Short answer
No. Leucine flips the switch that starts muscle building, but in studies of leucine taken on its own, muscle mass and strength did not grow any more. What kept muscle building going after exercise was 25 g of protein, not leucine.
Can leucine alone build muscle?
'Leucine → muscle-building switch ON → more muscle' — does this formula hold?
Leucine is one of the amino acids that make up protein, and it is famous for turning on the signal to build muscle. Some people take leucine powder or pills instead of protein. We checked whether leucine on its own grows more muscle.
Leucine is a switch, that part is true
The muscle-building rate rose, muscle mass did not
An analysis that pooled 9 randomized trials found leucine raised the rate at which muscle protein is built. But lean mass (muscle and everything except fat) and leg lean mass did not differ from the groups without leucine. The switch flipped, yet muscle did not grow more.
Why didn't muscle grow?
Leucine alone did not last
After exercise, 24 people drank either 25 g of whey protein or 6.25 g of whey topped up with leucine to match the 25 g dose. For 1-3 hours, muscle building rose with no difference between drinks. At 3-5 hours, only the 25 g whey drink stayed high (184% vs 55%). The other building blocks had to be there for it to last.
What if you take leucine while training?
No extra strength, no extra muscle
20 people trained for 8 weeks and took 3 g of leucine or a placebo after workouts. Leg strength rose 33% and 37%, and thigh muscle grew 8.9% and 9.6%, with no difference. In 19 people who trained for 12 weeks while eating enough protein, muscle grew 2%, and 7.5 g of leucine a day added nothing.
What about taking it for months without training?
After 3 months, muscle and strength were unchanged
30 people took 2.5 g of leucine with each meal (7.5 g a day) or a placebo for 3 months. Neither group changed in muscle mass or strength. An analysis pooling 17 randomized trials with 1,418 people found leucine alone changed lean mass by just 0.03 kg, with no difference in grip or leg strength.
Bottom line: leucine alone does not grow more muscle
Training and protein did the work
Leucine flipped the muscle-building switch, but in studies of leucine taken on its own, muscle mass and strength did not grow more. What kept muscle building going for 3-5 hours after exercise was 25 g of whey protein. Training and eating enough protein-rich food works better than adding leucine pills.
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