If a man at moderately active work takes a diet which furnishes 3000 Calories and 75 grams of protein, he is taking 10 per cent of his calories in the form of protein. Of course the protein requirement cannot bear a fixed relation to the calorie requirement, since the latter is largely influenced by activity, while the former is not. Most men, when at complete rest, would require more than 10 per cent of their calories in the form of protein because the lack of exercise would not reduce the protein requirement to the same extent as the energy requirement. On the other hand, most Americans are accustomed to take more than 10 per cent of their calories as protein regardless of whether they require it or not. If, then, the active man's need for protein is met by supplying him with 10 per cent of his needed calories in the form of protein, this will serve as a convenient starting point in considering the requirements of a child. Let this be compared with the normal dietary of an infant. Human milk averages about 1.6 per cent protein, 4.0 per cent fat, 7.0 per cent carbohydrate. Here about 9 per cent of the calories are taken in the form of protein, or about the same proportion as has been allowed for the full-grown active man. Furthermore Hoobler has shown experimentally that this is as high a proportion of protein as the infant will utilize with the highest efficiency in growth of body tissue. During the suckling period the growth is relatively more rapid than at any other age. Mendel * gives the following figures:

The Relative Daily Gain In Body Weight Of Children

In the first month is about......... 1.00 per cent.

At the middle of the first year........ 0.30 per cent.

At the end of the first year......... 0.15 per cent.

At the fifth year ............ 0.03 per cent.

Maximum in later years for boys.............. 0.07 per cent for girls .............. 0.04 per cent.

If, then, the full-grown man and the child at the time of most rapid growth each requires but 10 per cent of his calories in the form of protein, it seems probable that this proportion is also sufficient for any intermediate age, if the diet is of ample fuel value, and the protein is of the right kind. But the proper selection of the protein is of very great importance in the feeding of children, who differ from most other young mammals in that their period of growth is so many times longer than the suckling period. Even the child that is nursed for a year and attains three times his birth-weight before weaning will still have much the greater part (probably five sixths) of his growth to make on other food. By the time growth is complete he will probably have about twenty times the body weight and more than twenty times the body protein with which he was born.

Growth at the normal rapid rate of early childhood involves the conversion of a very considerable part, sometimes as much as one third, of the protein of the food into body protein. This can be accomplished to the best advantage only when (1) the protein of the food is largely of the kind most efficient in supporting growth,i.e. milk protein; (2) the protein is well "protected" by the protein-sparing action of liberal amounts of carbohydrate and fat.

* Childhood and Growth, p. 18.

That the child needs a diet of high fuel value to meet the requirements of his energy metabolism has already been pointed out (Chapter VII (Conditions Governing Energy Metabolism And Total Food Requirement. Basal Metabolism Of The Adult)). It is because the high protein requirement of childhood (for young children more than twice as much per unit of weight as for adults) is paralleled by an equally high energy requirement that the diet of the child need not contain a higher percentage of its calories in the form of protein than does the ordinary diet of the adult, if the protein for the child is well chosen.

Usually, however, a well-planned dietary for a child will show a somewhat more than average proportion of its calories in the form of protein because after weaning the main feature of the child's diet should be cows' milk which furnishes about 19 per cent of its calories in the form of protein. A child, fed mainly upon cows' milk and taking enough food to amply cover his energy requirement, will therefore receive a safe surplus of protein in the best available form. With a full quart of milk in the daily dietary of the growing child the other foods may be selected chiefly with reference to other qualities than their protein content; without a liberal use of milk the proper feeding of a growing child becomes a very difficult problem.

Having discussed the protein requirements of ordinary adult maintenance and of growth, the requirements of the aged should also be considered. This does not require extended discussion, since advancing age involves no new features but only a gradual modification of those pertaining to middle life.

In general, elderly people show a somewhat diminished protein requirement and likewise a diminished power of dealing with excess. Surplus protein taken in the food is not so rapidly absorbed and catabolized, and, while there appears to be no essential difference in the form in which the nitrogen is finally excreted, the susceptibility to excessive putrefaction of protein appears to be increased. It would seem that in the dietary of the aged the protein should be reduced to at least as great an extent as are the calories.