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. [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 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.