Whey or plant — the argument always lands on one question. Which builds more muscle?
Forty-three randomized trials answered. They measured mass, strength, and performance separately — and the three results didn't agree.
Plant Protein vs Whey for Muscle: Three Tests, Three Answers
Animal protein has a small, statistically significant advantage over plant protein for muscle mass — but the difference is trivially small and does not translate to muscle strength or physical performance. When total protein intake reaches 1.6 grams per kilogram of body weight per day, soy protein produces identical results to whey across seventeen head-to-head trials.
— Reid-McCann et al. 2025 · Nutrition Reviews · n=1,538
On mass alone, animal protein won. Across thirty pooled trials, the advantage was real — small, repeatable, favoring whey and other animal sources over plant.
The gap was tiny. A margin that sits at the bottom of what counts as meaningful — the kind of difference a caliper might catch in a lab but your training log would never show.
Strength told a different story. Plant protein users got exactly as strong as animal protein users — across fourteen head-to-head comparisons. Zero difference.
Physical performance — the tests that measure what a body can do outside a lab — came back the same. No difference across five more trials.
The protein that won the mass measurement didn't win the measurement that predicts what you actually train for. More tissue on a scan, not more weight on the bar.
The explanation starts with leucine — the amino acid most supplement labels lead with. Whey delivers more per gram. The tubs advertise it in bold.
Leucine has a ceiling. Past a daily threshold, more of it adds nothing to the muscle-building signal. At roughly 1.6 grams of protein per kilogram of body weight each day, both plant and animal eaters clear that ceiling. The amino acid advantage printed on the whey label becomes irrelevant above a protein intake most dedicated lifters already exceed.
One controlled trial, led by Hevia-Larraín, tested the threshold directly — plant-based and omnivore lifters matched at 1.6 grams per kilogram, identical training for twelve weeks, muscle measured five different ways. Every result came back the same.
"Plant protein" was never one thing. Across seventeen trials comparing soy protein to milk protein, the difference was zero — functionally identical. The gap that gave animal protein its mass advantage came almost entirely from non-soy plant sources like rice, chia, and potato, tested in just five trials.
The variable the entire debate ignores is the one that drives outcomes: total daily protein intake. At 1.6 grams per kilogram, the source question fades. Whether that protein comes from a whey shake or a soy blend matters far less than whether the total lands.
Non-soy plant proteins — rice, pea, oat — may genuinely trail animal protein for mass. The evidence behind that gap is thin, resting on five trials, but it exists. Dismissing it would be exactly the kind of one-sided confidence this debate already suffers from.
The argument was never source versus source. It was source versus amount — and amount won by a margin the supplement labels never printed. The full evidence across every protein type starts where those labels stop.