Food is that which, when taken into the body, repairs waste, forms tissue, or yields energy in the form of heat and muscular power. Most food material contains a portion which is non-edible in addition to the nutrients. In fish, the bone and skin form the non-edible portion; in eggs, the shell; in meat, the bone, - and so on. Foods present a great variety of appearances and flavors, but chemical analyses show that each one is composed of a portion or all of the following compounds: Protein, fats, carbohydrates, and mineral matter. Water is usually present in food, and while not classed among nutrients, is absolutely necessary in the body, because it carries the nutrients, and, as it bathes the different tissues, each appropriates to itself such nutrients as are needed for repairs or growth.
The nutrients are made up of the following elements: Nitrogen, carbon, oxygen, hydrogen, phosphorus, sulphur, calcium, magnesium, sodium, potassium, silicon, chlorin, fluorine, and iron.
Probably no single nutrient contains all of these. Of the nutrients named above, protein alone supplies nitrogen to the body, and it is therefore necessary that we take food containing some protein, as the muscular tissues and the fluids of the body need it for repairs and growth. Protein is capable of forming fatty tissues, and can be used to produce heat and energy, but it is not wise to use it so, because this entails unnecessary work on the organs, thus, in time, often causing disease. Protein is also more expensive than fats and carbohydrates, which should be used for pro-producing heat and energy. Mineral matter is the only nutrient which cannot supply carbon, hydrogen, and oxygen to the body.
Foods may be classed, with reference to their origin, as animal foods and vegetable foods. The principal animal foods are eggs, meats (including fish of all kinds, poultry, and game), milk, and cheese. Vegetable foods include all edibles commonly called "vegetables," as well as nuts and fruits. Animal foods are in general richer in protein, easier of digestion, and higher in price than vegetable foods, though there are some exceptions to this rule. Milk, cheese, and the cheaper cuts of meat are inexpensive sources of protein food, and when properly prepared are well assimilated, though tough meats and all kinds of cheese require careful preparation.
Vegetable foods in general contain a small amount of nutritive material in proportion to their bulk. Legumes, such as peas, beans, and lentils, are an exception to this rule. Cereals are also rich in protein, starch and fat. Vegetables and milk are especially valuable in giving needed bulk to the food, and many fruits and young vegetables aid digestion much by furnishing needed acids and mineral matters. Almost all animal and vegetable foods furnish some mineral matter, but milk and cereals are especially valuable in this way. Water is also a source of mineral matter.
Fat is derived from both the animal and the vegetable kingdoms, - as the fat of meat, milk, etc. Its chief source among vegetables is nuts, as the peanut, walnut, etc.; fruits, as the olive; and grains, as corn and wheat.
With reference to their use in the body, foods may be classed as protein foods, - as eggs, meat, milk, etc.; carbo-hydrate foods, - as rice, potatoes, etc.; and fatty foods,- as pork, butter. etc.