Effect of Excess Levels of Individual Amino Acids on Growth of Rats Fed Casein Diets

Abstract

The adverse effects of excess levels of 18 individual L-amino acids were studied with growing rats. When rats were fed a 10% casein diet containing 5% of individual amino acids for 3 weeks, growth depression of varying degrees was produced. With respect to the depression in growth, amino acids tested were divided into three groups and ranked in increasing order of the adverse effect. The first group, producing no or slight growth depression, was: alanine, proline, glutamic acid, leucine and arginine. The second group, which caused a marked growth depression, was: isoleucine, aspartic acid, lysine, valine, threonine, glycine, cystine, serine, tryptophan and histidine. The third group, which caused more severe growth depression, was: tyrosine, phenylalanine and methionine. In general, liver protein, RNA and DNA concentration of these test groups did not differ markedly from those of the control, but total liver protein and RNA content were lower in many of the amino acid-supplemented groups than in the control group. In particular, total protein, RNA and DNA contents in livers of rats fed excess methionine were reduced significantly. When L- or D-methionine was added in graded levels to a 10% casein diet, a sharp fall in growth was produced with increasing levels above 2%, and even the initial body weight could not be maintained with 5% addition of methionine. Toxicity of L-methionine tended to be somewhat higher than that of the corresponding D-form. When glycine was added in graded levels to a 10% protein diet, the growth rate fell gradually with increases above the 3% level of supplementation. The severe depression in growth due to excess amino acids was fairly well or almost completely counteracted by elevation of the protein content of the diet to 25 or 50%, with the exception of the depression produced by excess methionine.