Leucine supplementation enhances integrative myofibrillar protein synthesis in free-living older men consuming lower- and higher-protein diets: a parallel-group crossover study.

Parallel-group crossover study in 20 generally healthy, community-dwelling older men aged 65-85 years in Hamilton, Ontario, Canada. Participants were assigned to lower-protein or higher-protein habitual intake groups, stayed on a controlled 9-day diet in a free-living setting, and completed the leucine and placebo periods with and without resistance exercise.

Leucine was provided orally as a taste-matched beverage, 5 g per meal, taken midway through each of 3 daily main meals (breakfast, lunch, and dinner) for 3 days (days 3-5 of the treatment period). The active regimen was compared with placebo after an initial placebo period (days 0-2).

Leucine co-ingestion increased integrated myofibrillar protein synthesis versus placebo in both rested and resistance-exercise conditions, with no meaningful difference between the lower-protein and higher-protein groups. In the rested condition, MyoPS was 1.57% ± 0.11%/d with leucine versus 1.48% ± 0.08%/d with placebo (P < 0.001); after resistance exercise, it was 1.87% ± 0.09%/d versus 1.71% ± 0.10%/d (P < 0.001). The authors concluded that leucine also amplified the anabolic response to resistance exercise and may be a practical strategy to help maintain muscle mass in older adults. No adverse effects of leucine were reported.

Small sample size with 10 participants per active arm, short 3-day supplementation window, and a narrow population of healthy older men limit generalizability. The study did not assess longer-term outcomes such as changes in muscle mass or physical function, and results may not extend to women or to older adults with major comorbidities.