Cciv. the Biological Value of Proteins. Vii. the Influence of Variation in the Level of Protein in the Diet and of Heating the Protein on Its Biological Value. by Harriette Chick, Margaret Averil Boas-fixsen,

MITCHELL [1924, 2], using young growing rats as experimental animals, found that the biological value of proteins was lessened as the proportion of the protein in the diet was increased. He considered that this result might in part be due to the fact that proteins could be used more economically for maintenance than for growth. When supplied in the diet in a low proportion, almost all the protein would be used for maintenance, whilst, as the level was increased, an increasing proportion would be used for growth. Martin and Robison [1922], working on themselves, found that the value of wheat proteins for the maintenance of nitrogenous equilibrium remained steady when these were present in the diet at different levels; the values they obtained for milk proteins, on the other hand, were most irregular. Boas-Fixsen and Jackson [1932], using adult male rats as experimental animals, found that the biological value of maize protein was increased when its level in the diet was lowered, but Boas-Fixsen [1930] found no progressive change in the nutritive value of a sample of dry-heated, purified caseinogen when the amount present in the diet was raised from 5 to 15 %, the values obtained being uniformly low. The experiments to be described were designed to find out whether the biological value of proteins for maintenance was increased at very low levels in the diet for all proteins, or whether this effect was peculiar to a few. Experiments were therefore planned using diets containing only a low proportion (3-4 %) of the proteins used by Boas-Fixsen and Jackson [1932], viz. wheat, white flour, wheat germ, maize endosperm, milk and lactalbumin, for comparison with the results of their previous experiments with diets containing 5-7 % of these proteins. A series of experiments was also performed with caseinogen in various forms, i.e. "light white" (commercial), heated, heated and purified, and roasted, to determine the effect of heat and purification on its biological value as well as that of variation in the proportion contained in the diet. The levels of the caseinogen in these diets varied from 3 to 10 %. The effect of heat on lactalbumin was also investigated.