The principal proteids of animal origin have their corresponding proteids
in the vegetable kingdom. Some kinds, whether of animal or vegetable
origin, are more easily digested than others. They have the same
physiological value from whichever kingdom they are derived.
The Osseids comprise ossein, gelatin, cartilage, &c., from bone, skin,
and connective issue. They approach the proteids in composition, but
unlike them they cannot form flesh or fulfil the same purpose in
nutrition. Some food chemists wish to call the osseids, albuminoids; what
were formerly termed albuminoids to be always spoken of as proteids only.
Jellies are of little use as food; not only is this because of the low
nutritive value of gelatin, but also on account of the small quantity
which is mixed with a large proportion of water.
The Vegetable Kingdom is the prime source of all organic food; water,
and to a slight extent salts, form the only food that animals can derive
directly from the inorganic kingdom. When man consumes animal food--a
sheep for example--he is only consuming a portion of the food which that
sheep obtained from grass, clover, turnips, &c. All the proteids of the
flesh once existed as proteids in the vegetables; some in exactly the same
chemical form.
Flesh contains no starch or sugar, but a small quantity of glycogen. The
fat in an animal is derived from the carbohydrates, the fats and the
proteids of the vegetables consumed. The soil that produced the herbage,
grain and roots consumed by cattle, in most cases could have produced food
capable of direct utilisation by man. By passing the product of the soil
through animals there is an enormous economic loss, as the greater part of
that food is dissipated in maintaining the life and growth; little remains
as flesh when the animal is delivered into the hands of the butcher. Some
imagine that flesh food is more easily converted into flesh and blood in
our bodies and is consequently more valuable than similar constituents in
vegetables, but such is not the case. Fat, whether from flesh or from
vegetables is digested in the same manner. The proteids of flesh, like
those of vegetables, are converted into peptone by the digestive
juices--taking the form of a perfectly diffusible liquid--otherwise they
could not be absorbed and utilised by the body. Thus the products of
Formerly, proteid matter was looked upon as the most valuable part of the
food, and a large proportion was thought necessary for hard work. It was
thought to be required, not only for the construction of the muscle
substance, but to be utilised in proportion to muscular exertion. These
views are now known to be wrong. A comparatively small quantity of proteid
matter, such as is easily obtained from vegetable food, is ample for the
general needs of the body. Increased muscular exertion requires but a
slight increase of this food constituent. It is the carbohydrates, or
carbohydrates and fats that should be eaten in larger quantity, as these
are the main source of muscular energy. The fact that animals, capable of
the most prolonged and powerful exertion, thrive on vegetables of
comparatively low proteid value, and that millions of the strongest races
have subsisted on what most Englishmen would consider a meagre vegetarian
diet, should have been sufficient evidence against the earlier view.
A comparison of flesh and vegetable food, shows in flesh an excessive
quantity of proteid matter, a very small quantity of glycogen (the animal
equivalent of starch and sugar) and a variable quantity of fat. Vegetable
food differs much, but as a rule it contains a much smaller quantity of
proteid matter, a large proportion of starch and sugar and a small
quantity of fat. Some vegetable foods, particularly nuts, contain much
fat.
Investigation of the digestive processes has shown that the carbohydrates
and fats entail little strain on the system; their ultimate products are
water and carbon dioxide, which are easily disposed of. The changes which
the proteids undergo in the body are very complicated. There is ample
provision in the body for their digestion, metabolism, and final
rejection, when taken in moderate quantity, as is the case in a dietary of
vegetables. The proteids in the human body, after fulfilling their
purpose, are in part expelled in the same way as the carbohydrates; but
the principal part, including all the nitrogen, is expelled by the kidneys
in the form of urea (a very soluble substance), and a small quantity of
uric acid in the form of quadurates.
There is reciprocity between the teeth and digestive organs of animals and
their natural food. The grasses, leaves, &c., which are consumed by the
herbivora, contain a large proportion of cellulose and woody tissue.
Consequently, the food is bulky; it is but slowly disintegrated and the
nutritious matter liberated and digested. The cellulose appears but
slightly acted upon by the digestive juices. The herbivora possess
capacious stomachs and the intestines are very long. The carnivora have
simpler digestive organs and short intestines. Even they consume
substances which leave much indigestible residue, such as skin, ligaments
and bones, but civilised man, when living on a flesh dietary removes as
much of such things as possible. The monkeys, apes, and man (comprised in
the order _Primates_) have a digestive canal intermediate in complexity
and in length to the herbivora and carnivora. A certain quantity of
indigestible matter is necessary for exciting peristaltic action of the
bowels. The carnivora with their short intestinal canal need the least,
the frugivora more, and the herbivora a much larger quantity. The
consumption by man of what is commonly called concentrated food is the
cause of the constipation to which flesh-eating nations are subject. Most
of the pills and other nostrums which are used in enormous quantities
contain aloes or other drugs which stimulate the action of the intestines.
Highly manufactured foods, from which as much as possible of the
non-nutritious matter has been removed is often advocated, generally by
those interested in its sale. Such food would be advantageous only if it
were possible to remove or modify a great part of our digestive canal (we
are omitting from consideration certain diseased conditions, when such
foods may be useful). The eminent physiologist and bacteriologist, Elie
Metchnikoff, has given it as his opinion that much of man's digestive
organs is not only useless but often productive of derangement and
disease. In several cases where it has been necessary, in consequence of
serious disease, to remove the entire stomach or a large part of the
intestines, the digestive functions have been perfectly performed. It is
not that our organs are at fault, but our habits of life differ from that
of our progenitors. In past times, when a simple dietary in which flesh
food formed little or no part, and to-day, in those countries where one
wholly or nearly all derived from vegetable sources and simply prepared is
the rule, diseases of the digestive organs are rare. The Englishman going
to a tropical country and partaking largely of flesh and alcohol, suffers
from disease of the liver and other organs, to which the natives and the
few of his own countrymen, living in accordance with natural laws are
strangers.
Indigestible Matter--Food is never entirely digested. As a reason
against confining ourselves solely to vegetable food, it has been stated
that such is less perfectly digested than animal food and that it
therefore throws more work on the digestive organs. It is also urged that
on this account a greater quantity of vegetable food is required. We have
shown elsewhere that, on the contrary, vegetarians are satisfied with a
smaller amount of food. Man requires a small quantity of woody fibre or
cellulose in his food to stimulate intestinal action and prevent
constipation.
It is difficult to determine how much of a food is unassimilated in the
body. This is for the reason of the intestinal refuse consisting not only
of undigested food, but also of residues of the digestive juices, mucus
and epithelial debris. These latter have been shown to amount to from
one-third to one-half of the whole of the fæces, which is much more than
had previously been supposed.
John Goodfellow has shown that of very coarse wholemeal bread quite 14 per
cent. was undigested, whilst bread made from ordinary grade wholemeal
showed 12.5 per cent. Such a method of analysis was adopted as it was
believed would exclude other than the food waste. The experiments were
made on a person who was eating nothing but the bread. It seems probable
that a smaller proportion would have remained unassimilated had the bread
not formed the sole food. It is advisable that wheatmeal he ground as
finely as possible, the coarse is not only to a less extent assimilated
but apt to irritate the bowels. Notwithstanding that fine white bread gave
only 4.2 per cent. and a coarse white bread 4.9 per cent. of waste, a fine
wheatmeal bread is more economical as the same quantity of wheat produces
a greater weight of flour richer in proteid and mineral matter. From a
large number of experiments with man, it has been calculated that of
proteids there is digested when animal food is eaten 98 per cent., from
cereals and sugars 8 per cent., from vegetables and fruits 80 per cent.
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