Use of Free Plasma Amino Acid Levels for Estimating Amino Acid Requirements of the Growing Rat

The validity of using plasma amino acid levels for estimating amino acid requirements of the growing rat was investigated. A series of diets was prepared (equivalent to 10% protein, provided by whole egg and an amino acid mixture simulating egg proteins) having graded levels of the amino acid under test. These test diets were fed to groups of 10 male Wistar rats, 28 to 30 days old, for 3 days. The amino acid under test was determined in the plasma of individual rats by microbiological assay. The dose-response curve relating plasma level and dietary level of the test amino acid was drawn. The requirement was considered to be the dietary level at which the corresponding plasma level was equal to the normal fasting level. Requirements expressed as percentage of diet were estimated to be: lysine, 0.65; tryptophan, 0.13; threonine, 0.42; leucine, 0.82; isoleucine, 0.50; and histidine, 0.20. Estimates of amino acid requirements of the rat were in close agreement with average values obtained by other methods reported recently in the literature.