NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
THE DIGESTION AND UTILIZATION OF CRUDE FIBRE
159
anus, the abnormal conditions set up prevent the digestibility coefficients obtained
from being altogether reliable (cf. also 41,78).
From our knowledge of the nutrition of wild birds, their ability to digest fibre
must be regarded as highly probable. Further, the two long and capacious caeca
indicate the chief locus of fibre digestion and the nature of the caecal faeces, which
are always excreted separately from the intestinal faeces, and in the Scottish
grouse (124) as well as in the fowl (139, 81) and the goose (Heinroth), form a pasty,
evil smelling mass, gives a similar indication. By comparison of chemical analyses
of caecal and intestinal faeces, and by metabolism experiments on normal fowls and
fowls after operative removal of the caeca, Mangold (81) with Roseler (99) and
Radeff (97) showed that the caeca are the chief sites of fibre digestion, and this was
confirmed with Meyer (90) by microscopic examination of the caecal contents. Ex-
tending these studies, Mangold and his collaborators, Henning (46), Bruggemann (11),
Stotz (128) and Peters (94) in this Institute, determined the coefficient of digesti-
bility for fowls of fibre in many feedstuffs, using a modification of the Weende
method. In these studies the new method of Stotz (127) was used for the chemical
separation of urine from the faeces excreted with it.
From these investigations it appears that the digestibility of fibre by poultry is so
high for many feedstuffs that it must be taken into account in calculating the value
of a ration. For barley, maize, oats, peas and cabbage, digestibility coefficients for
fowls, pigeons, geese and ducks of over 20 per cent., and for maize and barely as high
as over 30 per cent. were found.
It also appeared, however, that the digestibility of fibre in different varieties of
the same cereal varied greatly. In the experiments of Stotz and Bruggemann
(128), who in each experiment used several birds, usually Rhode Island hens, the
differences between different varieties were so great that the values varied, e.g., for
barley, between 0 and 31.5 per cent. For oats, Halnan (37) also found a variation
in fibre digestibility by Leghorns from 0-6.6 per cent. Errors in technique can
scarcely be responsible for these results since the experiments were carried out with
the same methods in the same Institute. It should also be noted that other nutri-
tive constituent of the ten varieties of barley studied by Stotz and Bruggemann
showed a general uniformity in their coefficients of digestibility.
It may therefore be taken as true in general of the digestibility of fibre in feeds-
t uf f s and foods that even with the same species of plant, the variety and origin,
in addition to all other factors, are of importance. The reason of this is probably
to be found in varietal differences in chemical and structural make-up of the cell
membrane. This is probably true not only for the cereals studied, but also for
different varieties of the same vegetables and fruits, of importance in human
diet.
160 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ IV, II.
The supplementary observation by Mangold and Schaff (109) may be men-
tioned, that, of other Sauropsida,.the tortoise has a very considerable power of di-
gesting fibre. For Testudo graeca the average digestibility of fibre in barley was
10; in white maize, 30; hay, 45 ; yellow maize, 71; lettuce, 84; and in carrots,
93 per cent. The locus of fibre digestion here also was the caecum and proximal
colon. The long time taken for food to pass through the gastro-intestinal canal,
in Testudo 24 days, and up to 38 and even 55 days for the last traces, greatly faci-
litates the action of bacteria on the fibre.
PENTOSANS.
Of the substances closely associated with orthocellulose in the cell membrane
and in fibre, the hemicelluloses, including pentosans, are of most importance. These
polysacharides are also acted on by bacteria (122, p. 496) and fermented by those
of the rumen (8, 9). According to Kellner (66, p. 162) pentosans promote fat forma-
tion. Experiments of Iwata (61) and of Furth and Engel (28) on rabbits show that
xylan, a pure pentosan, can be broken down in the organism and converted into
reducing sugar.
According to Rubner (101) pentosans are food constituents of importance for
plant eaters. In the sheep, Stone and Jones (126) found pentosans of grass and
bran 40-90 per cent. digestible; Konig (70) found those of grass hay 82.06, of clover
hay, cut before flowering 76.41 and of pea straw 44.02 per cent. digestible. Thomas
and Pringsheim (134) found those of potatoes 72.79 per cent. digestible.
In man, pentosans are of less importance, since only a few grams of pentoses
and pentosans are ingested daily. Grafe (32, 33) carried out experiments with
xylose of which quantities up to 30 g. daily were well tolerated. It appeared that
in man as in the dog, individual variations occurred, from 20-60 per cent. being
excreted in the urine ; and that pentose metabolism is largely independent of dex-
trose metabolism, the increase in blood sugar after xylose being wholly as pentoses
and not as dextrose. Respiration experiments, however, showed that the xylose
that was not excreted was burned, and had a protein sparing action.
PECTINS AND LIGNIN.
Pectins also, which act as intercellular cementing substances, are chiefly acted on
by bacteria in the animal organism (43, 120, 121, 122); such bacteria were shown by
Ankersmit (2) to occur in the digestive tract of the horse and, by means of the pec-
tinase, they produce, to separate the plant cells and make the digestion of cellulose
possible.
Lignin is the most resistant part of the cell membrane, and is regarded as, in
general, non-digestible (44, 77), although Konig (70) gives the digestibility of lignin by
THE DIGESTION AND UTILIZATION OF CRUDE FIBRE
161
sheep as 16-23 per cent. in hay, 4-13 in clover hay, and 28 in pea straw, the corres-
ponding figures for cutin being 7.27, 10.67 and 20.93 per cent. , while those for pure
cellulose, pentosans and total fibre varied between 42 and 83 per cent. According
to Henneberg and Stohmann (44) and Dietrich and Konig (18) the non-digestible
residue of fibre corresponds to the lignin content. Konig considers that the utilisa-
tion of fibre and of the organic constituents of plant feedstuffs as a whole depends
on the content of lignin and cutin, but Rubner (106) and Woodman and Stewart (148)
have shown that the digestibility of cellulose does not depend only on the degree of
lignification or the lignin content, since, for instance, in grass, the decrease in diges-
tibility with age is associated with only a relatively small increase in lignin values.
According to Rubner (106), of the small quantity of lignin ingested daily in
bread by a human experimental subject, about half was digested. Thomas (133) on
the other hand considered that the apparent partial digestion of a lignin preparation
depended on the presence of cellulose. Experiments of Rogozinski and Starzewska
(100) on ruminants with pure lignin or lignin in straw, gave completely negative
results.
Cork substance, suberin, is also regarded as non-digestible (44).
FACTORS AFFECTING DIGESTIBILITY.
Certain observations on the effect of different factors on digestion of fibre may be
briefly summarised. Reference has already been made to the influence of indivi-
duality. What has been said is equally true for animals and man. Henneberg and
Stohmann (44, p. 451), recognised the importance of individuality and of nutritional
condition. Kellner (66, p. 49) considered that the most important reasons for poor
utilisation of food lay in faulty dentition, too rapid eating or pathological conditions.
Individual differences in capacity to digest fibre should, nevertheless, be regarded as
fundamentally physiological, for every test of digestibility of feedstuffs in animals of
the same species and origin shows that they are organisms differing in fine details of
constitution. In studies on digestion and food value, this fact is not sufficiently
considered.
It would take us too far to attempt to define the range of variation due to
individuality from the coefficients of digestibility found by different workers in
studies on digestion and utilisation of fibre. But such individual variations occur
throughout.
In man, Rubner and Thomas (106, 107) have shown that, for instance, cellulose
may be digested to different degrees by two experimental subjects while pentosans
are equally well digested; that, in fact, the digestibility of fibre in man is ex-
162 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ IV, II.
tremely variable and consequently the experimental results of different workers on
e. g., the digestibility of different kinds of bread, can scarcely be compared and used
to give averages valid for the general case. These differences are considered to be
due to individual differences in intestinal flora, and the environmental conditions of
the bacteria in the intestine. Jansen and Muller (60) have made similar observations
on fibre digestion in 5 experimental subjects who gave coefficients varying between
79.1 and 90.0 per cent. for wheat bread with a low fibre basal diet, and between 58
and 75 per cent. for potatoes.
The variability of results for fibre digestion is probably also affected to a signi-
ficant degree by the nature of the rest of the food and of the total diet fed. This
will have an effect on the intestinal flora, altering the culture medium for the bacte-
ria (101, 133, 134).
These factors must be of special importance in ruminants, in which, on account
of the large quantities of fibre to be dealt with, the possibility of such quantitative
changes is greater. We have already referred to the depression of fibre fermentation in
the rumen due to simultaneous feeding of easily soluble carbohydrates. It has been
shown by Henneberg and Stohmann (44), that this effect on fibre utilisation is
bound up with the nutritive ratio (protein to other nutritive constituents); this has
recently been confirmed by Mertins (89), who found in sheep that addition of
protein to the ration decreases fibre digestibility.
Similar effects of changes in the composition of the ration occur, according to
Rubner (103) in man and the dog, and according to Bollmann (6) in the pig.
It seems likely that the degree of mechanical division of the food would have an
effect on the utilisation of fibre and would be of practical importance in man and
animals. In contrast to protein, fat and starch, which can be attacked and ren-
dered soluble at once by the digestive enzymes, it seems natural to expect that the
chemical digestion of fibre, depending as it does on the action of bacteria, would be
affected by an increase in the surface area, open to attack by the bacteria, by
mechanical subdivision of the food. It is true that machanical treatment in the
animal body takes place in chewing, rumination and grinding in the gizzard of
poultry. But, since the work of chewing consumes a part of the food energy and so
reduces the food value (see above), it is of importance to know whether breaking up
the food before ingestion, so as to reduce the work of chewing, would increase diges-
tibility and utilisation of fibre.
Haberlandt and Zuntz (36, p. 707) in their experiments with birch wood meal,
found that the grinding of the wood, which they adopted to secure availability of
the cell contents, served also to make available the cell walls. The digestibility of
THE DIGESTION AND UTILIZATION OF CRUDE FIBRE
163
the fibre is, however, not necessarily increased in proportion to the degree of sub-
division. Kellner (66, p. 278) both with wheat and barley straw in cattle, found no
difference whether the straw was fed chopped or as meal. In the horse, however,
Gay (cf. Kellner 66, p. 279) found an increase in digestion of fibre from whole oats
42.0 per cent.) to ground (48.9 per cent.) and crushed grain (63.6 per cent.). In
pigs the advantage of milling has frequently been observed. Thus Haberhauffe (34)
working with maize, found a fibre digestibility of 52.39 per cent. with whole grain,
55.66 per cent. with coarsely ground, 75.86 with medium ground and 78.05 per cent.
with finely ground grain, while Honcamp, Schram and Stotz (58) found a higher
starch equivalent for oatmeal than for whole oats ; Woodman an increase of 15
per cent. in the feed value of ground as compared with whole barley, and Nils
Hansson (38, 39) an increase in feed value of maize of 5-6 per cent. by crushing,
10-12 per cent. by coarse grinding and 15-18 per cent. by fine grinding.
Similar observations have been made on fibre digestion in man.
In addition, numerous other factors affect the digestion of fibre, above all, differ-
ences in structural and chemical composition of the cell membrane as determind by
variety, origin, age and conditions of growth of plants ; the effect of drying (grass
and hay), the temperature used (cf. Watson and Ferguson, 142), ensiling, cooking
boiling and so on. Although these factors are of greater importance in the feeding
of animals than of man, yet in human nutrition methods of preparation of plant
food will have a similar action in modifying the digestion and utilisation of fibre as
well as on the availability of cell contents.
In conclusion, reference may be made to the numerous attempts which have
been made to treat fibre in straw and wood so as to increase digestibility and utilisation
of types which are otherwise non-digestible or only slightly digestible. It may be
recalled that both with straw, treated with alkalis by the method of Lehmann and
others (cf. 66, 23, 53), or without chemicals (84, 54, 56), and also with wood quite
apart from conversion into sugar as proposed by Bergius), treated by mechanical
and chemical means (35, 36), or by the more recent colloid chemical methods of
Schwalbe (123) and Mangold (86), feedstuffs may be prepared with greatly increased
fibre digestibility, comparable with that of other feedstuffs. But, since the questions
involved in these process are chiefly technical problems of treatment and preparation,
there is no need to discuss here the various methods used and the results of
testing the products as feedstuffs.
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