60.  The Different Foods.  The body is very exacting in its demands, requiring certain definite foods for the formation and maintenance of its cells, and other foods, equally definite, but of different character, for heat; our diet therefore must contain foods of high fuel value, and likewise foods of cell-forming power.

Although the foods which we eat are of widely different character, such as fruits, vegetables, cereals, oils, meats, eggs, milk, cheese, etc., they can be put into three great classes:  the carbohydrates, the fats, and the proteids.

61.  The Carbohydrates.  Corn, wheat, rye, in fact all cereals and grains, potatoes, and most vegetables are rich in carbohydrates; as are also sugar, molasses, honey, and maple sirup.  The foods of the first group are valuable because of the starch they contain; for example, corn starch, wheat starch, potato starch.  The substances of the second group are valuable because of the sugar they contain; sugar contains the maximum amount of carbohydrate.  In the sirups there is a considerable quantity of sugar, while in some fruits it is present in more or less dilute form.  Sweet peaches, apples, grapes, contain a moderate amount of sugar; watermelons, pears, etc., contain less.  Most of our carbohydrates are of plant origin, being found in vegetables, fruits, cereals, and sirups.

Carbohydrates, whether of the starch group or the sugar group, are composed chiefly of three elements:  carbon, hydrogen, and oxygen; they are therefore combustible, and are great energy producers.  On the other hand, they are worthless for cell growth and repair, and if we limited our diet to carbohydrates, we should be like a man who had fuel but no engine capable of using it.

62.  The Fats.  The best-known fats are butter, lard, olive oil, and the fats of meats, cheese, and chocolate.  When we test fats for fuel values by means of a calorimeter (Fig. 26), we find that they yield twice as much heat as the carbohydrates, but that they burn out more quickly.  Dwellers in cold climates must constantly eat large quantities of fatty foods if they are to keep their bodies warm and survive the extreme cold.  Cod liver oil is an excellent food medicine, and if taken in winter serves to warm the body and to protect it against the rigors of cold weather.  The average person avoids fatty foods in summer, knowing from experience that rich foods make him warm and uncomfortable.  The harder we work and the colder the weather, the more food of that kind do we require; it is said that a lumberman doing heavy out-of-door work in cold climates needs three times as much food as a city clerk.  Most of our fats, like lard and butter, are of animal origin; some of them, however, like olive oil, peanut butter, and coconut oil, are of plant origin.

[Illustration:  FIG. 27.—­a is the amount of fat necessary to make one calorie; b is the amount of sugar or proteid necessary to make one calorie.]