Effect of Excess Amino Acids on the Utilization of the First Limiting Amino Acid in Chick Diets

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Abstract
A crystalline amino acid reference standard diet (RS) was modified in various ways to examine the effect of excess amino acids on the utilization of the first limiting amino acid. When a basal diet containing 60% of the amino acids in the RS was supplemented with graded amounts of an amino acid mixture (RS minus lysine), neither weight gain nor efficiency of lysine utilization was adversely affected. In contrast, supplementing a diet containing 50% of the amino acids in the RS with graded amounts of an amino acid mixture (RS minus isoleucine) progressively depressed weight gain and food intake. Nevertheless, gain was commensurate with isoleucine intake. Chicks fed a low nitrogen diet (60% RS with a void of isoleucine) consumed more feed and grew much better than chicks fed a high nitrogen diet (RS minus isoleucine) at comparable concentations of isoleucine at or below 0.4%. Efficiency of isoleucine utilization, as measured by the slope-ratio technique over the linear part of the response curves, was identical in both series. An assay of the same type involving leucine yielded comparable results. Efficiency of isoleucine utilization was found to be identical whether growth was depressed by an imbalanced mixture of amino acids or by an anorexic agent (Quillaja saponin). In all instances reported here, gain in weight was a function of the absolute intake of the first limiting amino acid.