In the selection of foods one of the questions that will come up will be that of the relative value of animal and vegetable foods. An increasing number of people are confining their diet largely, if not exclusively, to vegetable products, while others add to these such animal substances as do not imply the taking of life, such as milk and eggs. Is a mixed diet essential for health? Or may we at will choose exclusively from the animal or the vegetable kingdom?
Certain broad distinctions between animal and vegetable food will immediately present themselves. Speaking generally, animal foods are richer in nitrogenous matter, while vegetable foods are the chief source of carbohydrates. This becomes much more evident if we compare the two in a dry condition. Milk, for instance, makes a poor showing in proteid as compared with dried peas and lentils, or even with rice. But if we take the total solids of the milk as a basis of comparison, eliminating the 87 per cent of water, the case is quite otherwise. This is the fair method, for the dried peas and rice absorb many times their weight of water in the process of cooking, so that the analysis of the raw material is quite different from that of the cooked food.
Hutchison gives the following composition of a few typical dried foods:
One hundred parts of dried lean beef contain 89 parts of proteid.
One hundred parts of dried fat beef contain 51 parts of proteid.
One hundred parts of dried pea flour contain 27 parts of proteid.
One hundred parts of dried wheat contain 16 parts of proteid.
One hundred parts of dried rice contain 7 parts of proteid.
To this we may add:
One hundred parts of dried milk contain 25 parts of proteid.
On the other hand we find our carbohydrates almost wholly in the vegetable kingdom. Milk is the only important exception to this. In milk, dried, we find 38 parts of carbohydrate to 100 of the total solids.
Another difference between animal and vegetable food is found in their comparative cost. Animal food as a rule is much more expensive than vegetable. This is not difficult to understand when we remember that our animal food has been put through a further process of manufacture than the vegetable food. If the grain raised, instead of going directly to man as food, is used to feed cattle, and these in turn are slaughtered to furnish nourishment for human beings, the process necessarily adds to the cost of the food. This process, as well as the fact that plants are in general builders of material, while animals break down the complex compounds built up by the vegetables, is graphically shown by the accompanying diagram.
The same intermediate process which adds to the cost of food increases also its digestibility, though the less complete absorption by the system of vegetable than of animal proteid seems to lie in the fact that in the plant the proteid is enclosed within cellulose walls and ordinary processes of cooking do not always free it, rather than in any difference in the proteids themselves.
In deciding from which kingdom we shall choose our diet, we consider almost wholly the proteid. As we have seen, carbohydrates must necessarily be obtained chiefly from vegetable sources, and it seems to be a matter of indifference whether the fat of the diet is of animal or vegetable origin. With the addition of milk, butter, cheese, and eggs, it is not difficult with care to provide a satisfactory dietary without the use of meat.
The case is different when vegetables form the only source of food supplies. Because of the great excess of carbohydrates and the presence of indigestible matter in the form of cellulose, a great bulk of food must be taken in order to get the necessary proteid. As a matter of fact, nearly all purely vegetarian diets are deficient in proteid. The extra cost of the animal proteid is justified by its availability since it may be obtained without an excess of other substances and since it is easily assimilated.