. 64 L27 | 75.56 16 168
(Whole grain before processing). | 876 |?1237] 207) 74.92 1, 88
165 Semolina,® milied from durum
wheat, Kubanka. _ 7. 57 |? 11.57 0.89 | 79,06 |----..-- 0.91 168
(Whole grain befor } 10.48 |21245| 248) 72.92] 283) 1.67 |_..-.-_-
Flour mixtures: ,
173 “Bran flour,” 8 mixture of 86 per-
cent straight grade and 14 percent
very finely ground bran from hard
Weissenburg wheat, -| 969]11296 | 1.48] 73. 62 1.25] 3. 876 168
174 “Germ flour,” a mixture of 93 per-
cent straight grade flour and 7
percent finely ground germ milled
From hard ‘winter Welssenburg
1.66 | 72.97 |....-.-- 87 | 3.962 168
475 1.49 4
179 1. 89 1
180 1.47 1.
182 1.35 1.
184 1.94 1.
185 2.27 1
187 2.12 L
189 1.37 L
190 1.35 1
Refined:
192 Endosperm, granulated. -| 9.35 | 110. 54 85 - 50 129
193 Endosperm, granulated. 7) 1268 |?11.81 | 240 - 65 126
194 Endosperm, farina | 10.55 | 1970) 1.36 - 42 62
195 Endosperm, farina. | 1058] 1918] 110 154 82
196 Endosperm, farina. -| 11,37 | *13.03 TT . 56 97
Wheat breakfast food fs
198 Wheat and barley malt_._.-..._-_- 6.47 | 111.63 77 2.17 9
199 Wheat, whole grain and barley
mait, “Force”. .__..---.-..----- 10.86] #9.86] 1.65 2.86 | 3. 822 9
200 Wheat, whole grain and barley
malt mixture, “Force”... 7.37, 1981) 218 2.40 62
201 Whest and barley malt, “Malta
Vita” ---| 11.32 ]11220] 152 2.93) 3.841 9
202 Branny portion, fraction of 73-9:
percent 2 _ | 147 | 157 42 43 [2222 2- 30
203 Branny portion, fraction of 82-95
percent 3___ 22. _ eee. 14.3 | *15.0 44 5.0 jr------- 30
LEGUMES AND NUTS
Beans, dry (Phaseolus vulgaris): i H
210 Common white, navy beans_. 11.21 [718.25] 1.63) 64.89 4.02 | 3. 885 184
211 Common white, navy beans — i19;?2069! 158! 62.54 4.00} 3922 184

25,
1 Item No. 164 was prepared from this sample.
% 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.

1 Ttems No. 165 and 187 were prepared from this sample.

2 Estimated by authors of article.

11 The wheat was milled to 73 percent and the branny
fraction obtained from the remaining portion.
original grain with a 5 percent loss from cleaning.

*N-conversion factor tot reported.

Refers to

103


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72


TasLy 9.—Daily food intake in the experiments from which Atwater originally obtained the calorie-nitrogen
ratio of 7.9 for urine

Metaboitsm experments | Digestion experiments

| Respiration experiments
Food iter |
| 1

[?ley*

‘ 1

Grama

Grams

Grams
96

Bread, rye.
Crackers, rail
Oatmeal.-.

Wheat breakfast food
Sugar____.

Beaus, bake:
Potatoes, boiled in skins_
Apples

Peaches.
Pears, canned
Alcohol...

Grame

Crome! Grama| Grams | Grama; Grams

150 421 100 5170
- 25

Grams | Grama
10

10 |169

Grams
18

5 107 52 |l44
850 775 850 [575

ga | ad | 75 P15 35

1 Includes experiments 4P, 4A, 4B, 4C, and 48. (See table 8.)

Jn 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 experi-
ments 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 scareely 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 already 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. Bene-
dict 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 experi-
ments 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

16

calories and in the levels of protein, fat, and carbo-
hydrate. The urinary calorie-nitrogen ratio for
these experiments varied from 6.44 to 10.36. Both
extremes were within those observed for experi-
ments 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 (148) 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 (152) he reported
that the ratio was between 7 and 8, although he
had found variations outside this range. Among
othe * problems Rubner (/48) studied the influence
of level of fat, single foods, and periods of rapid
growth on cvaiv..e-nitrogen ratio of the urine and
summarized the results as follows:

|Calories per] Duratlon of
Food gram N | experiment

Days

Mother's milk__. 12.10 7
Cow's milk, infants 6. 93 7
Cow's milk, aduits_ i 771 7
Diet poor in fat. | gert 2

Boo | 933 4
Diet rich in fat_ 8 87 2

Do. & 44 4
Boys’ mi i | 6,42 4
Boys’ mixed diet 7. 50 4
Meat__ 7. 69 1
Potatoes. 7.85 i



Composition and Heat of Combustion of Foods

Composition and heat of combustion dats are recorded
in table 24 for test foods used in digestion experiments
deseribed 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 ame item number in both tables 23
and 24, The composition data are recorded aa reported
by the authors and the factors they used for converting
nitrogen to protein are noted in footnotes. More complete
deseription of the food items is given in table 23.

Tan.e 24.—Composition and heat of combustion of food items used in experiments on human digestibility

(table 28
Carbohydrate Heat of
Test food, deseription Water | Protein Fat Ash | qmbor | Ret. No.
GAGS | Fiber eam
GRAINS, GRAIN PRODUCTS
Barley Products (Hordeum vulgare) Percent | Percent | Percent | Per Percent
1 Barley, flaked.......-_- | o-79| 1887] 1.24 | yo 221 0.77 62
2 Barley, germinated, flaked -} 10,18 |! 10. 62 121 76. 89 1.27 62
Buckwheat ‘Products (Fagopyrum escus
11.34) 7813 1,22 | 77. 69 |..------ 1, 62 |__-.-.-. 134
Corn, Corn Products (Zea mays)
8 Hominy. 10. 96 ¥9, 44 . 67 | 78. 61 . 37 +82! 3, 986 120
9 (Hominy)
steamed_ - oO 79.96 5. 30 76.77 1,61 121
5. . .
5.
35 . 62
14
120
15 Mesl, waxy variety
ported from China. 67
16 Meal, white. 97
20 Corn’ endosperm, toasted, adde
sugar and galt__---- = 7.49 | 77.38 126
Oats, Oat Products (Avena sativa’
25 Rolled oats. 8.66 |? 14, 69 164
27 Rolled oats 11. 02 | 115. 69 126
28 Rolled oats. 7. 90 |? £5, 10 |.. 88
32° Rolled oats, quick-cooking. 7. 36 | 216.13 129
33 Meal, granulated or pinhead. 751 | 112,43 62
34 Meal.._.. 7.35 | 113.17 62
35 Meal, rolled. 7.45 | 112.21 62
38 Mi 8.12 | 113.25 62
8. Coarsely ground pinhead. is. [11.70 102
b. Medium ground. 15. 78 60
Average_____ 15. 10.10
Rice, Rice Producta (Oryza sativa)
49 Polished or white. (12, 92)) #8. 55 167
50 _Undescribed_ 3.50 | *8 27 144
Rye, Rye Producta
54 Flour No. 18. 9.25 | *15. 48 2.01 2. - 141
55 Flour III, a comercial flo 11.40 | *14, 92 1. 89 1. 141
56 Flour No. 17. 10. 96 | *15. 78 1. 89 1 - 141
57 Flour No. 9. 30 | *15. 67 1.78 1. - 141
58 Flour No. 10.06 | *16. 14 1.76 1 - 141
59 Flour No. 11. 00 | *15. 38 L7 1, - 141
60 Flour I__ 11. 26 | *15. 33 1,89 1 - 441
tN X 5.7, tN X 5.95,
2N X 6.25. *N-conversion factor not reported.
tN X 5.83.

100


balance, the potential energy of food and unoxi-
dized excreta, the kinetic energy of heat given off,
and the external work performed.

Each subject had a rather simple diet consist-
ing 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, un-
dressing, 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 rodea sta-
tionary 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, in-
cluding 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 digesti-
bility of protein was a little larger in the experi-
ments 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 differ-
ence was taken as the amount of alcohol oxidized
in the body. Previous research by another
worker had indicated that alcohol was not ex-
ereted by way of the intestine even when con-
siderable 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 com-
pared 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 utine
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 trans-
formed completely into kinetic energy and ap-
peared either as heat or as muscular work, or both.

Atwater and Benedict made some deductions
concerning the protecting effect of alcohol on
body inaterial, 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 glyco-
gen stores for each individual at the beginning and
end of the experiment were the same. hoy
found some gains and some losses of body fat on
either kind of diet but on the average there was
again. 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. Com-
parisons made between the ordinary and alcohol
periods indicated that alcohol was slightly inferior
to carbohydrate or fat in protecting body protein;
that is, a Jarger average daily loss, 3.9 grams, of
body protein occurred tn the alcohol periods then
the average loss, 3.5 grams per day, for the ordi-
nary 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 experi-
ments (totaling 15 balances on 2 men) in which
the alcohol and nonalcobol 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 urinar:
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 not effect of alcohol on gain
or loss of body protein and fat in terms of totel
energy change is shown in column 13. The last
twvo 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 measurements
of the heat given off by the body plus the heat
equivalent of the muscular work performed (in
the work experiments).

21


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eliminating this factor. In other experiments
there was more variety in the diet but on the
whole the diets were simple. Results indicate no
appreciable differences in digestibility when sub-
jects had rye bread alone or as part of simple
mixed diets.

Rye flours have been described as dark, whole-
grain meal, medium, and light, and the following
data on their composition have been cited as
being representative for these products (100, 185):

whok

H ole-
Constituent { Dark grain Medium] Light
mh
|

Percent | Percent | Percent | Percent
11.0] 11.0) 11.0 11.0
1.4 94
1.7 Lo
11 (7
1.0 4
74.8) 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, Jight rye has
been described as ranging from white to medium-
light flour with a comparatively wide range in

roximate composition. In these circumstances,
it was difficult to determine the type of flour used
in the early digestibility studies. However, there
is & 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 ia 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
telationship of the digestibilities of either protein
ér 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 inchide intermediate
extractions of rye and wheat flours, digestibility
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 coeffi-
cients 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, 3.28, and 3.41

34

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 digestibility data are available.
Among 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), Maine, and
Minnesota Agricultura] Experiment Stations.

For pearled and flaked barleys, Woods and
Snyder applied their estimated coefficients of
digestibility of barley products, 78 percent for
proteim and 94 percent for carbohydrate. These
products had undergone some refinement and in
chemical composition were much like modern
pearled barley (18). The energy factors for

earled barley, 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 buckwheat flour, “farina,” 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 buck-
wheat flour. The above coefficients of digestibility
applied to tho 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 car-
bohydrate.

For dark buckwheat flour, we have found no
experimental work on digestibility. The figures
in table 13 have been calculated arbitrarily from
data for light buckwheat flour, assuming that the
ratio of digestibility 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 cnergy 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.



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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 parti-
tion of nitrogen in foods between true protein and
other forms. Although this procedure may re-
sult in an appreciable error in the caloric 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
nitrogen (mostly vegetables and fruits) contain
relatively small amounts of total nitrogen.

TarLe 4.—Average determined heats of combustion
of proteids and nonproteids and calculated heat of
combustion of protein

Heat of combustion per gram

Rind of material

Determined Assumed or
Calories Calories
Beef, fat-froc muscle ._.. i
Beef, fat-free muscle, extract- :
ives removed__. - 5.78
Veal, fat-free musele__- 5.6!
Mutton, fat-free muscle 5.

Protein of meat.
Egg albumin_
Egg, protein of yolk
Vitellin.
Protein of egg
Milk casei
Milk protein
Protein of dairy
Gliadin_
Glutenia

products.

Protein of cereals (96% pro-
teids). an
Protein of legumes (96% pro:
ids) -

‘0

proteids)... 00.2202. 222) eee ee eee 5,00
Protein of fruits (70% pro- |
teids) . 5.20

pt
teids of animal foods__
Asparagin, as type of n
protelds of vegetable foods.

Note.—This table appears as table 6 in The Availability
and Fuel Value of Food Materials (17).

The effect of method of calculation on estimated
energy values for the nitrogenous compounds in a

food.can be shown by using potatoes as an ex-
ample—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 nitro-~
en should be 28.2 calories and the heat of com-
ustion 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. protein 5.8 cal./em.=21.75 calories

.49 calories

28.24 calories

5.02 calories

§.63 gm. nitrogenous compound:
1.0 gm. nitrogenous compounds:

Tf, however, all of the nitrogen is assumed to be
protein (6.25 gm. protein) and to this is applied
the factor 5.02 calories (corrected as shown above
for the lower heat of combustion for the nonpro-
tein portion}, an energy value of 31.4 calories per
gram nitrogen results (1 X 6.25 X 5.02). This
result is about 11 percent higher than that ob-
tained in the first calculation because the content
of protein is overestimated. If no correction is
made for the presence of nonprotcin nitrogenous
compounds and if the higher heat of combustion
of potato protein, 5.8 ealorics per gram, is applied,
an energy value equivalent to 36.25 calories per

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

his result is nearly 30 percent higher than the
first caleulation because there has been overesti-
mation ia both the content of protein and the heat
of combustion of the nonprotein fraction. This
illustration shows that we should have data on
the actual partition products, but until we do, it
seems best to continue the rather arbitrary pro-
cedure shown here as the second calculation,
namely, to apply a weighted calorie factor to total
nitrogen treated as protein,

Determined versus calculated gross
energy values of foods

Gross energy may be determined directly by
burning a sample of food, or it may be calculated
by applying previously determined heats of com-
bustion to composition data on the energy-yielding
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 in
preceding paragraphs, and in view of the assump-
tions made in deriving heat. of combustion values
to apply to each fraction, Atwater recognized the
importance of checking the gross energy values
calculated for foods. He compared results for
calculated and determined gross heats of combus-
tion for 276 samples including foods of animal

5


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94


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 ill effects—his excellent
physical condition at the end of the period was
confirmed by four physicians. The apparent
digestibility of the potato protein in this case
averaged 84 percent.

In a 10-day experiment Rose and Cooper (142)
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 pro-
vided a relatively low nitrogen intake, 4.8 grams
per day. The apparent digestibility was 74
percent,

The detailed analyses reported on the nitro-
genous fractions of potatoes by Street, Kenyon
and Watson, 1946 (178), Crook and Watson,
1948 (44), Neuberger and Sanger, 1942 (180),
and Headden, 1927 (65), indicated that most of
the nitrogenous matter in potatoes would be
available. The proportions of the different nitro-
genous 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 frac-
tion, 2 to 6 percent has been determined as
ammonia and nitrate nitrogen,

Of more practical significance than nitrogenous
compounds in estimati energy values for
potatoes are data on digestibility and composition
of the carbohydrate fractions, Results from six
studies reporting digestibility of total carbohy-
drate ranged from 92 to more than 99 percent,
with an average of 96 percent. These values
appear reasonable in view of the high proportion

starch, sugar, and dextrin in potatoes, Also

present are lignin, cellulose, pentosans, pectins,
and other hemicelluloses, but the complex carbo-
hydrate constitutents of doubtful availability
appear to make up only about 1 to 3 percent of
the potato. Sugars have been found in varying
amounts, from less than 1 to as much as 6 percent,
The quantities of dextrin present are small.
Starch makes up nearly all of the remaining carbo-
hydrate.

In a recent compilation the following average
data were obtained: Carbohydrate by difference,
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 carbo-
hydrate constituents not determined as crude
fiber by the Weende method. It has been shown
by Remy (189), Williams and Olmsted (787), and.
Weinstock and Benham (1/86) 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 of 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 calorimeter for
the same samples. Data from protein, fat, and
carbohydrate analysis and from the bomb calo-
rimeter were taken from a study of Bryant and
Milner (83). Close agreement between the deter-
mined 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, respec-
tively.

Tasie 17.—Comparison of determined and caleulated gross energy values of potatoes

th Heat of combustion per 100
Composition rest
Sample number Caleatated
siculated *
Water Protein Fat eens’) Biber Ash Bomb ealo- | “trom com-
position
Pereent Percent Percent Percent Percent Cotories Catories
2. 0. 17.4 04 0.8 84. 85.
- . 2.3 al 18.4 J... - 9 90.0 89.7
- . Lg 3 11 78. 2 TTA

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 legumes
to immature shelled beans, peas, and other
legumes, those for fruit to rhubarp and tomatoes,

38

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 adjustment in the
heat of combustion factor generally used for


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

le 23)—Continued
Carbohydrate Heat of
Test food, description Water | Protein Fat ash | comes: | Ref, No,
Sa 3 | Fiber gram.
GRAINS, GRAIN PRODUCTS—Con.
Rye, Rye Products (Secale cereale) —Con.
Flour, whole grain, most external | Percent | Percent | Percent | Percent | Percent | Percent | Calorice
layer removed. 13.0 | # (8 40)/---.---.|-------- 5 (2, 03) 21, 62 |___.-. | 99
(Whole-grain Swiss rye bi
‘i (13.0) | ® (& 28)). 002-2 fe lee 6(1,65)| (1. 61)/...-2..-
64 Flour,
blended in equal amounts........| 9.16 | ¥12. 42 1.30] 75,72 |..-...-. 1.40 |... 2-2. i41
65 Flour, finely ground flour com-
mobly used in Wurzburg... --.- w6 | #938 |. |e fle 1.24 |... 2 99
66 Flour, flours 0 and III blended in
equal amounts _ 11. 32 | *10. 76 1. 38 75. 84 |. --- 1.20 |----__.-! 141
71 Flour, made by Steinmetz process
from rye of Bilesia 13.0) | # (9. 629/29 fe * 87} 38)|------_ 99
(Whole-grain rye of Bilesia before
processiny fe iy
72 Flour, mat
milling from 99
73 Flour No. 13_ M41
74 Flour No, 12_ 11
75 Flour, 75 percent yield_ 99
77 Flour I... Mi
78 Flour No. 10. 141
79 Flour No. 2. 141
80 Flour No. 7. 141
81 Flour No. 6_ 141
82 Flour, 62 percent yield. 99
83 Flour 0__ 141
84 Flour No. 1_ 141
85
30
86
percent 4, 30
87 Branny portion, fraction of 85-95
percent & -| 14.2 | “16,2 3.6 61.1 75 4.9 |-----7 30
Wheat, Wheat Products (Triticum aesti-
‘oum):
Flours, whole grain and nearly whole
grain:
92° Graham, 100-percent extraction,
Scotch Fife, hard spring _ - 8 61 | 112. 65 2.44 1,72 4,148 164
{Whole grain before processing) - 850 | 112. 65 2, 36 1.80 | 4,140
93 Graham, 100-pereent extraction,
Scotch Fife. -__..._..- 13.21 | 114.21 2.01 2.01 3.971 186
(Whole grain before processing)" -| 10.41 | 215, 50 2. 28 1.93 4. 023
94 Graham, 100-percent extraction. 10. 51 | 1 14. 00 2. 52 2.00 4, 004 195
96 Graham, milled from Oregon white |
winter wheat__-----.-- 815] 18.18 1. 68 172 3. 990 168
(Whole grain before processing)". 8.99 18.32 1.83 176 4, 008
97 Graham, milled from hard wioter
Weissenburg wheat from Okla-
7.73 | 115. 33 1.79 | 73. 83 1,32 | 4.178 168
(Whole grain before processing). 8.65 | 115. 33. 1. 83 72. 87 1.32 4.110
98 Graham, milled from Michigan soft
winter wheat _. 112. 24 1.41 1.85 3. 906 166
(Whole grain bef z 112.34 1.36 1.83 4, 000
102 Graham. _..---_------------- *10. 63 LZ. . 25 o7
IN X 57. from this sample. Also from this wheat were 2 prepared
7N X 6.25. items No. 94, 114, and 137 appearing in another report.
tN X 5.83. © Items No. 93, 113, and 13@ were prepared from this
5 Reported as cellulose. sample.

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

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

* The rye was milled to 67 percent and the branny frac-
tion. obtained from the remaining portion. Refers to
original grain with 5 percent loss from cleaning.

¥Ttems No. 92, 112, 135, 146, and 147 were prepared

ms
1 Items No. 96 and 115 were prepared from this sample.
4 Items No. 97, 116, 139, 173 and 174 were prepared
from this sample.
38 Items No. 98, 118, and 141 were prepared from this

sample.
*N-conversion factor not reported.

101


which the proportions 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 known 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 mn Agriculture Handbook No.
8 (185), are shown in the sample calculation below.

Sample calculation of energy factors for food mixtures (baking powder biscuits)

| Protein

Carbohydrate (hy difference)

‘Kind of data Weight Specific Specific Specifi
z B specific |
weigne | ‘chery | Ea | welgnt / chergy | Pastey | weigns | ‘enemy | Energy
Ingredients: Gm, Gm, Catigm. Cat. Gm. Cetjom. {Cat om. Cal fgm. Cat,
336| 363] 405] 147.02] 30) 837] 2511 | 255.0) 4 12 |1,050. 60
55 |. . 55.0] 8 84 | 486.20 |. wenees
244 12] 879: 176 | 1k 4] 387

Weighted factor (per

653 | 44.8 |. -- - 183. 32 58. 2 I -------- 513. 07 267.4 |_-..--..
|
| weep 882 ff 4.41 |e.
i 4
! | eon 267.4 |---|.
i 8 82 93. 49 52. 2 4.41 214. 54

The factors 4.09, 8.82, and 4.11 calories per
gram were applied to the protein, fat, and carbo-
hydrate values of the baked biscuits, with the
resultent calorie value 342 calories per 100 grams.

PART IV. APPLICATION

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 avail-
able 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 fecel 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. When 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 satis-
factory for estimating available energy from the
different nutrients. As a result of changes in
method, however, no big changes in actual total

Similar calculations were made for the other units
of weight given for this item in tables 2 and 3 of
Handbook No. 8.

OF CALORIE FACTORS

available calorie values for the foods are antici-
pated.

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 metabolic 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 literature 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