The hypothesis that excess dietary Leu affects growth performance and metabolism of branched-chain amino acids (BCAA) in growing pigs was tested. Forty barrows (30.0 ± 2.7 kg) were placed in metabolism crates and randomly allotted to 5 diets that contained 100, 150, 200, 250, or 300% of the requirement for standardized ileal digestible Leu. Initial and final body weight of pigs and daily feed provisions were recorded. Urine and fecal samples were collected for 5 d to measure N balance and biological value of diets. At the conclusion of the experiment, blood, brain, liver, and muscle samples were collected and average daily gain (ADG), average daily feed intake (ADFI), and gain to feed ratio (G:F) were calculated. Orthogonal polynomial contrasts were used to determine linear and quadratic effects of increasing Leu in the diets.

Results indicated that ADG, ADFI, and G:F decreased (linear, P < 0.05) as dietary Leu increased (Table 1). A trend (linear, P = 0.082) for decreased N retention and decreased (linear, P < 0.05) biological value of protein was also observed. Plasma urea N increased (linear, P < 0.05) and a quadratic reduction (P < 0.05) in plasma serotonin and a linear reduction (P < 0.05) in cerebral serotonin were observed with increasing dietary Leu. Concentrations of BCAA in liver increased (linear, P < 0.001), concentrations of BCAA in muscle decreased (linear, P < 0.05), concentration of α-keto-isovalerate was reduced (linear and quadratic, P < 0.001) in liver, muscle, and serum, and α-keto-β-methylvalerate was reduced (linear and quadratic, P < 0.001) in muscle and serum, whereas α-keto-isocaproate increased (linear, P < 0.05) in liver and muscle, and in serum (linear and quadratic, P < 0.001) with increasing dietary Leu.