Leucine is a branched chain amino acid (BCAA) that has become quite popular, largely because it directly stimulates muscle protein synthesis by activating the mTOR pathway. However, the effects of leucine supplementation on muscle mass and strength gains throughout the course of a resistance training program are not yet fully understood.
This study evaluated the impact of leucine supplementation (two, 5g doses per day) versus a placebo in young, resistance-trained men over a 12-week duration. One dose was consumed in the morning (8am), and the other after training (6pm).
Both groups participated in a lower-body resistance-training program 2x/week for 12-weeks; the program consisted of leg press and leg extensions, and progressive adjustments in volume and intensity were made throughout. Strength was assessed via leg press 1RM, and quadriceps (vastus lateralis) muscle size was measured using B-mode ultrasound.
Both strength and muscle size increased from pre- to post-intervention in both groups, but no significant differences were observed between them. Dietary protein intake pre- and post-intervention remained consistent (~1.7-1.8 g/kg; about double the RDA) for both groups throughout the duration of the study.
The bottom line: These results suggest that additional leucine supplementation does not enhance hypertrophy or strength gains in young, resistance-trained individuals when dietary protein intake is sufficient. Leucine requires the presence of the other essential amino acids in order to maximize muscle protein synthesis, so ingestion of leucine alone probably isn’t going to induce massive benefits in the context of a diet that is already rich in leucine and essential amino acids. So, you should definitely aim to have sufficient protein in your diet, but beyond that, additional leucine doesn’t appear to be necessary.