Influence of Amino Acid Level in the Diet upon Amino Acid Oxidation by the Rat

A series of experiments was conducted to develop and evaluate a nondestructive method for estimating the amino acid requirements of the growing rat. Rats fed graded levels of lysine were injected with radioactively labeled lysine via heart puncture. The production of radioactive carbon dioxide was used as an index of the amount of lysine oxidized. Results obtained indicated that the oxidation of lysine did not increase markedly until the dietary lysine intake was increased above that level at which average daily gain and gain/feed were maximal. This level was approximately 115 mg lysine consumed per day for a 200-g rat fed ad libitum. The adaptation of the oxidative mechanisms in the rat to changes in the dietary amino acid intake was shown to be relatively rapid. The lysine requirement expressed in terms of daily lysine consumption did not appear to change with time for a period of up to 28 days. The oxidation of a second amino acid, methionine, by rats fed graded levels of lysine was shown to be independent of lysine intake. It was concluded that the oxidation technique provides a means for estimating the dietary amino acid requirement for a specific growth rate in the growing rat.