When a young animal is growing there is a constantly increasing need for food, and a progressive increase in consumption is observed. If two animals are fed diets which are alike except for their protein content, or in the nature of their proteins, one may grow because the protein of its food is well constituted, while the other may fail because the protein in its food is deficient in some respect. Comparison of food intake and consequently of protein consumption is not practicable in these animals since the difference in size is becoming constantly more pronounced. Osborne and Mendel have made an elaborate study of food consumption in rats under different dietary restrictions, and their data are of great interest in connection with the present question (8).

They found that rats growing well on a diet containing 9 per cent of casein when a small amount of cystin was added, frequently ate no more food in terms of calories, and ingested no more protein than did rats which were growing no better on even larger percentages of casein without the addition of cystin. They concluded in this instance: "It thus appears that a marked deficiency in any essential ingredient of the diet does not lead to a corresponding compensatory increase in food intake."