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,