"Muscle protein synthesis" is the phrase behind most claims about protein timing, protein shakes and the "anabolic window". About 1,800 searches a month ask what it is. It is not a slogan; it is a measured rate, and the handful of studies that measured it explain most of what is said about protein and training — including where the popular versions go further than the data.
This page sets out how it is measured, what the key studies found and where they disagree. It reports the amounts the studies used; it does not tell anyone how much to eat.
How it is measured
The method has been the same since the 1990s. Volunteers receive a constant infusion of an amino acid — usually leucine or phenylalanine — tagged with a heavy, non-radioactive isotope such as carbon-13. Muscle biopsies taken hours apart show how much of the tagged amino acid has been built into muscle protein. The result is the fractional synthetic rate (FSR): the share of the muscle's protein made new per hour.
The numbers are small. In Witard 2014, resistance-trained men given no protein after exercise had a myofibrillar FSR of 0.041% an hour (PMID 24257722). At that rate, if it held all day, about 1% of the muscle's contractile protein would be remade in 24 hours — our arithmetic, and only an order of magnitude, because the rate rises and falls with meals and training. Breakdown runs at a similar pace in the other direction; muscle grows only when synthesis stays ahead of breakdown over weeks.
Newer studies also use deuterium oxide ("heavy water"), drunk daily, which lets synthesis be measured over days of normal life rather than hours in a laboratory bed.
How long it stays raised after training
| Study | Who | Session | MPS above control |
|---|---|---|---|
| MacDougall 1995 (PMID 8563679) | 6 young men | 12 sets of 6–12 RM arm curls, other arm as control | +50% at 4 h · +109% at 24 h · within 14% (not significant) at 36 h |
| Phillips 1997 (PMID 9252485) | 8 untrained adults (4 men, 4 women) | 8 × 8 leg reps at 80% 1RM, concentric or eccentric | +112% at 3 h · +65% at 24 h · +34% at 48 h |
These two studies are where the "24 to 48 hours" figure comes from. Both are small, both measured people who were untrained or lightly trained, and both measured fasting.
The finding that is usually left out. In Phillips 1997, breakdown rose too, and net protein balance stayed negative at every time point — less negative than at rest (−0.030 versus −0.057 %/h at 3 hours), but below zero. Training without food raised synthesis without making the muscle gain protein. Feeding is what tips the balance positive, which is why most later studies test protein after exercise.
How much protein it responds to
| Study | Who | Protein after exercise | What happened |
|---|---|---|---|
| Moore 2009 (PMID 19056590) | 6 young men, leg exercise | 0, 5, 10, 20 or 40 g egg protein | Synthesis rose with dose and plateaued at 20 g; leucine oxidation rose at 20 and 40 g |
| Witard 2014 (PMID 24257722) | 48 resistance-trained men, ~80 kg, leg exercise | 0, 10, 20 or 40 g whey | +49% at 20 g, +56% at 40 g vs none; the authors judged 20 g sufficient for maximal stimulation; oxidation and urea rose at 40 g |
| Macnaughton 2016 (PMID 27511985) | 30 trained men, whole-body exercise | 20 or 40 g whey | 40 g significantly greater; group means 0.048–0.051 vs 0.059 %/h — about 19% more (our calculation) |
| Trommelen 2023 (PMID 38118410) | volunteers recovering from exercise, quadruple-tracer method | 25 or 100 g protein | 100 g gave a larger response lasting more than 12 hours, with little rise in oxidation |
The "20 g per meal" rule came from the first two studies, which trained one or two leg muscles and measured for four hours. The next two changed two things at once — more muscle trained, longer measurement — and found more protein produced more synthesis. The 2023 authors titled their paper "no upper limit in magnitude and duration"; one author was an employee of a dairy company, which the paper declares, stating the company had no role in the study.
What the four studies together support: the response depends on how much muscle was trained and how long it was measured, and a 20 g ceiling measured over four hours after leg exercise does not describe a whole-body session followed by a large meal. What they do not show is that larger single servings build more muscle over months; that requires training studies, set out on protein for muscle recovery and creatine vs protein powder.
Why a big rise is not the same as growth
The most useful caution in the literature comes from Damas 2016 (PMID 27219125). Ten young men trained their legs for 10 weeks; integrated myofibrillar synthesis was measured with heavy water after sessions in weeks 1, 3 and 10, alongside biopsies for muscle damage and fibre size.
- Week 1: the largest rise in synthesis, and the most muscle damage — but the rise did not correlate with eventual growth.
- Weeks 3 and 10: damage had faded, and the rise in synthesis correlated strongly with hypertrophy (r ≈ 0.9).
- Fibre size increased significantly only at week 10.
In someone new to training, much early synthesis goes to repair. That limits every short study above, most of which measured untrained or newly loaded muscle once. Soreness and damage markers are covered on creatine for recovery and creatine kinase.
What creatine does — and does not do — to it
Creatine is often described as "boosting protein synthesis". The tracer studies that tested it found otherwise:
- Louis 2003, at rest (PMID 12475751): six men, 21 g a day for 5 days; muscle creatine +30%; feeding doubled synthesis and cut breakdown by 40%, but creatine had no effect fasting or fed.
- Louis 2003, after exercise (PMID 12824083): seven men, same loading; muscle creatine +21%; exercise raised myofibrillar and sarcoplasmic synthesis two- to threefold, with or without creatine.
- Parise 2001 (PMID 11509496): 27 men and women, 20 g a day for 5 days then 5 g; no rise in synthesis, but leucine oxidation fell 19.6% in men, not women — a possible reduction in breakdown.
So creatine's lean-mass effect in training trials is not explained by a direct push on synthesis. The usual explanations are more work done in each session and water drawn into muscle, set out on creatine for muscle growth and does creatine make you look bigger.
Collagen against whey
Protein quality matters to synthesis, and collagen is the clearest test. Oikawa 2020 (PMID 31919527) gave 22 healthy older women 30 g of whey or collagen twice daily for six days, with one leg trained. Whey raised both acute and multi-day synthesis; collagen did not raise multi-day synthesis above baseline in either leg. Collagen lacks tryptophan and is low in leucine. Where collagen does have trial support — tendons and joints, not contractile protein — is on collagen vs whey protein and collagen peptides for muscle growth.
What is not established
- The size of the "anabolic window". The timing studies measured hours to two days in untrained people, fasting; how trained lifters on normal diets differ is less well measured.
- A per-meal protein ceiling. The studies disagree because they trained different amounts of muscle and measured for different lengths of time.
- That acute synthesis predicts growth. Damas 2016 says it does not in the first week.
- Any supplement that raises synthesis beyond protein itself. Creatine did not in the tracer studies; the peptide and secretagogue claims are on peptides for muscle growth.
This page reports what the studies measured; it does not tell anyone how much protein or creatine to take.
Sources
- MacDougall JD, et al. The time course for elevated muscle protein synthesis following heavy resistance exercise. Can J Appl Physiol 1995. PMID 8563679.
- Phillips SM, et al. Mixed muscle protein synthesis and breakdown after resistance exercise in humans. Am J Physiol 1997. PMID 9252485.
- Moore DR, et al. Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men. Am J Clin Nutr 2009. PMID 19056590.
- Witard OC, et al. Myofibrillar muscle protein synthesis rates subsequent to a meal in response to increasing doses of whey protein. Am J Clin Nutr 2014. PMID 24257722.
- Macnaughton LS, et al. The response of muscle protein synthesis following whole-body resistance exercise is greater following 40 g than 20 g of ingested whey protein. Physiol Rep 2016. PMID 27511985.
- Trommelen J, et al. The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Rep Med 2023. PMID 38118410.
- Damas F, et al. Resistance training-induced changes in integrated myofibrillar protein synthesis are related to hypertrophy only after attenuation of muscle damage. J Physiol 2016. PMID 27219125.
- Louis M, et al. Creatine supplementation has no effect on human muscle protein turnover at rest in the postabsorptive or fed states. Am J Physiol Endocrinol Metab 2003. PMID 12475751.
- Louis M, et al. No effect of creatine supplementation on human myofibrillar and sarcoplasmic protein synthesis after resistance exercise. Am J Physiol Endocrinol Metab 2003. PMID 12824083.
- Parise G, et al. Effects of acute creatine monohydrate supplementation on leucine kinetics and mixed-muscle protein synthesis. J Appl Physiol 2001. PMID 11509496.
- Oikawa SY, et al. Whey protein but not collagen peptides stimulate acute and longer-term muscle protein synthesis with and without resistance exercise in healthy older women. Am J Clin Nutr 2020. PMID 31919527.
- All abstracts read via PubMed on 2026-10-26.