running the engine. When an engine is not working, fuel is not consumed, but the body requires food for mere existence, regardless of whether it does active work or not. When we work, the cells break down more quickly, and the repair is greater than when we are at rest, and hence there is need of a larger amount of food; but whether we work or not, food is necessary.
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.
63. The Proteids. The proteids are the building foods, furnishing muscle, bone, skin cells, etc., and supplying blood and other bodily fluids. The best-known proteids are white of egg, curd of milk, and lean of fish and meat; peas and beans have an abundant supply of this substance, and nuts are rich in it. Most of our proteids are of animal origin, but some protein material is also found in the vegetable world. This class of foods contains carbon, oxygen, and hydrogen, and in addition, two substances not found in carbohydrates or fats—namely, sulphur and nitrogen. Proteids always contain nitrogen, and hence they are frequently spoken of as nitrogenous foods. Since the proteids contain all the elements found in the two other classes of foods, they are able to contribute, if necessary, to the store of bodily energy; but their main function is upbuilding, and the diet should be chosen so that the proteids do not have a double task.
For an average man four ounces of dry proteid matter daily will suffice to keep the body cells in normal condition.
It has been estimated that 300,000,000 blood cells alone need daily repair or renewal. When we consider that the blood is but one part of the body, and that all organs and fluids have corresponding requirements, we realize how vast is the work to be done by the food which we eat.
64. Mistakes in Buying. The body demands a daily ration of the three classes of food stuffs, but it is for us to determine from what meats, vegetables, fruits, cereals, etc., this supply shall be obtained (Figs. 28 and 29).
Generally speaking, meats are the most expensive foods we can purchase, and hence should be bought seldom and in small quantities. Their place can be taken by beans, peas, potatoes, etc., and at less than a quarter of the cost. The average American family eats meat three times a day, while the average family of the more conservative and older countries rarely eats meat more than once a day. The following tables indicate the financial loss arising from an unwise selection of foods:—
FOOD CONSUMED—ONE WEEK
| FAMILY No. 1 | FAMILY No. 2 | ||
| 20 loaves of bread | $ 1.00 | 15 lb. flour, bread home-made (skim milk used) | $ 0.45 |
| 10 to 12 lb. loin steak or meat similar cost | 2.00 | Yeast, shortening, and skim milk | 0.10 |
| 20 to 25 lb. rib roast or similar meat | 4.40 | 10 lb. steak (round, Hamburger and some loin) | 1.50 |
| 4 lb. high-priced cereal breakfast food, 20¢ | 0.80 | 10 lb. other meats, boiling pieces, rump roast, etc. | 1.00 |
| Cake and pastry purchased | 3.00 | 5 lb. cheese, 16¢ | 0.80 |
| 8 lb. butter, 30¢ | 2.40 | 5 lb. oatmeal (bulk) | 0.15 |
| Tea, coffee, spices, etc. | 0.75 | 5 lb. beans | 0.25 |
| Mushrooms | 0.75 | Home-made cake and pastry | 1.00 |
| Celery | 1.00 | 6 lb. butter, 30¢ | 1.80 |
| Oranges | 2.00 | 3 lb. home-made shortening | 0.25 |
| Potatoes | 0.25 | Tea, coffee, and spices | 0.40 |