While the elements of the food are being oxidized, the latent (potential) energy released by the oxygen creates mental and physical force and keeps active the metabolic changing of food into tissues and cells, also the changing of cells and tissues into waste.
Scientists have measured the energy latent in food material, also the amount of heat given off in the oxidation of a given quantity of waste. The unit of measurement is the calorie—the amount of heat which will raise one pound of water 4 degrees Fahrenheit.
The fuel value of any food denotes the total number of calories which may be derived from a pound of that food if it be completely oxidized in the body.
C. F. Langworthy gives the fuel value of proteins, fats, and carbohydrates as follows:
| 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.