The leucine trigger for muscle protein synthesis
Leucine is the branched-chain amino acid most directly tied to switching on muscle protein synthesis, acting partly through the mTOR signalling pathway. The threshold concept holds that a meal must deliver enough leucine to cross an activation point before synthesis responds meaningfully. Research often places this practical leucine threshold near 2.5 to 3 grams per meal for younger adults, though the figure is an approximation drawn from controlled feeding studies rather than a fixed biological constant.
Below that range the synthetic response is modest. Once the threshold is reached, adding still more leucine yields diminishing returns, so the dose-response curve flattens toward a plateau rather than climbing indefinitely.
Why older adults may need more
Anabolic resistance, the blunted response to amino acids that tends to accompany aging, appears to raise the leucine threshold, so older adults may need a larger per-meal protein dose to reach the same effect. This is one reason per-meal protein distribution, not just daily totals, is discussed for older populations.
These values are general educational reference points, not personalized targets. Individual requirements depend on body size, training, and health status, and anyone with kidney disease or other conditions affecting protein handling should consult a qualified professional.