Purified diets: some cautions about casein.

Growth and intake of weanling rats on five casein-based purified diets and matched cereal-based diets were compared. The results support Lát's conjecture that the failure of animals to exhibit normal growth on some purified diets may be attributed to their failure to consume sufficient casein. Given a choice, the rats consumed both types of food, composing a diet in which the ratio of purified to cereal-based diet was 2:1, and they grew normally. Although the growth-promoting characteristics of caseins from different sources varied, the preference ratio between casein- and cereal-based diets was the same with one exception: a reduced preference for purified diet was shown by animals who had previously experienced subnormal growth for an extended period of time on a purified diet. These results show that all caseins are not equivalent in their growth-promoting properties. Further, they show that rats are exquisitely sensitive to the nutritional properties of their diets and, given the opportunity, can compose a diet which yields normal growth.