Study finds no upper limit to muscle protein synthesis after protein intake

A 100-gram hit of isotope-labeled milk protein kept muscle protein synthesis climbing long after a standard shake would have been expected to top out. In a controlled trial of 36 healthy, recreationally active young men aged 18 to 40, all of whom first completed a standardized full-body resistance workout, Jorn Trommelen and Luc van Loon’s team compared 0 grams, 25 grams and 100 grams of protein over a 12-hour recovery window and found a clear dose-response pattern.

The paper, published in Cell Reports Medicine in December 2023, used a comprehensive quadruple isotope tracer feeding-infusion approach to track how dietary amino acids moved into circulation and into muscle tissue. The result was not just a bigger early rise from the larger dose. The 100-gram serving produced a more sustained anabolic response than 25 grams, and the authors reported about 40% higher synthesis rates in the later hours. Their conclusion was blunt: the anabolic response to protein ingestion during recovery from exercise has no apparent upper limit in magnitude and duration in humans, at least under the conditions tested.

That is a direct challenge to one of sports nutrition’s most repeated assumptions, the idea that there is a per-meal ceiling where extra protein stops being useful. The International Society of Sports Nutrition has long described a general target of about 0.25 grams per kilogram of body weight per serving, or an absolute 20 to 40 grams of high-quality protein, with acute doses supplying roughly 700 to 3000 milligrams of leucine. A 2017 review also reflected the prevailing view that 20 to 35 grams of high-quality protein might be enough to maximize the anabolic response in one sitting.

The new data do not erase that guidance, but they do narrow its reach. Previous work often used shorter observation windows and lower protein doses, which made it easier to assume the response flattened early. This study suggests that, after lifting and in young men, a much larger bolus can keep driving muscle-building signals well beyond the usual 3 to 4 hour meal pattern that many athletes have been told to follow.

Later commentary put a hard brake on overgeneralizing the result. A 2024 analysis in the International Journal of Sport Nutrition and Exercise Metabolism argued that the no-upper-limit interpretation does not appear to carry over to resistance-trained young women, underscoring a real sex-specific gap. Older dose-response work also remains relevant, including findings that 30 grams of protein was enough to maximize myofibrillar protein synthesis after endurance exercise. For athletes, aging consumers, and product makers, the practical message is not that more protein is always better; it is that the old 20 to 40 gram ceiling is no longer safe to treat as universal, especially when designing high-dose recovery formats.