More protein means more muscle — but only a little

How much protein healthy adults actually need to build muscle and strength is one of the most contested questions in nutrition. Recommendations vary widely, and it is unclear whether extra protein helps everyone equally, or whether age and resistance training change the picture. It also matters whether any gains show up in outcomes people care about — strength and physical function — rather than only in body composition measurements. This registered systematic review set out to quantify the effect across the whole randomised-trial literature in healthy, non-obese adults.

The authors registered a protocol (PROSPERO) and systematically searched Medline, Embase, CINAHL and Web of Science for randomised controlled trials in which healthy, non-obese adults increased their daily protein intake. Three-level random-effects meta-analyses and meta-regressions were conducted on data from 74 trials. Subgroup analyses split studies by whether participants performed resistance exercise, by age (under 65 versus 65 and over), and by the level of daily protein ingestion. Outcomes included lean body mass, muscle strength and performance on physical and functional tests.

Increasing daily protein enhanced lean body mass gains in studies where participants performed resistance exercise (SMD 0.22, 95% CI 0.14 to 0.30, 62 studies, moderate-level evidence). The lean-mass effect was significant in adults 65 and over ingesting 1.2-1.59 g of protein per kg per day, and in younger adults at 1.6 g/kg/day or more, when combined with resistance training. Lower-body strength gains were slightly higher at intakes of 1.6 g/kg/day or more (SMD 0.40, 19 studies, low-level evidence), and bench press strength was slightly increased in under-65s (SMD 0.18, 32 studies). Effects on handgrip strength were unclear, and effects on physical function tests were only marginal.

This synthesis confirms that extra protein does help, but frames the benefit honestly: the additional gains are small and appear mainly when resistance training is part of the picture. It also extends the evidence by suggesting age-specific intake thresholds, with older adults appearing to benefit at somewhat lower intakes than younger adults. The strength findings rest on low-level evidence, so they are less secure than the lean-mass result. Open questions remain about handgrip strength and functional performance, where the data were unclear or marginal, and several authors declared food-industry relationships.

For a healthy adult who lifts, this is a reason to think total daily protein matters — but as a small amplifier of training, not a substitute for it. The pattern in the data suggests the benefit depends on age and on actually doing resistance exercise, rather than protein working in isolation. It is also a useful corrective to expectations: the additional gains from eating more protein were modest across dozens of trials. The training stimulus remains the main event; protein intake shapes how much you get out of it.