If you're already eating enough protein, taking extra leucine won't help you build more muscle.

Taking extra leucine (a protein building block) won’t help healthy, trained young adults build more muscle from weight training, even if the dose is big enough to trigger a key muscle-growth signal in the body.

Taking extra leucine pills won’t help you build more muscle, get stronger, or recover faster from weight training if you’re already eating enough protein.

Leucine is a type of amino acid that scientists think helps your muscles start growing right after you eat it, but we’re not sure if taking extra leucine over time actually makes you stronger or bigger muscles in the real world.

Taking leucine supplements won’t help young athletes build more muscle, get stronger, or recover faster—even though leucine is known to trigger a muscle-growth signal in the body.

If you're a young man who already eats enough protein and works out with weights, taking extra leucine won't help your muscles grow any more — your muscles just can't use more than a certain amount, no matter how much you give them.