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.]

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