Growth potential, but not body weight or moderate limitation of lysine intake, affects inevitable lysine catabolism in growing pigs.

Inevitable catabolism contributes to the inefficiency of using dietary lysine intake for body protein deposition (PD). This study was conducted to determine the effects of true ileal digestible (TID) lysine intake, body weight (BW), and growth potential on lysine catabolism in growing pigs. Starting at 15 kg BW, 16 female Yorkshire pigs were offered a purified diet providing all nutrients in excess of requirements for maximum protein deposition (PDmax). At approximately 25 kg BW, the pigs' PDmax was determined using the N-balance method. Thereafter, 4 pigs were allocated to each of 4 diets, first-limiting in lysine, providing lysine intakes corresponding to 60, 70, 80, and 90% of estimated requirements for PDmax. The pigs were surgically fitted with catheters in the jugular and femoral veins. Lysine catabolism was determined at 2 BW (40-45 kg, low; 70-75 kg, high) either directly (oxidation) using a primed, constant infusion of l-[1-(14)C]-lysine or indirectly (disappearance) using the N-balance method. There was no effect of BW on the rate (g/d) or fraction of TID lysine intake catabolized. Lysine catabolism decreased with increasing growth potential. Lysine disappearance and lysine oxidation (% of TID lysine intake) were independent of lysine intake, except for the lowest lysine intake level, where they were lower. When lysine catabolism was independent of intake, lysine oxidation based on plasma free lysine specific radioactivity (SRA) was lower (9.9% of TID intake) than lysine disappearance (17.4% of TID intake) or lysine oxidation based on liver free lysine SRA (13.4% of TID intake).