Amino Acid Imbalance and the Growth Requirements for Lysine and Methionine

Abstract

As the protein level of the chick's diet is increased, the lysine and methionine plus cystine requirements also increase but at a slower rate. Thus, a protein which is slightly deficient in lysine or the sulfur amino acids at the 20% protein level may prove satisfactory when the protein level is increased to 30%. Attempts were made to simulate the effect of high protein levels by adding free amino acids to diets containing barely adequate amounts of lysine or methionine plus cystine. The growth depressions caused by these amino acid excesses were not correctable by raising the methionine level of the diet. The growth-depressing effects of dl-methionine levels above the optimum are overcome by high protein levels. When the diet contains 20% protein, the chick's requirements for methionine and cystine appear to be met by levels of approximately 0.5% and 0.3% of the diet, respectively, or 0.8% methionine alone.