Leucine, along with isoleucine and valine, is one of the three branched chain amino acids (BCAAs). Out of all of the other amino acids, leucine is the most potent activator of protein synthesis. Cells are able to sense leucine levels, and in response turn on protein synthesis via the enzyme protein mammalian target of rapamycin (mTOR), a master-regulator of protein synthesis.
Given the well-established protein-synthesis activating properties of leucine, a number of human trials have been conducted to determine whether adding supplemental leucine to various protein sources can augment muscle protein synthesis (MPS).
Results in elderly individuals have been consistently positive. Anabolic resistance is a well-established phenomenon during aging, with cells becoming more resistant to turning on protein synthesis. This can be partly ameliorated with extra leucine, as research indicates that older adults need twice as much leucine compared to younger adults for similar activation of muscle protein synthesis (MPS).
This increased leucine requirement for MPS in the elderly can be partially explained by increased retention of orally ingested leucine in the gut, which is retained twice as much compared to young adults, limiting the amount of leucine that makes it into the blood stream. Resistance to leucine-stimulated protein synthesis also occurs at the cellular level in aging cells.
Studies on leucine supplementation in younger adults have been positive, with some caveats. In a study that compared a low (6 g) dose of whey protein alongside 3 or 5 g of extra leucine to 25 g whey protein alone, the 6 g whey plus 5 g leucine meal stimulated MPS to a similar, but lesser extent than 25 g whey protein. Another study comparing the same 25 g dose of whey protein to 6 g whey with an amount of leucine equivalent to 25 g of whey found that only the 25 g whey protein group had elevated MPS 3 to 5 hours post-exercise in young adults. Taken together this works suggests that supplemental leucine can augment MPS in the context of limited total protein, but a 25 dose of whey alone is likely better, at least in non-elderly adults.
Research suggests that supplemental leucine in older adults can augment muscle protein synthesis. For non-elderly adults, leucine can increase MPS when overall protein intake is limited. Additional leucine will not likely increase MPS in non-elderly adults beyond that obtained with moderate amounts (20+ g ) of whey protein.