Leucine is one of nine essential amino acids and the primary trigger for muscle protein synthesis (MPS) in humans. Of the three branched-chain amino acids (BCAAs), leucine alone is responsible for most of the MPS-stimulating effect; isoleucine and valine are secondary.
The threshold concept comes from research showing that MPS response per meal is non-linear. Below a certain leucine dose, the response is muted. Above the threshold, response saturates and additional protein in the same meal does not produce proportionally more synthesis. The number that emerges from this research is roughly 2.5-3 g of leucine per meal as the saturation point.
Translating leucine grams into food protein grams:
- Whey protein: ~10-12% leucine. 25 g whey delivers ~2.5-3 g leucine. Hits the threshold cleanly.
- Casein protein: ~9-10% leucine. 28-30 g casein delivers the threshold.
- Beef, chicken, fish: 7-9% leucine. 30-40 g of meat protein delivers the threshold.
- Eggs: ~8% leucine. About 3-4 large eggs (or roughly 25 g protein) delivers the threshold.
- Soy, tofu, tempeh: 6-8% leucine. Slightly more total protein needed (35-45 g) for the same effect.
- Other plant proteins (rice, pea, hemp): variable, often 5-7%. Combining sources or using slightly larger doses (40-50 g) closes the gap.
Practical implications:
- Plan 25-40 g of protein per meal across 3-5 meals for general muscle preservation and growth goals. This consistently meets the leucine threshold from animal sources and is close enough from plant sources.
- A small snack with 8-10 g of protein does not maximally stimulate MPS even if it tastes high-protein. The leucine dose is too low.
- BCAA supplementation alone does not maximize MPS because the other essential amino acids must also be present. Whole protein sources beat isolated BCAAs.
- Older adults benefit from slightly higher per-meal protein (often described as anabolic resistance). 35-50 g per meal compensates.