Every protein ranking she has ever read arrived at one answer. Whey on top. Casein close behind. Plant protein somewhere lower, functional but not quite equal. The hierarchy felt settled because every source agreed, and not one of them mentioned whether it applied to women specifically.
The absence never registered as a gap. The comparisons said "plant versus animal" and reported a winner. She absorbed the result and moved on. It never occurred to her that the data might look different when someone actually checked women separately.
Does Plant Protein Work as Well for Women?
A 2025 meta-analysis pooled thirty trials and more than 1,500 participants, comparing plant protein against animal protein for muscle mass. The overall result: animal protein had a small but real advantage. The gap was modest, but it was there, and the direction matched everything she had heard. Plant protein builds muscle. Just not quite as much.
The finding is legitimate. Across thirty trials, the pooled data confirmed what she already believed.
Then someone split the data by sex.
For women, the difference between plant and animal protein for muscle mass was zero. Not small. Not trending toward zero. The measured gap was 0.00 — and the confidence in that number was so high that the probability of it being a fluke was less than one in a hundred. Plant protein and animal protein produced identical muscle outcomes in women.
When the largest meta-analysis of plant versus animal protein split its results by sex, the difference in muscle mass for women was exactly zero. Thirty pooled trials showed a small overall advantage for animal protein, but that advantage was carried entirely by the male data. For women specifically, protein source made no measurable difference.
— Reid-McCann et al. 2025 · Nutrition Reviews · n=1,538
The finding she had been carrying — the one that made her feel like she was settling every time she reached for a plant-based option — had nothing to do with her. It described an average she happened to be folded into, one that masked the fact that her results looked nothing like the pool.
The pooled result that favored animal protein was driven almost entirely by the male data. Men showed a trend toward better muscle outcomes with animal protein, though even that trend fell short of statistical significance. When the sexes were averaged together, the male trend pulled the overall number down and created a small measurable effect. For women, there was no signal to pull.
The headline number was real. It just described two populations at once, and one of them — hers — showed nothing. She had been reading a finding shaped by men's muscle response and treating it as her own, because the content she consumed never told her otherwise.
ALL PARTICIPANTS
A small but statistically significant advantage for animal protein for muscle mass.
WOMEN ONLY
Zero difference. Plant and animal protein produced identical muscle outcomes.
One caveat belongs here. The sex-stratified analysis was not the study's primary comparison — it was a secondary subgroup analysis, which means the trial was not designed to test this exact question. A precise zero in a subgroup of a large meta-analysis is a strong signal, not a final verdict. The best current evidence shows no difference for women, and the evidence is substantial. But it was not the question the study set out to answer.
The protein-source question is settled for her, and the evidence opens a sharper one. Soy matched milk across seventeen head-to-head trials. Non-soy sources — rice, pea, oat — had far fewer. Her next comparison is not plant versus animal. It is soy versus everything else.