Leucine is the amino acid most closely tied to the muscle protein synthesis signal, acting through the mTOR pathway that regulates protein synthesis in skeletal muscle.
The 'leucine threshold' idea has direct human support in older adults: in a controlled trial, a 6.7g essential amino acid dose supplying about 1.7g leucine did not raise muscle protein synthesis in older adults but did raise it in young adults, while a version supplying about 2.8g leucine raised it in both.
Leucine is more abundant in animal protein than in plant protein.
The acute synthesis signal is reproducible, but it does not carry through to long-term lean mass or strength when leucine is supplemented on its own, and pooled analyses suggest it may add nothing when total protein intake is already adequate.
Where combination products supplying leucine, whey protein, and vitamin D have been tested in sarcopenic older adults, the between-group gains have been small.