Every positive result came from a trial that did not control calories. That is the finding of this Bayesian meta-analysis of 75 trials and 1,206 athletes, and it is the sort of detail that usually disappears somewhere between a study's results section and a supplement label.
What they did. A search across seven databases turned up 6,129 studies, narrowed to 75 randomized controlled trials. The pooling used a Bayesian hierarchical model rather than the conventional approach - a reasonable choice for a messy literature full of small trials, though it makes the numbers a little harder to line up against older reviews.
On paper, protein won. Protein-carbohydrate supplements beat placebo for endurance performance (SMD 0.57, 95% CI 0.20 to 0.93). Pure protein beat placebo for endurance (0.37) and muscle strength (0.72, 95% CI 0.18 to 1.27). Protein also beat carbohydrate alone for glycogen resynthesis after exercise (0.83). Taken at face value, a clean sweep.
Then the catch. All of those significant effects came from trials where total energy intake was not matched between groups. When the supplement group eats more calories than the control group, you have not tested protein - you have tested extra food. The authors say so plainly, and it means the honest reading is not "protein supplements work" but "we still lack clean evidence that they do anything beyond the calories they carry."
The number worth keeping. The most practical line in the paper sits in its conclusion: an additional 1 g/kg/day from supplements, bringing total intake to roughly 2 g/kg/day, appeared most effective for performance. For an 80 kg athlete that is about 160 g of protein a day from all sources combined. Note what kind of target that is - it is a target for total intake. The powder is a delivery mechanism, not an ingredient with independent magic.
Where this sits against what we already know. None of this contradicts the well-established finding that adequate protein supports training adaptation and helps preserve muscle. It narrows the claim. Hitting your protein target matters. Whether the last 30 grams arrive as a shake, chicken or lentils is not something this literature can answer right now, because the trials were not built to.
The practical read. If you already reach roughly 1.6-2 g/kg/day from food, this review gives you no reason to add a supplement and no good evidence one would help. If you do not reach it, a powder is a cheap and convenient way to close the gap - and convenience is a perfectly legitimate reason to use one. Just do not expect the tub itself to do something your kitchen could not.
Medical note. This article is for general educational purposes and is not personal medical advice. People with kidney disease should discuss protein targets with their own clinician.