Plant protein builds less muscle. You have read the headline, heard the gym-floor verdict, maybe even used it to settle an argument. And without thinking about it too hard, you filed the next conclusion alongside: if it builds less muscle, it must make you weaker.
That conclusion felt so obvious you never checked it. Mass and strength travel as a pair in your head — more of one means more of the other. The word muscle covers both, and the single word does the conflating for you.
Does plant protein affect strength or just muscle size?
The mass finding is real. A 2025 meta-analysis pooling forty-three randomized controlled trials found a small but statistically significant edge for animal protein when it comes to how much tissue accrues. The effect is classified as small — close enough that you would struggle to spot it in a mirror — but consistent enough across the data to be real.
Then the same analysis measured strength.
Upper body: identical results from either protein source. Lower body: identical. The protein that produced less tissue produced the same force. Same participants, same training programs, same meals minus one ingredient — opposite verdicts depending on which measurement you read.
One word — muscle — had been hiding two answers. Size: small animal advantage. Strength: dead even.
Plant protein produces slightly less muscle mass than animal protein, but it produces identical strength gains. Across randomized controlled trials, no significant difference emerged in upper or lower body strength regardless of protein source, despite a small but real mass advantage for animal protein. The answer splits depending on which outcome you measure.
— Reid-McCann et al. 2025 · Nutrition Reviews · 43 RCTs
The split traces to the lab test everyone leans on when ranking proteins. After a meal, your body spikes in new protein production — and that spike reliably favors animal sources. Higher leucine content, faster absorption, bigger immediate response. For years, the post-meal spike was treated as the scoreboard. The problem is that this short-term production burst turns out to be a poor predictor of what muscles actually do over months of training. The signal that crowned one protein source above another never predicted the long-term outcome that matters in a gym.
One group broke the pattern. Adults over sixty showed a lower-body strength advantage from animal protein that younger adults did not. Whether this reflects a genuine age-related shift or the small number of trials in the subgroup is an open question — and the kind that deserves naming rather than hiding.
For everyone under that line, the split is clean: protein source nudges how much tissue grows. It leaves how much force that tissue generates untouched. The cost of switching turns out to be far less than a single word ever let on.
How the older-adult subgroup breaks the pattern — and why age redrew the line for strength — is the question the full 43-trial analysis pulls apart next.