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 |
| Miscellaneous canned goods | 2.00 | Apples | 0.50 |
| Milk | 0.50 | Prunes | 0.25 |
| Miscellaneous foods | 2.00 | Potatoes | 0.25 |
| 3 doz. eggs | 0.60 | Milk | 1.00 |
| Miscellaneous foods | 1.00 | ||
| 3 doz. eggs | 0.60 | ||
| $23.45 | $11.30 | ||
"The tables show that one family spends over twice as much in the purchase of foods as the other family, and yet the one whose food costs the less actually secures the larger amount of nutritive material and is better fed than the family where more money is expended."—From Human Foods, Snyder.
The Source of the Different Foods. All of our food comes from either the plant world or the animal world. Broadly speaking, plants furnish the carbohydrates, that is, starch and sugar; animals
