1 pound of protein yields 1860 calories 1 ” ” fats ” 4220 ” 1 ” ” carbohydrates yields 1860 ” That is, according to fuel value—the capacity of the nutrients for yielding heat and mechanical power—a pound of the protein of lean meat or egg albumen just about equals a pound of starch or sugar, and about two pounds of these would about equal a pound of the fat of meat or of the body fat. The calculation has been made, based on experiments, that one who does no muscular work needs only an amount of food which will produce 2700 calories. One doing light muscular work needs 3000 calories. An individual doing moderately heavy work should take 3500 calories, while heavy muscular work takes 4500 calories. One hundred grams of protein food, however, gives only fifteen per cent. of the amount of energy required. About 500 grams of carbohydrate and 50 grams of fat are needed to make up the 3000 calories which must be furnished by the daily supply of food for one doing light muscular work. The brain worker, who is using brain tissue more rapidly than the day laborer, should have a diet equally as rich in protein, though less fat and carbohydrates are needed. It has been estimated that an ordinary man on full diet excretes about twenty grams (about five-eighths of an ounce) of nitrogen a day. As protein material contains about sixteen per cent. of nitrogen, such an individual needs to take about 120 grams of protein a day to supply the nitrogen needs of the body. Because of its need for protein, the body does not store it. A day laborer needs 0.28 of a pound of protein a day with enough fat and carbohydrate to give a fuel value of 3500 calories. A professional man requires 0.25 (1/4) of a pound of protein a day. Much more than this is usually taken. This means from 1/3 to 1/2 a pound of lean meat. _Nothing is lost in Nature’s distribution of force and energy. Everything accomplished in life, either in the physical handling of material, the brain work in planning the constructions, the mental movements of thought in art, literature, or science, are all representatives of the heat and energy released from the body, and every man and woman should endeavor to make the body yield as large an income as possible in the expression of this energy. In order that it may do so, it must be used with intelligence, just as any other great machine must be used intelligently; it must be fed, exercised, and rested judiciously._ CHAPTER V REPAIR AND ELIMINATION OF WASTE (METABOLISM) The work of the body never stops. If it is to be kept in thorough working order its tissues must be rebuilt as incessantly as they are torn down in the process of producing heat and energy. These chemical changes are called collectively _metabolism_.