“How much protein do I need” and “which protein” are two different questions, and the industry has an unfortunate habit of answering the first and selling the second. The evidence base for each is in a different state. One is well populated; the other has a scoring system that almost no consumer ever sees.
The quantity question, answered by 74 trials
Nunes and colleagues, in the Journal of Cachexia, Sarcopenia and Muscle 2022, registered a protocol (PROSPERO CRD42020159001), searched Medline, Embase, CINAHL and Web of Science, and included only randomized controlled trials in healthy, non-obese adults who increased their daily protein intake. Three-level random-effects meta-analyses and meta-regressions ran on data from 74 RCTs. Most of the selected studies tested the effect of additional protein during resistance exercise training.
The conclusion is deliberately modest, and the modesty is the point: increasing daily protein ingestion results in small additional gains in lean body mass and lower body muscle strength gains in healthy adults enrolled in resistance exercise training. Slight effect on bench press strength, minimal effect on physical function test performance, unclear effect on handgrip strength.
“Small additional gains” and “lower body muscle strength gains” is not the same as “more protein builds more muscle”. The first is a marginal effect on top of a training programme; the second is a claim about absolute outcomes that these trials do not support.
A disclosure note belongs here too, because the author list is long and the journal requires it: Stuart Phillips reports being an inventor on a patent and an unpaid advisory board member, and has received honoraria and travel expenses in the last five years from the National Dairy Council, the National Cattlemen’s Beef Association and the US Dairy Export Council. Philip Atherton has received research funding or honoraria from Abbott Nutrition, Fresenius-Kabi and Ajinomoto. Francesco Landi has received financial support from Abbott Nutrition and Nutricia. Four authors are employees of Abbott Nutrition, International Flavors & Fragrances and Givaudan respectively. Nine authors declare no conflict. This does not invalidate the meta-analysis; it does mean the conflict-of-interest section is part of reading the paper, not an appendix.
The quality question, and the score that answers it
The tool is the DIAAS, the digestible indispensable amino acid score. Burd and colleagues, in Frontiers in Nutrition 2019, lay out why it exists: current recommended dietary allowances and population reference intakes are set to cover basic needs, a higher intake may be needed to optimise muscle mass, and any accurate assessment of protein quality such as the DIAAS has to be balanced against methods that give a physiological correlate — established measures of protein synthesis, substrate oxidation, lean mass retention or accrual.
The 2026 review in Nutrition Reviews by Nielsen and colleagues states the methodological case directly. Protein quality used to be assessed with the protein digestibility-corrected amino acid score, the PDCAAS. The DIAAS is more accurate because it accounts for the true ileal digestibility of the indispensable amino acids. On a gram-for-gram basis, excellent-quality proteins stimulate muscle mass and health outcomes, particularly in the context of age-related anabolic resistance and sarcopenia.
The mechanism is unglamorous and it is the whole point: what limits a plant protein is not what is missing from it in total, it is how much of the limiting amino acid survives digestion before it reaches the blood. A score that measures grams per serving cannot see that. A score that measures what is digested at the end of the small intestine can.
What the score predicted, in real people
Ciuris and colleagues, in Nutrients 2019, put the score against measured athletes. Seven-day food records from 38 omnivores and 22 vegetarians, all endurance athletes, with lean body mass and leg-extension peak torque measured rather than self-reported. DIAAS scores reflect the true ileal digestibility of the indispensable amino acids present in food items and can be used to compute the available protein in a diet plan.
DIAAS scores were 11 per cent higher and available protein 43 per cent higher in omnivores (both p < 0.05). Omnivores had 14 per cent higher lean body mass. Available protein correlated with strength (r = 0.314) and with lean body mass (r = 0.541). On the available-protein calculation, the vegetarian athletes in this study would need on average an additional 10 grams of protein daily to reach 1.2 grams per kilogram of body weight per day, and an additional 22 grams to reach 1.4 grams per kilogram per day.
Note what that is and is not. It is a seven-day dietary record in 60 athletes with a cross-sectional comparison of two groups that differ in more than their diets. It is not a trial that fed vegetarians meat and measured what happened.
The part that cuts against the industry
The 2026 review is worth reading past the muscle paragraph. It reports that some evidence suggests specific amino acid enrichments — cysteine, glycine, arginine, glutamine, glutamate — may improve cardiometabolic health outcomes, whereas enrichments common in higher-quality proteins, such as leucine, isoleucine, valine, lysine and methionine, may increase cardiometabolic disease risk. The same amino acids that make a protein score well are the ones the review flags in the other direction.
And it is explicit that effects on cardiometabolic disease progression are less clear than the effects on muscle. “High-quality protein builds muscle” is a narrower claim than “high-quality protein is better for you”, and the review is careful not to conflate them.
What this evidence does not show
It does not show that protein quality rankings translate into a measurable difference in muscle gain between two people eating the same total protein. The 74-trial meta-analysis addressed quantity. It does not show that the DIAAS is validated against muscle outcomes as an endpoint — the 2026 review proposes it as “the most accurate assessment of protein-quality effects on anabolism” and calls for future studies, which is not the same as a demonstrated predictive model. It does not show that vegetarians need more food to meet their targets: the extra 10 grams and 22 grams figures come from one dietary-record study and are calculated, not administered. And it does not license the trade on the shelf, where “high quality”, “complete” and “premium blend” are not regulated terms.
This article summarises published meta-analyses, reviews and a dietary-record study. It is not medical advice and it is not a recommendation for any individual.
