Walk into any supplement aisle and the packaging tells a tidy story: whey is for building muscle, plant protein is for people who feel bad about it. A meta-analysis published on 29 October in the Journal of the International Society of Sports Nutrition tested that hierarchy against the trials that have actually run head to head, and the pooled result came out flat. Across five randomised controlled trials and 372 participants, plant-based and whey protein supplements produced no statistically significant difference in lean body mass or fat mass.

The result has been read in some places as “plant protein is just as good as whey”. That is not what the paper says, and its authors go out of their way to say so.

What the review actually pooled

The search was narrow by design. The authors wanted trials in which the only meaningful difference between groups was the protein source, so they restricted inclusion to healthy adults aged 18 to 59 who had no resistance-training experience, who supplemented for at least six weeks, and whose body composition was measured by dual-energy X-ray absorptiometry (DEXA) rather than by bathroom scales or skinfold calipers. Whey had to be the active comparator, and total calorie intake had to be reported. The protocol was registered in advance on PROSPERO (CRD420251145410).

Three databases — PubMed in April 2025, Web of Science and ScienceDirect in July — returned 70 records. After screening, five trials qualified: 372 people, ages 18 to 59, two of the five recruiting men only. The plant arms were not interchangeable. Two trials used soy protein isolate, one a soy protein concentrate, one pea protein, and one chia flour. Whey appeared as isolate in two trials and concentrate in three.

The dosing and the training varied as much as the products. One trial gave 75 men roughly 3 grams of leucine per serving across 12 weeks. Another supplemented 72 adults with about 22 grams a day for nine months, pushing their intake to roughly 1.4 grams of protein per kilogram of body weight. A third put 84 sedentary adults aged 30 to 59 through six resistance sessions a week for 12 weeks, comparing 20 to 22.5 grams of pea protein with 20 to 21.1 grams of whey. A fourth matched the two supplements at about 2 grams of leucine — 19 grams of whey isolate against 26 grams of soy isolate — for 12 weeks.

The numbers behind “no difference”

For lean body mass, the pooled mean difference between plant and whey was −0.35 kg, with a 95% confidence interval running from −2.46 to 1.76 kg. Because that interval straddles zero, the result is formally “no detectable difference”. Heterogeneity was essentially absent (I² = 0%), which means the five trials were not quietly pulling in opposite directions.

Individually, the estimates were loose. The trial with the smallest difference came in at 0.00 kg (95% CI −3.12 to 3.12); the widest ran from −4.46 to 7.06 kg. One trial pointed the other way: after nine months of supplementation, whey produced larger gains in lean mass, with a mean difference of about −3.20 kg favouring whey — but its confidence interval (−9.54 to 3.14) was wide enough to include no effect at all.

Fat mass told the same story across the four trials that reported it, again with no heterogeneity and confidence intervals crossing zero. The review’s risk-of-bias assessment rated the included trials moderate to high in quality overall, with concerns about allocation concealment, missing outcome data and selective reporting.

The other 2026 answer

A Bayesian network meta-analysis in Frontiers in Nutrition earlier this year is the natural counterweight. Pooling 36 trials and 1,552 participants, it ranked whey first for lean mass (SUCRA 0.91), concluded that plant proteins “showed smaller effect estimates”, and rated that evidence low to very low in certainty. Look at its direct, trial-to-trial estimates, though, and whey’s edge over soy was 0.72 kg (95% credible interval −0.08 to 1.52) and over pea 0.80 kg (−0.05 to 1.65) — both intervals crossing zero.

A ranking asks which arm looks best across a mixed network; a head-to-head pooled difference asks whether one beat the other. On the second question both syntheses draw the same blank. Our earlier piece on the guidelines’ protein target covered the intake side of the same argument.

Where the trials diverge

The detail is more useful than the headline. Duration looks like one of the levers: the only trial that found whey ahead ran for nine months, while the shortest — eight weeks, using 50 grams of chia flour providing 20 grams of protein against 30 grams of whey concentrate providing 23 grams — found no change in lean mass in either arm. The authors treat the chia result as an open question rather than a verdict, since no other trial has tested it.

Matching on leucine, the amino acid most often credited with whey’s reputation, was done in exactly one of the five trials. There, 26 grams of soy isolate and 19 grams of whey isolate produced comparable gains. The review notes that this single trial cannot be generalised to every plant product on the shelf, and points out that multi-source blends of pea, rice, hemp and potato have separately been shown to stimulate muscle protein synthesis at doses around 30 grams — while single-source options such as isolated hemp or peanut protein may fare worse.

What the meta-analysis cannot do is separate protein source from total protein. Everyone in these trials was resistance training, and where intake was reported it sat around 1.2 to 1.4 grams per kilogram a day — above the 0.8 g/kg floor for healthy young adults that the review’s own introduction cites. That is precisely the condition under which the source of the powder matters least.

Why the authors refuse to call it equivalence

The paper’s own summary is blunt: the finding “should not be interpreted as evidence of physiological equivalence”. The reasons are structural rather than rhetorical. Five trials is a small evidence base; wide confidence intervals mean a real difference of a kilogram or two in either direction has not been ruled out; formal tests for publication bias were skipped because they are unreliable below ten studies; and the search covered three databases, with CENTRAL, Scopus, Embase and SPORTDiscus excluded — a limitation the authors state plainly. A leave-one-out sensitivity analysis was also judged uninformative with four or five studies per outcome.

So the defensible reading is narrower than the headlines: in untrained adults who resistance train and eat enough protein, a soy- or pea-based supplement has not been shown to produce worse body-composition results than whey. Whether that holds for trained lifters, for people over 60, for longer than nine months, or for every plant product sold, remains untested here.

What the label still does not tell you

Two consumer reference points are worth keeping beside the paper. Harvard’s Nutrition Source puts the National Academy of Medicine minimum at 0.8 grams of protein per kilogram of body weight per day, and notes that protein powders — like other dietary supplements — are not regulated by the U.S. Food and Drug Administration for safety. Powders can carry non-protein ingredients: added sugars, thickeners, non-caloric sweeteners, flavourings. Reading the ingredient panel is not optional, whatever the protein source.

MedlinePlus makes the complementary point that plant foods are sufficient on their own: “You do not need to eat animal products to get all the protein you need in your diet.” It also notes that essential amino acids “do not need to be eaten at every meal” — the balance over the whole day is what counts. For most protein-rich foods, one ounce carries about 7 grams of protein.

None of this makes a powder necessary. It makes the choice between two powders less dramatic than the marketing suggests — and, if the review is right, mostly a matter of taste, tolerance and price. Our earlier look at creatine and the evidence beyond the gym made a similar point: the training stimulus does most of the work.

What we did not find

There was no trial in this review of trained athletes, no trial in adults over 59, and no head-to-head of a modern multi-source plant blend against whey. Nothing in the pooled data suggests plant protein is superior to whey; the correct reading is that a difference was not detected. And there is no evidence here about cost, digestibility in people with sensitive guts, or how any of these powders behave over years rather than months.

People with kidney disease, liver conditions, or a diagnosed eating disorder should not reshape their protein intake around a supplement comparison; that is a conversation with a clinician. As always, this article reports what the cited sources say and is not medical advice — see our medical disclaimer for what that means.