The leucine threshold
Muscle protein synthesis is triggered largely by leucine, one of the essential amino acids found in protein-rich food. Most research suggests you need somewhere around 2.5–3g of leucine in a single meal to maximally trigger that response — which works out to roughly 25–40g of high-quality protein per meal for most adults, depending on body size and the specific protein source.
Why one big meal isn't as effective as spreading it out
Eating 120g of protein in one sitting doesn't trigger four times the muscle-building response of a 30g meal — once you're past the threshold that meal has already maximised its contribution, and the excess is used for other things (energy, or general amino acid needs) rather than an extra boost to muscle repair. Spreading the same total protein across 3–4 meals through the day, each individually crossing that threshold, generally produces a better result than one or two very large meals.
This threshold rises with age
Older adults generally need a larger dose of protein per meal to trigger the same muscle protein synthesis response — a consequence of the same anabolic resistance that raises overall daily protein needs after age 50. Where a 25-year-old might reliably cross the threshold with 25g of protein, someone in their 60s or 70s may need closer to 35–40g in a single meal to get the same effect.
Putting this into practice
Rather than guessing, aim for a rough per-meal target based on your daily total split across 3–4 meals, and check it against real food combinations. Our 30g, 40g, 50g and 60g protein meal pages show exactly what that looks like with real ingredients, and the protein meal calculator can suggest a combination for any specific gram target.
FAQs
What is the leucine threshold?
It's the amount of the amino acid leucine (found in protein-rich food) needed in a single meal to maximally trigger muscle protein synthesis — roughly 2.5–3g of leucine, which works out to about 25–40g of protein for most adults.
Is it wasteful to eat more than 40g of protein in one meal?
Not wasteful exactly — the extra protein is still used by the body for other purposes — but it doesn't produce a proportionally bigger muscle-building response compared to spreading that protein across another meal instead.
Do older adults need more protein per meal?
Generally yes — due to anabolic resistance, older adults often need a larger per-meal dose (closer to 35–40g) to trigger the same muscle protein synthesis response that a smaller amount would produce in a younger adult.
Want your daily target split into real meals?
HitProtein's Protein Coach can suggest meals sized to hit your target evenly across the day.