, an essential and branched chain amino acid, has been recently shown to play a pivotal role in turning on protein synthesis, especially after resistance exercise.
Although leucine makes up only a fraction of the body's proteins, its levels are known to decline by about 30% during strength training and 11 to 33% during fat burning cardio sessions (i.e., aerobic exercise), making leucine a "must have supplement" for athletes.
Although the recommended dosing of leucine varies from study to study, it is reported that supplementation with leucine at 50 mg/kg bodyweight (split into two doses and taken immediately before and after exercise) prevents the decrease in blood levels during intense strength training. However, most athletes report taking much larger doses based on a "more is better" mentality and shotgun approach to supplementation. As with most supplements, throwing caution to the wind is not the safest or economical means to muscular development. Nevertheless, in a recent registered clinical trial, published in the American Journal of Clinical Nutrition, researchers sought to determine the safe tolerable upper limit for leucine intake in healthy adults aged 20 to 35 years old. Subjects were given graded increases in leucine intake from 50 mg/kg/day up to 1250 mg/kg/day and based on blood and urinary analysis the researchers concluded that 500 mg/kg/day was the safe upper limit.
Elango R, Chapman K, Rafii M, Ball RO, Pencharz PB. Determination of the tolerable upper intake level of leucine in acute dietary studies in young men. Am J Clin Nutr. 2012 Oct;96(4):759-67. Epub 2012 Sep 5.