There is a number that has done a lot of quiet damage in this field, and it is worth spending a paragraph on where it comes from and what it is not.
The 2025 meta-analysis in Nutrition Reviews pooled 43 randomized controlled trials, 30 of which reported muscle mass. Headline: compared with animal protein, plant protein resulted in lower muscle mass after the intervention, standardized mean difference −0.20, 95% confidence interval −0.37 to −0.03, p = 0.02.
Minus 0.20 is a small effect. In a field where headlines get rounded, that is the difference between “plant protein builds muscle slightly less well” and “plant protein does not build muscle”. It is also, on the same page, alongside a confidence interval that touches zero and a set of subgroups that make the headline considerably more complicated.
Reading the meta-analysis properly
The 43-trial analysis has a structure worth walking through, because each layer changes the answer.
Overall, plant versus animal on muscle mass: SMD −0.20 (−0.37 to −0.03), p = 0.02. Statistically significant, and small.
By age. In adults under 60 the estimate is the same −0.20 (−0.37 to −0.03), p = 0.02. In adults 60 and over it is −0.05 (−0.32 to 0.23), p = 0.74 — that is, no detectable difference. The significant overall result is being carried by younger adults.
By which plant protein. Soy versus milk: SMD −0.02 (−0.20 to 0.16), p = 0.80, across 17 trials. So a properly formulated soy protein concentrate is statistically indistinguishable from milk protein for muscle mass. But non-soy plant proteins — rice, chia, oat and potato — sit at −0.58 (−1.06 to −0.09) across five trials, and whole plant-based diets at −0.51 (−0.91 to −0.11) across seven trials.
On strength: no difference, across 14 trials. On physical performance: no difference, across five trials. And the sentence at the end that no summary ever repeats: no trial examined sarcopenia as an outcome.
So the accurate statement of the current position is not “plant protein is worse than animal protein”. It is: a small deficit on muscle mass that lives entirely in younger adults and disappears above 60, is entirely absent for soy, is concentrated in the less complete plant sources and in whole-diet comparisons, and produces no difference in strength or performance at any age.
The 2021 systematic review in Nutrients, covering 3,081 screened records with 18 articles selected and 16 meta-analysed, points the same way and more quietly: protein source did not affect changes in absolute lean mass or muscle strength. There was a favouring effect for animal protein on percent lean mass, and in younger adults under 50 an absolute lean mass difference of 0.41 kg. A third of a kilo, in the group where it appears, against no strength difference anywhere.
The trial that settles the acute question
If the debate is about mechanism, there is a 2024 trial in the American Journal of Clinical Nutrition that is better designed than anything else in the field, and its result deserves to be read twice.
Thirty-one healthy older men, aged 72 ± 4, on a controlled diet set at the recommended protein allowance for a seven-day control phase. Then, double-blind and randomized, they were assigned to add a further daily supplement of whey, pea or collagen — a large amount, split between breakfast and lunch — for eight days. Integrated myofibrillar protein synthesis was measured using deuterated water and muscle biopsies.
Results:
- Whey: integrated rate 1.59 ± 0.11 % per day, p < 0.001 against baseline of 1.46 ± 0.09
- Pea: integrated rate 1.59 ± 0.14 % per day, p < 0.001 against baseline of 1.46 ± 0.10
- Collagen: no change
Pea and whey are the same number to two decimal places. The plant protein and the dairy protein produced identical daily rates of muscle protein synthesis in the same people, over the same week, measured in the same muscle with the same tracer. And collagen — which is a complete protein, and which is the active ingredient in the best-selling version of this claim — did nothing, because it is almost entirely glycine and proline and carries very little of the essential amino acid profile that drives synthesis.
That last point is the real lesson of the whole category. The distinguishing variable between a protein that works and a protein that does not is not animal versus plant. It is whether the essential amino acid profile reaches a threshold, and leucine is the amino acid that has been studied most because of its role in triggering the acute response.
It is also worth noting what the trial’s own authors concluded: the additional protein was sufficient to overcome the anabolic signalling deficits of older age, but the source mattered for overcoming age-related anabolic resistance. A protein that does nothing adds nothing, whatever its origin.
The mechanism, and why absorption is the honest caveat
Two reviews lay out the plant protein position clearly, and both land in the same place from different directions.
The 2021 review in Sports Medicine states that ingesting plant-derived proteins such as soy and wheat results in lower postprandial muscle protein synthesis than an equivalent amount of animal-derived protein, and attributes the lesser anabolic properties to differences in protein digestion and amino acid absorption kinetics, and to differences in amino acid composition. Most plant proteins have low essential amino acid content and are often deficient in one or more specific amino acids, such as lysine and methionine. There are also large differences between plant sources.
The 2015 review in the Journal of Nutrition gets more mechanistic still: the lower anabolic properties of plant sources may be attributable to lower digestibility, greater splanchnic extraction, and consequent urea synthesis of plant protein-derived amino acids. Most plant proteins have a relatively low leucine content.
Splanchnic extraction is the one that almost never gets explained to the person buying the tub. A meaningful fraction of plant protein amino acids is burned in the gut wall and in the liver on first pass, before it ever reaches the muscle, and shows up as urea. Animal proteins pass more of their amino acids through.
The 2023 trial in the European Journal of Nutrition measured this directly, in 50 young men doing one-legged resistance exercise. After a bolus of cricket, pea or whey protein, blood amino acid concentrations were significantly higher with whey than with pea or cricket. And yet: no difference between any of the three in mTORC1 activation in muscle, either at rest or after exercise.
That is the whole argument in one paper. Lower amino acid in the blood, identical signalling in the muscle. Whether that gap closes over a whole day of eating is a different question, and it is the question the integrated synthesis trial in older men answers — with pea and whey level.
The diet-level trial, and the number that is actually reassuring
The 2023 trial in the Journal of Nutrition is the one that people who eat a plant-based diet actually live with, because it does not supplement anything. It compares an omnivorous high-protein diet with a non-animal-derived, mycoprotein-rich high-protein diet, isonitrogenous in both.
Phase one, sixteen young adults over three days with daily unilateral leg resistance exercise: daily myofibrillar protein synthesis rates were about 12% higher in the exercised leg than at rest, and there was no difference between the omnivorous and the plant-based group.
Phase two, twenty-two young adults over ten weeks of high-volume progressive resistance training, five days a week:
- Lean mass: +2.6 ± 1.1 kg omnivorous, +3.1 ± 2.5 kg plant-based, p > 0.05
- Thigh muscle volume by MRI: +8.3 ± 3.6% against +8.3 ± 4.1%, p > 0.05
- Muscle fibre cross-sectional area: +33 ± 24% against +32 ± 48%, p > 0.05
- Strength across multiple muscle groups increased comparably
Note the standard deviations. The lean mass gain of 2.6 kg carries a standard deviation of 1.1, and the plant-based gain of 3.1 kg carries 2.5 — a 22-person study does not resolve a half-kilogram difference, and the authors correctly report p > 0.05. What twenty-two people over ten weeks can resolve is that eight percent more thigh muscle volume happened in both groups, and that a fully plant-based, mycoprotein-rich diet did not prevent it.
The authors’ conclusion: omnivorous and vegan diets can support comparable rested and exercised daily myofibrillar protein synthesis rates in healthy young adults consuming a high-protein diet, and this translates to similar skeletal muscle adaptive responses during prolonged high-volume resistance training.
Recovery is a different story from growth
The 2025 systematic review in Nutrients looked specifically at recovery from resistance exercise-induced muscle damage, screening 1,407 records down to 24 studies, 22 of them randomized, with interventions using soy, pea, rice, hemp, potato and blended plant sources at 15 to 50 grams, mostly taken after exercise.
The finding: single-source plant proteins generally offer limited benefits compared with animal proteins such as whey, particularly in acute recovery settings — a limitation, the authors note, consistently documented in the literature. But well-formulated plant protein blends perform better than single sources.
This is the same split that runs through the whole field, and it is the most useful thing to take away. If you want a single plant source to do an animal protein’s job in an acute recovery window, the pooled evidence says it probably will not. If you want a blend, the evidence says it might.
What these trials do not show
No trial in the 43-trial meta-analysis measured sarcopenia as an outcome. That is in the paper, in the abstract, and it is the one that matters for anybody over 60, because that is the population where the plant-versus-animal difference was null (−0.05, p = 0.74).
No study has measured a hard outcome — no falls, no fractures, no functional independence, no hospital admission, no mortality. Everything in this literature is lean mass by DXA, muscle volume by MRI, fibre area by biopsy, and strength by dynamometer.
The acute synthesis trials run for days and the training trials for weeks. The 31-man trial measured eight days of supplementation. The diet trial measured ten weeks. There is no one-year plant-versus-animal protein trial with a body composition endpoint, and there is no one-year trial at all with a lean mass endpoint of this quality.
Almost none of these trials controls for total protein intake. The synthesis trial did it properly — a controlled diet at the recommended allowance, then a supplement on top — and that is why its result is interpretable. Most of the rest compare a plant protein powder against a whey powder without establishing that the two arms ate the same amount of protein overall, which means a difference in results can be a difference in total protein rather than a difference in source.
The participants are young healthy men and, in one case, young healthy adults who volunteered for a ten-week training study. Nobody over 70 is in a hypertrophy trial. Nobody training for a powerlifting meet is in a dietary trial. Nobody with a chronic condition is anywhere in this literature.
The protein blend question is under-studied relative to how much it is sold. The recovery review found blends better than single sources, and there is no equivalent head-to-head synthesis trial establishing that a blend beats whey.
Absorption has never been tested as a variable with a muscle endpoint in humans. Splanchnic extraction is a well-supported mechanism in the reviews and a real number in the blood, but the paper that found lower aminoacidemia with pea and lower nothing in the muscle is the whole of the direct human evidence.
Where that leaves the question. Plant protein builds muscle. The pooled deficit is 0.20 standard deviations, it lives in younger adults, it is absent for soy, it is larger for the less complete sources, and it produces no strength or performance difference anywhere. Pea protein matches whey on measured muscle protein synthesis in older men. What separates the good from the useless in this aisle is not the plant-versus-animal line — it is whether the essential amino acid profile is complete, whether the amount is large enough to matter, and whether you are eating enough protein that a supplement is not the wrong lever at all.
