The Chemistry of Food and Nutrition

by A. W. Duncan

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 digestion of both animal and vegetable proteids and fats are the same. 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.