In older men, how much leucine shows up in the blood after eating protein seems to be closely linked to how much muscle their body makes afterward — the more leucine, the more muscle building.

For older guys who lift weights, having a protein shake with 3 grams of leucine right after workouts and before bed doesn’t help build more muscle or strength than just having a placebo drink with the same calories.

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 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.

Older guys who lift weights and take a protein shake after workouts and before bed get the same muscle-building effect as those who take a fake shake with the same calories — so the protein might not actually help at the molecular level.

If you feed adult rats protein from sources like whey or eggs instead of wheat or soy, their muscles grow bigger and repair faster after eating, especially after the first meal of the day.