Editorial NexoScience
A protein shake made from peas. Another made from whey. Same calories, same grams of protein, same workout. To your muscles, according to the largest comparison of its kind, they are not quite the same thing.
Researchers at Queen’s University Belfast pooled data from 43 randomized controlled trials that pitted plant protein directly against animal protein, matched for calories and protein content, and tracked what happened to people’s muscle mass, strength, and physical performance. Thirty of those trials, covering 1,538 participants, had enough comparable data to combine into a single pooled analysis. It is the most complete side by side comparison of protein source and muscle health published to date, and it also breaks a common flaw of earlier work: most previous reviews only tested soy against animal protein, ignoring the growing list of other plant proteins people actually use, like pea, rice, oat, and chia.
Animal protein came out slightly ahead for building muscle mass. Across all 30 pooled trials, people on animal protein gained more muscle than people on plant protein, a small but statistically real difference (standardized mean difference of -0.20, 95% CI -0.37 to -0.03, P = .02). It is not a dramatic gap. On the scale researchers use, anything below 0.2 counts as a small effect, and this result sits right at that boundary. But with heterogeneity across so many different trial designs, the pattern held up.
The gap mostly disappears after 60. Split by age, the advantage of animal protein was clear in adults younger than 60 (SMD -0.28, P = .01) but vanished in adults 60 and older (SMD -0.05, P = .74, not statistically significant). That is a little counterintuitive, since older adults are usually described as needing higher quality protein the most to fight age related muscle loss. The researchers think it likely comes down to sample size: far more trials have been run in younger adults, giving that subgroup more statistical power to detect a small effect that may exist in older adults too, just not clearly enough to prove it yet.
Soy is the exception, not the rule. When the plant protein was soy, specifically soy protein isolate or concentrate, there was no measurable difference from milk protein at all, in 17 trials covering both younger and older adults, men and women. Soy protein has an amino acid profile close to animal protein, which likely explains why it performs like one. The story changes for other plant proteins: rice, chia, oat, and potato protein together showed a clearly weaker effect on muscle mass than animal protein (SMD -0.58, P = .02), and full plant based diets performed worse than omnivorous diets matched for protein (SMD -0.51, P = .01, based on 7 trials).
Strength barely cares where the protein came from. Muscle mass was not the whole story. When the researchers looked at muscle strength, upper body strength showed no meaningful difference between plant and animal protein (P = .53), and neither did physical performance, things like walking speed or the timed up and go test (P = .47). The one exception: in adults 60 and older specifically, animal protein beat plant protein for lower body strength, like leg press or squat performance (SMD -0.38, P = .03), even though it made no difference for younger adults.
Resistance training changes the math. The advantage of animal protein over plant protein for muscle mass was much larger in trials that combined protein intake with resistance training (SMD -0.45) than in trials without it (SMD -0.10, not statistically significant). In other words, if someone is not lifting weights, protein source barely matters for muscle mass either way. If they are, the type of protein starts to matter more.
None of this comes with the same certainty a single, perfectly controlled trial would offer. The 43 trials varied enormously in length (4 to 104 weeks), population, and protein dose, which is reflected in a fairly high statistical heterogeneity (I2 = 62% for the main muscle mass analysis). Most trials were also short term, in generally healthy adults, using isolated protein supplements rather than whole diets, and none of them measured sarcopenia itself as a diagnosed condition, only its building blocks: mass, strength, and function. The authors are careful to frame the difference they found as small, not a reason to avoid plant protein altogether.
The practical takeaway is less dramatic than the headline number suggests. Getting enough protein in the first place, combined with resistance training, still matters far more than its exact source. For anyone eating mostly plant based, choosing soy protein over other plant sources, and pairing it with strength training, appears to close most of the gap with an omnivorous diet. For everyone else, this is one more data point that protein quality, not just quantity, deserves a seat at the table.
This article is based on: Reid-McCann RJ, Brennan SF, Ward NA, Logan D, McKinley MC, McEvoy CT. “Effect of Plant Versus Animal Protein on Muscle Mass, Strength, Physical Performance, and Sarcopenia: A Systematic Review and Meta-analysis of Randomized Controlled Trials.” Nutrition Reviews, 2025;83(7):e1581-e1603. DOI: 10.1093/nutrit/nuae200.
