Critical evaluation of a method for estimating amino acid requirements for maintenance in the rat by measurement of the rate of14C-labelled amino acid oxidation in vivo

Abstract

1. The object of the experiments was to estimate the maintenance requirements for lysine and leucine by a radioactive method. Rats were given a single dose of¹⁴C-labelled lysine or leucine and groups of animals were killed after 15, 20 and 30 d.2. After 20 d the specific radioactivity (SR) of protein was approximately the same in liver, muscle and viscera; it was somewhat lower in skin. Once uniform SR is achieved, the rate of loss of radioactivity is a measure of the rate of endogenous loss of the amino acid.3. The rate of loss between 20 and 30 d was measured in two ways: from the daily output of expired¹⁴CO₂, and from the decrease, over the 10 d interval, of the total amount of radioactivity retained in the body.4. For the first 15 d after administration of the labelled amino acid, all rats were given a low-protein or low-amino acid diet on which body-weight was maintained constant. For the second 15 d period some rats were kept on this diet; others were transferred either to a protein-free diet or to a diet lacking the specific amino acid (lysine or leucine) which had been administered in the labelled form.5. The fractional rate of amino acid loss in the different experiments ranged from 1.5 to 3.5%/d, being greatest with the protein-free diet. The absolute rates of loss were calculated from measurements of the total lysine and leucine content of rats.6. The best estimates of the rate of endogenous amino acid loss obtained in this way, expressed as mg/kg^0.75per d were: lysine 136, leucine 80. These estimates are higher than most estimates of maintenance requirements obtained by growth or nitrogen balance methods and possible reasons for these discrepancies are discussed.