We may define a food to be any substance which will repair the functional waste of the body, increase its growth, or maintain the heat, muscular, and nervous energy.
Mineral Matter or Salts, is left as an ash when food is thoroughly burnt. The most important salts are calcium phosphate, carbonate and fluoride, sodium chloride, potassium phosphate and chloride, and compounds of magnesium, iron and silicon.
Flesh-formers, heat-givers, and bone-formers
Proteids are excreted from the body as water, carbon dioxide, urea, uric acid, sulphates, &c.
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.
A comparatively small quantity of proteid matter, such as is easily obtained from vegetable food, is ample for the general needs of the body.
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.