The cells of the body are constantly breaking down and in the growing body new cells are forming, hence food must furnish material for growth and repair.

As the cells contain nitrogen, a food which contains nitrogen in a form in which the body can assimilate it must be furnished. Protein foods contain 16 per cent nitrogen and are the only foods which do contain it, so they are absolutely necessary for the body.

The carbon and hydrogen of protein are first split off by the body and oxidized, yielding heat and energy before the nitrogen is available for tissue building. The broken-down protein of the cells leaves the body in the form of urea and other compounds excreted by the kidneys. The amount of protein thus lost to the body each day must be replaced by protein foods in order that the body may maintain a protein equilibrium. The amount required is variously estimated by different authorities. However, as a general estimate, about 10 to 12 per cent of the total energy requirement should be furnished by protein foods. Thus in a dietary requiring 3000 calories per day, from 300 to 360 calories should be furnished by protein foods. As each gram of protein furnishes 4 calories, the total amount required would be from 75 to 90 grams.

The average American dietary is very high in protein content, furnishing about 125 grams (about 4 ounces) of protein per day. Protein foods not required for cell growth and repair are not stored in the body, but are eliminated by the kidneys; hence an increased protein diet means increased protein elimination.

Note

Students compute food value of amount of common foodstuffs obtained for ten cents. Compute dietaries of amount of food required by persons of various ages and occupations, by the Percentage Composition Method, estimating cost.

Percentage Composition Method

Dietary for Man doing Light Muscular Work

Calorie Requirement, 3000 to 3500 calories. Protein Requirement, 300 to 420 calories.