In this double-blinded and placebo controlled study, healthy untrained men were assigned to one of two groups and given either leucine (50 mg/kilogram bodyweight/day, equaling 4 g/day) or a placebo. On training days subjects were instructed to take the entire daily dose immediately after training and on non-training days split the dose in 3 equal servings (to be taken in the morning, afternoon, and evening). Strength (sum of 5 rep max for 8 exercises) and body composition (using DXA) were tested before the training/supplementation program started and again after 12 weeks. The researchers reported that both groups saw similar gains in lean mass and body fat loss. However, those who received leucine during the training period had a 10% greater increase in strength compared to the placebo group. It was concluded that 4 g/day of leucine enhances strength performance during a 12 week resistance training program in previously untrained males.
Ispoglou T, King RF, Polman RC, Zanker C. Daily L-leucine supplementation in novice trainees during a 12-week weight training program. Int J Sports Physiol Perform. 2011 Mar;6(1):38-50.