Influence of dietary lysine concentration on the oxidation of an indicator amino acid by growing boars.

The influence of dietary lysine concentration on the oxidation of 14C-phenylalanine by growing boars was determined. Forty-five crossbred boars (30 to 40 kg) were fed a ground corn diet fortified with crystalline L-lysine to provide .28, .50, .85, 1.00, 1.25 and 1.54% total lysine. All other essential amino acids were supplemented to provide 135% of NRC (1979) recommendations. Release of 14CO2 from L-[1-14C]-phenylalanine was measured for 1 h following a meal of the experimental diet, which contained 20 mu Ci 14C-phenylalanine. Increasing dietary lysine concentration from .28 to .85% decreased 14CO2 production. Regression analysis of the data using a two-phase linear regression crossover model indicated that phenylalanine oxidation was minimized at a dietary lysine concentration of .65%. It was concluded that a concentration of .65% lysine minimized the oxidation of amino acids and provided them as possible substrates for protein deposition. The oxidation of an indicator amino acid can, therefore, be used to determine the effect of dietary lysine concentration on the partition of amino acids between metabolic fuels and body protein.