In attempting to set a standard for the amount of protein in the dietary we find no such definite and satisfactory basis for judgment as in the case of total food (or fuel) value. There is no indication that any kind of work necessarily increases the expenditure of protein as muscular work increases the expenditure of fuel, and the body cannot store up protein to anything like the extent that it stores fuel in the form of fat; the feeding of protein above what is required for maintenance increases only slightly the store of protein which the body carries.

* Large as are the appetites of growing children it is not uncommon for the "growth impulse" to outrun the food intake so that the child although always having had access to ample food may as the result of very rapid growth be brought into a condition somewhat resembling that of the young animals described in the preceding chapter (page 338) which become emaciated through "attempting to grow" on rations sufficient only for maintenance, i.e. through the growth of some tissues at the expense of others. As Aron points out a child in this condition has an abnormally low percentage of fat and high percentage of water in his body content. Hence he needs extra food not only to increase his weight up to that which corresponds to his height, but also to restore the normal percentage of fat in the bod} weight which he already has.

When one writer proposes an amount of protein but little above the minimum required for equilibrium, while another advocates a much larger amount, there is implied a difference of view regarding protein such as no longer exists with respect to the energy metabolism. The difference, it is true, is hardly so great as might appear from a casual examination of the proposed standards. It may perhaps be most fairly expressed in terms of the relation between protein and energy in the different standards. Protein would contribute, according to the standards of Voit, Playfair, and Gautier, about 16 per cent of the fuel value of the food; of Atwater, about 15 per cent; of Langworthy, 12 per cent; of Chittenden, 8½ per cent.

It will be of interest to examine some of the arguments which have been advanced in favor of a high protein or of a low protein diet. The following extracts, given in chronological order, are from writings of those who had given special study to the subject and chiefly from the literature of the first decade of this century, when Chittenden's investigation of the protein requirement was a subject of active discussion. The time of publication of these opinions must not be overlooked, since some of the phenomena then attributed to differences in protein intake might perhaps now be attributed, in part at least, to the ash constituents and vitamines of the food.