Dose response curve of MPS in elderly and young muscle with protein ingestion at rest. MPS in the young is stimulated above basal with ~2.5 g of crystalline EAA (found in ~5 g of intact protein) before reaching a plateau at ~10 g of crystalline EAA (found in ~20 g of intact protein). In the elderly, MPS is increased above rest after ingestion of 20 g of whey protein and, like younger adults, the response plateaus thereafter. Star indicates MPS in both young and elderly after 6.7 g of EAA (typically found in 15 g of whey protein) enriched with leucine (41% or ~2.8 g) . Finely dashed lines indicate the hypothesized leucine 'threshold' which must be surpassed in order to stimulate a robust increase in rates of MPS. The threshold may be considerably lower in the young (<1 g leucine in 2.5 g of crystalline EAA's) compared with the elderly (~1.5-2 g of leucine contained in 15-20 g of whey protein).