This claim survives on a real, narrow fact stretched into a broader conclusion it doesn’t support: individual plant protein sources can be lower in specific essential amino acids, but that doesn’t mean a plant-based diet is protein-inadequate.

A 2016 review in Nutrition & Metabolism ^[1] lays out the actual protein-quality picture: single-source comparisons (say, isolated soy protein versus whey protein) can show real differences in amino acid profile and digestibility. But those differences mostly wash out once you’re eating a properly varied diet across a full day — different plant foods are low in different amino acids (wheat is comparatively low in lysine, legumes in methionine, for instance), and a mixed diet fills those gaps without any special meal-pairing required.

A 2019 review in Nutrients ^[2] covers vegetarian and vegan diets specifically and reaches a similar conclusion: total protein intake and dietary variety are what matter, not combining specific foods in the same sitting. The old “combine rice and beans at the same meal” advice from decades ago has been superseded — the body maintains a pool of amino acids across the day, not just within a single meal’s digestion window.

The honest verdict is mixed, not a flat myth or fact — plant protein quality differences are real at the level of a single food, and irrelevant at the level of a varied day.

Where this verdict genuinely depends on context: someone eating a very limited range of plant foods, or trying to hit a very high protein target on a tight calorie budget, may need to pay closer attention to total intake and food variety than someone on an omnivorous diet would. That’s a real practical consideration — it just isn’t the same claim as “plant protein can’t build muscle.”