As explained in earlier chapters (text and figures, pages 55-68 and 224-226), it was shown by Osborne and Mendel that with a diet adequate in all other respects any one of a number of purified proteins such as casein, lactalbumin, or edestin might serve as the sole protein both for maintenance and for growth, while gliadin as sole protein food sufficed for maintenance but not for growth, and zein as sole protein did not suffice even for maintenance. Gliadin contains adequate tryptophane but only about 1 per cent of lysine; addition of lysine to the gliadin ration made it adequate for growth. Zein contains neither tryptophane nor lysine; addition of tryptophane to the zein diet makes it adequate for maintenance; addition of both tryptophane and lysine makes it adequate for growth.

When "adequate" proteins were fed in progressively restricted amounts, i.e. in diminishing percentage of the food mixture, Osborne and Mendel found that with different proteins different amino acids prove to be the limiting factors -e.g. lysine in the case of edestin, cystine in the case of casein. With 15 to 18 per cent of casein in the food mixture the rate of growth was normal; with 9 to 12 per cent of casein the rats grew more slowly but normal rate of growth was resumed upon adding 3 per cent of cystine to the food mixture. With only 4.5-6 per cent of casein the addition of the 3 per cent cystine did not make the growth normal, indicating that with casein reduced to this point the supply of some other amino acid had become insufficient.*

Another case in which cystine appears to have been a determining factor in tissue growth has been recorded by Evvard, Dox, and Guernsey in connection with their feeding experiments upon pregnant swine. Here a difference in the hair coats of the new-born pigs appeared to be due to the different intake of cystine in the food protein consumed by the mother, hair being rich in sulphur, and cystine the sulphur-bearing amino acid of the food.

A so-called incomplete protein, i.e. one which when fed alone is quite inadequate to meet the requirements of protein metabolism, may nevertheless contribute toward these requirements to an important degree and may even play a prominent part in promoting growth, as was strikingly demonstrated by Osborne and Mendel in experiments in which they added zein to a ration containing a small percentage of lactalbumin. (See Fig. 4, page 66.) Here the addition of zein to the ration more than doubled the rate of growth. Still more recently McCollum, Simmonds, and Pitz, feeding rats on rations composed of a single grain with supplementary additions, find that gelatin supplements wheat proteins excellently though it apparently does not appreciably improve the proteins of maize or oats. Since gelatin, although lacking tyrosine and tryptophane is relatively rich in lysine, these results are interpreted as indicating that lysine is probably the limiting factor in wheat proteins but not in the proteins of the maize or of the oat kernel.

* Journal of Biological Chemistry, Vol. 20, page 351.

In view of such evidence it is important to guard against the erroneous impression that incomplete proteins are useless for growth. The illustrations just given show that the growing organism may use such proteins to extremely good advantage; but the "incomplete" proteins must not be permitted to displace the "complete" proteins to too great an extent if the young organism is to grow and develop at a fully normal rate.

When growth is retarded by inadequate intake of protein or of a particular amino acid, the emaciated appearance characteristic of animals attempting to grow on an insufficient energy intake is not to be expected. Osborne and Mendel have recorded numerous cases of suspension of growth of young rats, especially when kept on rations containing gliadin as a sole protein food. Here the inadequacy of the lysine intake results in retardation or even complete suspension of growth, but the animal may remain quite healthy and symmetrical. Moreover rats may be subjected to this type of stunting for a remarkably long time (even as long as would normally cover the entire growth period) and still retain their capacity to grow when given an adequate diet.

In some cases * "after periods of suppression of growth, even without loss of body weight, growth may proceed at an exaggerated rate for a considerable period. This is regarded as something apart from the rapid gains of weight in the repair or recuperation of tissue actually lost. Despite failure to grow for some time the average normal size may thus be regained before the usual period of growth is ended." Statistical studies on children also indicate that retardation in early growth can usually be made up by extra rapid growth later.*

* Osborne, Mendel, Ferry, and Wakeman. American Journal of Physiology, Vol. 40, pages 16-20 (1916).

Mendel and Judson have studied the influence of different types of protein stunting upon the composition of the body in the case of the mouse. They find that when abundance of fat is furnished in the diet, but not enough protein to maintain normal growth, the percentage of fat in the animal becomes greater than when the food contains an adequate amount of protein with the same proportion of fat. They suggest that: "There seems to be a tendency to protect the limited amount of protein by increasing the available supply of fat in the body." "This does not occur when growth is arrested by lack of lysine, as in the use of gliadin as the only protein in the diet, since in this case the limiting factor lies not in the amount but in the nature of the protein."