[9] The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002.

Long-term leucine supplementation does not increase muscle mass or strength in healthy elderly men.

must be factored into the analyses to isolate the effects of supplemental leucine as a stand-alone, or if taken with other branched-chain amino acids (BCAAs).

Until then, dietary supplemental leucine cannot be associated as the prime reason for muscular growth or optimal maintenance for the entire population.

[18][19] Leucine toxicity, as seen in decompensated maple syrup urine disease, causes delirium and neurologic compromise, and can be life-threatening.

[22] As such, unofficially, a tolerable upper intake level (UL) for leucine in healthy adult men can be suggested at 500 mg/kg/d or 35 g/d under acute dietary conditions.

[22][23] Leucine is a dietary amino acid with the capacity to directly stimulate myofibrillar muscle protein synthesis.

[35] The vast majority of L-leucine metabolism is initially catalyzed by the branched-chain amino acid aminotransferase enzyme, producing α-ketoisocaproate (α-KIC).

[31][35] HMB-CoA is metabolized by either enoyl-CoA hydratase or another uncharacterized enzyme, producing β-methylcrotonyl-CoA (MC-CoA) or hydroxymethylglutaryl-CoA (HMG-CoA) respectively.

[37][35] Leucine is an essential amino acid in the diet of animals because they lack the complete enzyme pathway to synthesize it de novo from potential precursor compounds.

Leucine is a branched-chain amino acid (BCAA) since it possesses an aliphatic side chain that is not linear.