Little can be said with confidence regarding the best amount of protein for children after the nursing period. In practice well-planned dietaries for children usually contain between 10 and 15 per cent of the total energy in the form of protein. During the years of rapid growth a considerable fraction of the protein of the food is utilized in the synthesis of body proteins ; and since the amount of food protein required to form a gram of body protein is variable, depending upon the amino acid make-up of the former, it is evident that the kind of protein supplied becomes a matter of great importance. Here chemical and physiological laboratory evidence, clinical experience, and its evident place in nature all indicate plainly the superiority of milk as source of supply of protein for growth, whether the case be that of the growing child after weaning or of the nursling fed through the mother. The recommendation that family dietaries should whenever possible include "a quart of milk a day for every child" was aimed primarily to insure an appropriate protein supply. Needless to say, the milk also supplies important amounts of many other substances essential to growth.

Since the energy requirement is greatly increased by muscular activity and the protein requirement is not, it is evident that in the metabolism of normal adults the energy and protein requirements will not run parallel. The protein requirement of the healthy adult depends chiefly upon his size, while his energy requirement depends chiefly upon his activity.

In childhood both the energy requirement and the protein requirement are high - often two to three times as high per unit of weight as for adults without muscular work. Moreover the high protein and energy requirements of the child as compared with the man are found to run approximately parallel and as shown in a previous chapter the same proportion of pro-tdn in terms of the total energy which seems rational for the adult dietary suffices also for the food requirements of the child provided in the latter case the food is of appropriate kind.

In most family groups the differences in age and size will constitute a more prominent factor than the differences in activity, and since the former affect energy and protein requirements in about the same proportion, it becomes feasible and convenient to set the protein allowance for ordinary family groups in terms of a proportion of the total food value. To allow for varying conditions and for individual preferences as well as to provide a liberal margin for safety it is customary to consider that from 10 to 15 per cent of the total calories may be in the form of protein.

In cases where the nutritive requirements of growth, pregnancy, or lactation are to be met, the kind of protein is perhaps as important as the amount.