Muscle protein synthesis (MPS): the leucine threshold, the per-meal dose, and what changes with age

What it means

Muscle protein synthesis is the rate at which muscle cells assemble amino acids into new contractile protein. It rises after a protein-containing meal and after resistance exercise, stays elevated for a few hours (meal) to 24–48 hours (training), and falls back. Muscle grows when synthesis exceeds breakdown over days and weeks.

The trigger is the amino acid leucine acting through mTOR: about 2.5–3 g of leucine in a meal — roughly 25–40 g of high-quality protein — switches synthesis on fully. Less than that produces a smaller, shorter response; more does not add much per meal.

Why it matters for longevity

Older muscle shows "anabolic resistance": the same protein dose produces a smaller synthesis response, which is why sarcopenia develops even in people who eat "enough" protein by old guidelines. Understanding MPS turns protein from a daily total into a per-meal dose and a timing question — and makes resistance training the amplifier that restores the response.

What the evidence says

Moore 2009 and Witard 2014 established the dose-response in young men: MPS rose with protein up to about 20 g of egg or whey per meal and plateaued. Moore 2015 showed older adults needed about 0.4 g/kg per meal (roughly 35–40 g) to reach the same response — about twice the young threshold. Resistance training sensitises muscle to protein for at least 24 hours (Burd 2011).

Distribution matters: Mamerow 2014 found that spreading 90 g of protein evenly over three meals produced 25% more daily synthesis than loading it at dinner. Pre-sleep casein (40 g) raises overnight MPS (Res 2012). Total daily intake still sets the ceiling: meta-analysis puts the training-plus-protein benefit saturating around 1.6 g/kg/day (Morton 2018).

How to measure or use it

• MPS itself is measured only in research (stable-isotope tracers). The practical proxies are strength and DEXA lean mass over 12 weeks.
• The protocol the evidence supports: 1.6–2.2 g/kg/day, split over three to four meals of 30–50 g with 2.5–3 g leucine each (dairy, meat, fish, eggs, soy, or a leucine-enriched plant blend), one of them within a few hours of training.
• After 50, aim at the upper end per meal; creatine and resistance training are the two amplifiers with evidence.

Related

• Muscle mass and ageing — the full article
• The lean-mass protocol — MPS applied to a plan
• CJC-1295 / ipamorelin — a research peptide studied for the GH axis