Digestibility. Sugars. Cereals. Roots and Tubers. Peas and Beans. Green Vegetables. Fruits. Nuts. Fungi. Spices and Condiments.

Vegetable foods differ from animal foods in the large amount of sugar and starch and the relatively small amount of protein and fat that they contain. They also contain a large quantity of water. A cabbage is richer in water than is milk. The great water-content of vegetables makes them bulky foods, and their small nitrogen value necessitates the eating of a large quantity in order to supply the protein that is required. As vegetable albumin is so much less readily absorbed from the intestines than is animal albumin, the total quantity of vegetable matter that must be eaten to supply all the ingredients needed for the maintenance of life is increased still more. It is estimated that of dry lentils, which, for a vegetable food, contain a large amount of nitrogenous matter, 2000 grams will be needed to supply as much protein as is necessary for a day's rations for a man, or as much as will be supplied by 600 grams of cooked meat.

Digestibility

The sugars and starches of vegetable foods are very perfectly digested and absorbed. Most vegetable oils and fats are rapidly absorbed. The proteins are not. Cellulose, which, except when very young, is indigestible, seems to interfere with the digestion and absorption of the proteins. The protein of vegetables is contained in cells as starch is; but when cooked, starch swells and bursts the envelop of cellulose, while protein, like other albumins when heated, shrinks and remains inside its almost indigestible shell. Probably the quickening of intestinal movements that a vegetable diet produces also plays a part in preventing its perfect absorption. Forty per cent, of the protein of beans when they have been simply soaked and boiled until they are soft is lost in the feces. If they are ground to a fine powder and the cellulose shell about the protein is thus broken, it is almost as well digested and absorbed as is animal albumin.

Unquestionably a mixed diet provides the necessary constituents in the most digestible and absorbable form. The ratio that should exist between animal and vegetable food is variously estimated. It is probable that one part of raw animal food to three parts of raw vegetable food is as nearly accurate a ratio as can be established. In a large number of dietaries collected in the United States 45 per cent, of the food was of animal origin. This is a high percentage, and especially so for those who lead a sedentary life. However, for the latter class a large amount of vegetable food proves indigestible because its bulk is a tax upon the muscular power of the alimentary canal, and because, when not digested rapidly, it is especially liable to fermentation, forming acids that often provoke intestinal inflammation, acute or chronic. A man doing hard muscular work needs an abundance of carbohydrate to furnish him with strength. His work maintains good muscular activity of his alimentary canal. The free action of skin and lungs also produced by it enables him to dispose of the excess of water that a bulky vegetable diet makes unavoidable.

If such foods are eaten hastily by those who are weak, distention of the stomach and intestines is likely to be produced, and ultimately, because of slow digestion and fermentation of food, inflammation of these structures. Only those vegetables that are most digestible and least likely to ferment should be eaten under these circumstances, and then in small or moderate portions.

The sugar of vegetable foods is in their juices. It is the circulating form of carbohydrate in plants. Starch is the form in which it is stored. Sugar, is soluble, starch is not. In cold water starch undergoes no change, but in hot water it swells, breaks its envelop of cellulose, and is washed out. Cooking is, therefore, essential to make food containing starch capable of digestion. Dry heat can convert starch into dextrin, which is soluble.

Cellulose is a carbohydrate that is different from starch, being both insoluble and indigestible except when it is very young. It is, however, especially subject to attack by micro-organisms that generate marsh-gas from it. As sustenance for the human body it is of inappreciable importance. It does stimulate the muscular activity of the intestine by its bulkiness, and foods rich in it, such as brown bread, green vegetables, and fruits, are eaten to correct constipation.

Vegetable proteins are mostly globulins. Vegetable as compared with animal food is poor in nucleo-albumins. Like other proteins, those derived from vegetables coagulate with heat.

Chemical constituents resembling the extractives of meat are also found in vegetables. The fats or oils are for the most part digestible. Vegetables contain most of the salts needed for the maintenance of life, but they are comparatively rich in potassium salts and poor in sodium. Sugars

Sugar is the form of carbohydrate that needs least modification by digestion before it is absorbed. Dextrose needs more. The changes that must be effected in cane-sugar and other sugars have already been discussed in the chapter devoted to Digestion and Assimilation.

Cane-sugar, which in commerce is chiefly derived from sugarcane and sugar beet, is the form most used. Maple-sugar is cane-sugar, but contains certain ethereal substances that give it its characteristic taste. Maltose, or malt-sugar, lactose, or milk-sugar, glucose, or grape-sugar, levulose, or fruit-sugar, and invertose, or invert sugar, are other forms met with in foods more or less frequently.

Molasses and syrup are crude forms of cane-sugar. Honey consists of approximately 20 per cent, of water, 35 per cent, of grape-sugar, 39 per cent, of fruit-sugar, and 5 per cent, of substances other than sugar. It is obtained by bees from flowers and stored in cells of wax.

Since both sugar and starch must be transformed into dextrose before they are utilizable, they may be said to be identical as nourishment. Sugar is agreeable to the taste, which is important, as it creates an appetite or liking for food to which it is added. Starch is almost tasteless.