 Leucine and β-hydroxy β-methylbutyric acid, a minor leucine metabolite, exhibit pharmacological activity in humans and have been demonstrated to promote protein biosynthesis via the phosphorylation of the mechanistic target of rapamycin (mTOR).As a food additive, L-leucine has E number E641 and is classified as a flavor enhancer.The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002.