It may be well to point out here the distinction between the amount of protein actually required on the one hand, and, on the other hand, the amount which it may be thought best to allow in the planning of dietaries. The term "requirement" should preferably be applied only to the former; the latter would better be called the protein allowance or the standard for protein. The difference between the amount actually required and the amount which would ordinarily be allowed in planning a dietary is much greater with protein than with fuel value. Surplus fuel is stored as fat, and if excessive fatness is to be avoided, the fuel value of the food must not greatly exceed the energy requirements of the body; but surplus nitrogen is rapidly eliminated from the body and, so long as no injury to health results, leaves no evidence of having been taken in excess of body needs. The eating of a considerable surplus of protein has become habitual, and such a surplus of protein in the food is believed by many people to constitute a desirable "factor of safety," if not indeed to exert a directly beneficial effect upon health and stamina. Hence there is a tendency to set the protein allowance or standard for protein considerably higher than the actual requirement.

If the average daily food requirement of a man at rest be taken as 2000 Calories including 50 grams of protein, the same man at work may require 3000 or 4000 Calories while his actual requirement for protein will not be appreciably increased. If the protein be held at 50 grams while the food is increased from * 2000 to 3000 to 4000 Calories, the protein in percentage of total calories would be in the three cases 10 per cent, 7 per cent, and 5 per cent respectively. Thus it is plain that when the energy requirement is subjected to considerable variations by differences in muscular activity, the protein requirement cannot be taken as constituting a fixed proportion of the total calories, since muscular work increases the energy requirement very greatly and the protein requirement very little if at all. In practice, however, a diet of 2000 Calories would usually contain somewhat over 50 grams of protein; and when the man increased his activity and his total food consumption, he would probably increase his protein intake in almost the same proportion, for he would in most cases simply eat a larger quantity of his usual kind of food.

Moreover, those differences in food requirement which are due to differences in age and size will usually affect the energy requirement and the protein requirement in about the same proportion; and, as the majority of dietaries are planned for family groups, the differences in age and size are usually quite as important as the differences in muscular activity. Thus there is rational basis for the custom of allowing enough protein to furnish from 10 to 15 per cent of the total energy value of the diet.