Leucine

Leucine is an essential amino acid that stimulates muscle protein synthesis.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 222.} It’s one of three branched-chain amino acids (BCAAs) and apparently works by activating mTOR, a signal the other two BCAAs barely affect.^{1Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 222.} It also reduces muscle protein breakdown.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 223.}

Leucine triggers synthesis, but sustaining it depends on the other essential amino acids.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 223.} The leucine threshold describes the blood leucine level needed to maximally trigger synthesis.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 223.} In young, healthy people, that takes about 2 g of leucine, roughly 20 g of high-quality protein such as whey or egg.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 223.}

In practice

• A protein dose with 2 to 3 g of leucine, or 0.05 g/kg, maximally stimulates muscle protein synthesis in younger adults.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 228.} Leucine content, and possibly how fast it arrives, appears to set the acute response.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 228.}
• Larger servings can make up for low leucine content: rice protein matched a higher-quality protein once the two were equal in leucine.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 228.}
• Older clients need about twice the leucine per serving.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 223.} That works out to roughly 40 g of high-quality protein per meal, against about 20 g for younger people.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 232.}
• Leucine past the threshold adds no further benefit to synthesis, and leucine supplements alone haven’t increased protein gain in animal studies; clients need the full set of essential amino acids.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 223.}