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).

[Illustration:  FIG. 28.—­Table of food values.]

[Illustration:  FIG. 29.—­Diagram showing the difference in the cost of three foods which give about the same amount of nutrition each.]

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
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|         FAMILY No. 1      |       ||         FAMILY No. 2         |
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|20 loaves of bread         | $1.00 ||15 lb. flour, bread           |
|10 to 12 lb. loin steak    |       ||   home made (skim milk used) | $.45
|   or meat of similar cost |  2.00 ||Yeast, shortening, and        |
|20 to 25 lb. rib roast     |       ||   skim milk                  |  .10
|   or similar meat         |  4.40 ||10 lb. steak (round, Hamburger|
|4 lb. high-priced cereal   |       ||   and some loin)             | 1.50