Evaluation of Amino Acid Requirements by Observations on the Chick

Abstract

The level of an indispensable amino acid in the diet bears a linear relation to the rate of growth of the chick, from rapid loss of weight at complete deficiency to maximal growth at complete adequacy of the amino acid. All the curves converge toward a common negative rate of gain at complete deficiencies of any indispensable amino acids, since protein synthesis must stop in all such cases. Net gain of the chick depends upon the balance between protein synthesis and protein catabolism, over the entire range of the curves. While chicks grow better on diets in which the amino acids are supplied chiefly in whole proteins, as compared to diets compounded with amino acids or hydrolysed proteins, the maximal requirements determined with these 2 kinds of diets are in good agreement. This is compatible with the concept that on either kind of diet the chick must synthesize the same body proteins, hence requires the same proportions of amino acids for this purpose. These proportions of indispensable amino acids to each other remain the same for any sub-optimal rate of growth, even in the negative zone of growth. Reasons for the differences in the growth supported by whole proteins as compared to hydrolysed proteins and amino acids are discussed.