The value of a food depends partly on the amount of nutritive material it contains, and partly on whether the nutrients are in such form that they can be made use of by the body. Some foods are more easily digested and assimilated when raw than when cooked, as ripe fruits, melons, lettuce, cabbage, etc. Meat and white of egg are in many cases more easily used by the body when taken raw, but they have a better flavor when cooked. Most foods lose something both in flavor and food value by cooking, if they are perfectly palatable and wholesome raw.
Some protein foods are digested largely in the stomach, where the gastric juice dissolves the insoluble materials, and renders them capable of entering the circulation, to be carried to such parts of the body as need them. Others are digested largely in the lower part of the digestive tract, where the pancreatic juice performs a work similar to that done by the gastric juice in the stomach. Protein foods of vegetable origin are usually more difficult of digestion than those of animal origin, and many require cooking to render them fit for human food at all. Vegetable protein, as found in beans, peas, cereals, etc., is mixed with starch and cellulose, so that neither the flavor nor the food value is perfect unless thoroughly cooked.
Both sugars and starches, when taken in excess of what is needed to furnish the heat and energy required by the body, tend to lay on fat. A man doing hard work in the open air can assimilate much larger quantities of sugar than one of indoor life and sedentary habits. Sugar is generally regarded as adapted to the sick only in small quantities.
It is believed that sugar is more easily digestible when taken in liquids, as tea, coffee, and water, than in cake or other such food. A sufficient number of analyses have not been made, however, to prove this conclusively. Sugar, like butter, is highly prized for its agreeable flavor, and while starchy foods alone might furnish sufficient carbo-hydrates, the use of sugar in many cooked dishes is absolutely necessary to their palatability. There is, however, no excuse for eating sugar in sufficient quantities to injure the health.