Leucine

Leucine is one of the nine essential amino acids and the one most directly responsible for switching on muscle protein synthesis. It acts as a signal that enough building blocks are available to build new muscle.

It matters because reaching a sufficient dose of leucine at a meal (the so-called leucine threshold) helps maximally stimulate muscle building, which is why protein quality — not just quantity — matters, and why complete proteins like whey are effective.

In practice, getting enough total protein from leucine-rich sources across the day covers this automatically; it is the reason animal proteins and good plant blends are emphasised over incomplete protein sources.