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.

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

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.

In adult rats, how well their muscles use essential amino acids to build protein depends on whether they get their protein spread out through the day or all at once, and also on how much leucine is in the food they eat.

Taking extra leucine — a supplement found in protein — won’t help healthy, fit young adults build more muscle, get stronger, or recover faster from workouts, no matter how much they take or how long they take it, for up to 12 weeks.

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.