Leucine: the switch that turns on muscle growth

Leucine is one of the three branched-chain amino acids. Beyond being a building block, it acts as a signal: it activates mTOR, the pathway that switches on muscle protein synthesis (MPS).

The leucine threshold

Research suggests roughly 2.5–3 g of leucine per meal is needed to maximally stimulate MPS in young adults — and older adults may need a little more because of “anabolic resistance.”

How much food is that?

• Whey protein isolate: ~1 scoop (30 g) ≈ 3.0 g leucine
• Chicken breast: ~120 g cooked ≈ 2.6 g leucine
• Firm tofu: ~200 g ≈ 2.6 g leucine
• Cooked lentils: ~400 g ≈ 2.5 g leucine

Plant sources can reach the threshold — they just need bigger portions or a blend. Every food page in the ProteinMD Encyclopedia shows leucine per 100 g and per serving, and the Meal Builder has a live leucine meter.