Leucine is a key regulator that stimulates catabolism of branched-chain AA (BCAA; i.e., Leu, Ile, and Val) in skeletal muscle and liver. If diets fed to pigs contain excess Leu, catabolism of all 3 BCAA may increase because of the stimulating effect of the Leu metabolite, α-keto isocaproate, on the branched-chain α-keto acid dehydrogenase enzyme complex, which is responsible for degradation of the 3 branched-chain α-keto acids that originate from metabolism of the 3 BCAA. Serum Ile and Val concentrations were reduced by excess dietary Leu in growing pigs, and high dietary Leu reduces feed intake and growth performance in pigs, which may be a result of the imbalanced supply of BCAA that result from increased metabolism of Val and Ile. Recent data confirmed that excess dietary Leu reduced growth performance and tended to reduce protein synthesis, which is likely a result of reduced availability of Val and Ile.