Reviews/Dietary Protein and Muscle/Plant and animal protein similar gains
Source And Distribution · DPM-SRC-01 · v2
Borderline: within 4 points of the Established/Likely boundary, so a re-grade could land either side.
Across several randomized training studies, people who supplemented with plant protein (pea, soy, mixed plant blends, or a whole vegan diet) built muscle and strength to much the same degree as those using whey or an omnivorous diet [1][2][5][6][7][10]. The same pattern held when a blend was compared with whey [3], and a short study found pea raised muscle-building signals as well as whey in older men while collagen did not [4]. The result shows up in untrained, sedentary, trained and young groups and across different labs, which is why the panel treated it as a solid, repeated finding. It stops short of the top tier because the studies usually made sure both groups got the same amount of protein (and in one case the same leucine), some 'plant' arms were actually plant-animal blends, and a couple of 'no difference' results came from small trials that may simply have been unable to detect a gap. One study in older men also hints the protein source can still matter when the body resists muscle-building with age [4].
Six resistance-training RCTs report no meaningful between-source difference in muscle or strength gains: pea vs whey [1][5], soy vs whey matched for leucine [2], a plant blend vs whey [6], a soy-pea blend vs whey [7], and a vegan vs omnivorous diet [10]. A blend-vs-whey trial concurs that protein type did not affect myofiber hypertrophy [3], and a myofibrillar protein synthesis RCT shows pea matching whey (both 1.59 %/d) while collagen did not rise [4]. The convergence spans untrained, sedentary, trained and young populations and multiple independent labs. Parity is generally reported where protein quantity, and in [2] leucine, was equalised between groups. The contested point: The division is whether the corpus demonstrates genuine equivalence or merely fails to detect a difference under matched intake. The Sceptic notes [1] separated pea from placebo only in a weakest-subgroup sensitivity analysis and [5]'s whey arm gain was itself non-significant (p=0.06), and that [4] indicates source can still matter in older adults, whereas the Synthesist reads the repeated nulls across labs and proteins as consistent convergence.
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One reviewer's only job is to attack the finding. The challenges below were weighed before the grade was set.
Equivalence is demonstrated chiefly where protein quantity (and in [2] leucine) was matched; the bare claim implies gram-for-gram parity, which the corpus does not isolate given plant sources' lower leucine and digestibility.
Much of the 'similar' signal is a failure to detect a difference in modest-N trials rather than demonstrated equivalence; [1] was powered to separate pea from placebo only in a weakest-subgroup slice and [5]'s whey arm gain was itself non-significant (p=0.06).
Where it remains uncertain. Equivalence is generally shown under matched protein (sometimes leucine [2]) intake, several comparators are plant-animal blends [3][6][7], and [4] suggests source can still matter in older adults.
Aleatory
Trials run from N=31 [4] to N=161 [1] with completion attrition (48 of 61 in [2]), so random variation can mask a true between-source difference.
Epistemic
Equivalence is reported where protein quantity, and in [2] leucine, was matched; the corpus does not isolate whether unmatched, real-world plant intakes perform equally.
Inferential
[4] and part of [10] rest on muscle protein synthesis rather than measured hypertrophy or strength, requiring an inferential step to training outcomes.
The load-bearing papers from a corpus of 10, each classified by what it does to the claim. Every DOI is taken from the corpus the claim was graded on.
12-wk RT RCT: pea and whey produced equivalent muscle thickness/strength gains.
“Since no difference was obtained between the two protein groups, vegetable pea proteins could be used as an alternative to Whey-based dietary products.”
Panel auditInternal validity: high · scope of inference: Young males (18-35) doing upper-limb resistance training, biceps thickness and strength outcomes.Key limitations: between-group difference only a trend · sensitivity finding limited to weakest participants · upper-limb only
12-wk RT RCT: soy equals whey when leucine-matched.
“These data indicate that increases in lean mass and strength in untrained participants are comparable when strength training and supplementing with soy or whey matched for leucine.”
Panel auditInternal validity: high · scope of inference: Untrained young men and women (18-35), lean mass and leg strength outcomes, soy vs whey leucine-matched.Key limitations: untrained participants only · thickness outcome non-significant overall (p = 0.08) · protein doses differ by gram amount to match leucine
12-wk RT RCT: soy-dairy blend equals whey on hypertrophy outcomes.
“there was no effect on any outcome between protein supplement types (blend vs whey)”
Panel auditInternal validity: high · scope of inference: Healthy young men, myofiber CSA, satellite cell and myonuclear outcomes, soy-dairy blend vs whey.Key limitations: plant source is a soy-dairy blend, not pure plant · small per-group sizes · young men only
3 of 10 papers shown. The full review lists all of them, with the panel’s arbitration →
Claims are graded at each quarterly release, and re-graded between releases only when a pivotal trial lands. Every change is a new version of the claim.
Consistent and independent primary RCT evidence supports similar gains, but the dependence on matched intake, use of blends, and a population-specific counter-signal keep it at Likely rather than Established.
Release 2026-q4 re-graded every claim from its corpus under protocol v0.19. An earlier release's grade is shown for the record; it came from an earlier protocol and is not a movement.
Assumptions that carry weight but are not themselves established.
The panel records the gap that would most move the claim, and the study that would close it.
Equivalence outside matched intake. Parity is shown where protein quantity and sometimes leucine were equalised [2], leaving unmatched, real-world plant diets untested. The panel rates this the highest-importance open gap for the claim, and names the study design that would close it: RCT comparing habitual plant vs animal protein at equal total grams but without leucine-matching, measuring hypertrophy and strength.
Open question: Do plant and animal proteins produce similar training gains when total intake is equal but leucine is not deliberately matched? Why it is still open: Equivalence in the corpus is observed under matched quantity and, in [2], matched leucine, so unmatched conditions are untested.
Priors is a sceptical first reader, not peer review, and not medical advice.
Release 2026-q4 · graded 21 Oct 2026 · protocol v0.19