BLOOD AMINO ACID STUDIES: V. DETERMINATION OF THE LIMITING AMINO ACID IN DIETS

Diets containing limiting amounts of various amino acids were fed to male weanling rats for 3–8 days. Levels of plasma lysine, methionine, threonine, and tryptophan were then determined. Comparisons were made of methods for utilizing plasma amino acid (PAA) data to predict the limiting amino acid in the diet. A modified PAA ratio method appeared reasonably satisfactory but dividing the PAA pattern of rats fed the test diets by that of rats fed an egg diet gave erroneous results. The PAA pattern of rats on the test diets expressed as a percentage of that of fasted rats appeared to give the most satisfactory results. By this method, designated as the PAA score, lysine was limiting in egg or oatmeal, cystine in casein, methionine in peas, and tryptophan in a mixture of corn and fish flour. PAA scores and weight gain data of rats given whole wheat bread with and without supplementary amino acids indicated that lysine was the first and threonine the second limiting amino acid in this diet. In other studies, rats given diets of corn meal with and without supplements of amino acids did not gain weight during a 7 day test. The PAA score, in agreement with reports in the literature, indicated that lysine and tryptophan were about equally limiting in corn. It was concluded that the PAA score correctly predicted the limiting amino acids in each of the diets tested.