The leucine threshold is the per-meal dose of leucine, roughly 2 to 3 grams, needed to fully switch on muscle protein synthesis. Leucine is the amino acid that acts as the on switch: once a meal delivers enough of it, muscle building runs at full speed for a few hours. Meals that fall short trigger a weaker response, even if total daily protein looks fine on paper.
Why it matters
Muscle protein synthesis isn't a dimmer you nudge up with every bite. It's closer to a light switch. Flip it or don't. A meal with 25 to 40 grams of quality protein usually carries enough leucine to flip it. A handful of almonds and a slice of toast doesn't, and no amount of grazing adds up to one proper trigger. This is also why protein quality matters: whey, eggs, meat, and dairy are leucine-rich, while most plant proteins need a bigger serving to reach the same threshold.
How to use it in training
Aim for 25 to 40 grams of protein at each main meal instead of saving most of it for dinner. That range covers the threshold for nearly everyone, with a bit of margin.
Eating mostly plants? Go to the higher end, around 40 grams, or lean on soy, pea protein blends, and tofu, which carry more leucine than most whole plant foods. And older lifters should note the threshold creeps up with age. What triggered growth at 25 may need a larger dose at 60.
Related terms
Go deeper
Want the full breakdown with the actual doses and food examples? Read our guide: The leucine threshold explained.