The important food products derived from the vegetable kingdom are the Cereals, Legumes - Roots and Tubers -Green Vegetables - Fruits - Nuts - Fungi - Lichens.

Composition

Vegetable or plant foods contain the five proximate principles, namely, proteins, fats, carbohydrates, water and mineral matter. In this they resemble animal foods. The food principles in the two classes are, however, in different proportions, animal foods being rich in proteins or fats, and seldom containing carbohydrate in appreciable amounts; while vegetable foods are, as a rule, rich in carbohydrates, and poor in proteins and fats.

These differences are clearly shown in the following table:

The legumes and most nuts are exceptional in containing high percentages of protein or fat or both, but unlike meats, they also contain a very considerable amount of carbohydrate, as shown by the following analyses:

Protein

1 (1) Gluten of wheat and some of the other cereals.

(2) Legumin found in legumes.

(3) Vegetable albumin as found in some vegetables. Carbohydrates of Vegetables2 include - starch, sugar, vegetable gums and cellulose.

Fats of Vegetables 3 are in the form of oils and are similar, in composition to animal fats, but are more easily digested and equal in nutritive value.

Water

Dry vegetable foods, such as cereals, dried beans and peas, nuts, dried fruits, etc., do not, as a rule, contain as much water as animal foods, but fresh vegetables and fruits contain frequently from 90 per cent. to 95 per cent, of water.

This gives them a low nutritive value compared with their bulk. In cooking, water is added to most dry vegetable foods, so that when served they resemble fresh or succulent vegetable foods in this respect.

Mineral Matter

4 All vegetable foods contain valuable ash constituents. In cereals, these are largely in the outer part of the grain, and are lost if the entire grain is not utilized. Thus polished rice is much poorer in ash constituents than unpolished; and white flour than entire wheat flour. Fresh vegetables and fruits are desirable in the dietary for their salts and organic acids.

Digestibility

Vegetable foods are less easily digested than animal foods, owing to their complex composition, and especially to the intimate mixture of the nutrients with cellulose. This prevents the digestive ferments acting readily upon the food-stuffs so that they are digested more slowly and less completely than animal foods. Although cellulose is indigestible, its presence under normal conditions in proper amounts, is advantageous,5 but it should be thoroughly softened by cooking, except in the case of tender young vegetables such as lettuce.

1 Protein, see p. 10.

2 Carbohydrates, see p. 16.

3 Fats of vegetables, see p. 20. 4 Mineral matter, see p. 31. * See p. 17.

Comparative Value Of Animal And Vegetable Diets

An animal diet is concentrated and easily digested. If the animal foods taken are eggs and milk, no serious objection can be urged against them, except that they lack bulk. The food is so fully absorbed that the large intestine has no stimulus to action and no material for its muscles to act upon, and serious complications are likely to arise from constipation. If the animal food is largely meat, it is undesirable, not only for the reason above mentioned, but because excessive quantities of nitrogenous extractives are taken, which are likely to cause uric acid disorders and other disturbances, and also because important mineral salts are lacking (such as calcium, which carnivorous animals get by eating bone); and because the acids formed from the protein (phosphoric and sulphuric) tend to deprive the body of salts already present in the tissues.

A strict vegetable diet, on the other hand, while free from these objections, is necessarily very bulky, owing to the high percentages of water and cellulose, and to the fact that extra allowance must be made because it is less perfectly absorbed. Unless special care is taken, a vegetable diet is also likely to be low in protein. Legumes and nuts must form a considerable part of the dietary to provide this. When eggs, milk and milk products are added to the vegetable foods, the diet becomes mixed, and these objections no longer hold.

An Ideal Diet contains a mixture of animal and vegetable foods, so that protein, fat, carbohydrate and mineral matter are supplied in proper proportions and in available forms, and sufficient bulk is afforded for proper intestinal action.