Leucine-Enriched Essential Amino Acids in Clinical Practice: A Narrative Review

Muscle wasting driven by aging, chronic disease, or intensive exercise undermines mobility, metabolic health, and survival-yet the people who lose the most muscle are often those least able to eat more protein. Leucine-enriched essential amino acid (LEAA) mixtures combine a leucine signal that switches on muscle protein synthesis with the indispensable amino acids needed to sustain it. This narrative review examines 50 human studies. LEAA mixtures are defined here as oral supplements containing all nine indispensable amino acids with about 40% or more leucine by weight; studies of leucine-enriched branched-chain amino acids, free leucine, and leucine-enriched protein are treated as indirect evidence. The clearest findings are in older adults: 1.5-3 g raised muscle protein synthesis as much as 20-40 g of whey protein, an effect out of proportion to the amount given, and it was the leucine proportion, not the total amino acid content, that determined the response. Adding free leucine to a diet already sufficient in protein gave no further benefit, so the efficiency appears to lie in the combination rather than in leucine alone. Function improved when LEAA was combined with exercise, but exercise was given in all such trials, so the effect of LEAA alone is unclear. Evidence for long-term benefits that patients can feel is still limited, and safety has not been assessed systematically. Delivering the same stimulus in a few grams is therefore the main clinical argument for LEAA, and it now needs testing.