Leucine is an essential amino acid and one of the three branched-chain amino acids (BCAAs), found in protein foods such as meat, dairy and whey. As a supplement it is taken because it may switch on muscle protein building, mainly by lifters and by older adults trying to keep muscle.
Also known as: L-leucine, branched-chain amino acid.
There is no official upper limit. ODS cites studies suggesting an upper safe intake of 500 mg per kg of body weight a day in healthy young and older men, about 38 g a day for a 75 kg (165 lb) man. The studies behind that figure (Elango 2016) found blood ammonia rose above normal at higher intakes, and give a more cautious figure of about 24.5 g a day for a 70 kg older man. These are study protocols, not a recommendation.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling. ODS says leucine might help stimulate protein building in exercised muscle, and that whey protein has more leucine than casein or soy.
What studies show: ODS says taking BCAAs, or any one of them alone, has not consistently been shown to improve performance, build muscle or aid recovery. A 2022 meta-analysis of 17 trials in 1,418 older adults found leucine alone did not improve muscle mass, grip strength or leg press; combinations that included vitamin D did improve grip strength and walking speed slightly.
Safety: the upper-limit studies were short diet studies in men, and their authors say longer studies are needed (Elango 2016). At high intakes leucine raised blood ammonia above normal.
We could not find a single published half-life or time to peak for leucine that applies to a supplement dose, so we do not list one.
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