Protein Powder

Last reviewed October 2026 · Based on 15 studies

Bottom line

Protein supplements added to resistance training give modest gains in muscle and strength. Results for older adults and for timing are mixed. Two reviews together suggest total daily protein may matter more than the product. Kidney and heavy-metal data are limited.

What it helps with

Adds to muscle and strength gains from resistance training — Benefit, evidence grade B (Moderate). Meta-analysis of 49 RCTs (1,863 people) and a 2020 review of 65 trials; downgraded once because the gains are small and the older-adult evidence disagrees.
Gains stop rising above about 1.6 g of protein per kg a day — No effect, evidence grade B (Moderate). Meta-regression across 49 RCTs; downgraded once because the cut-off comes from comparing trials, not from randomised comparisons of intake levels.
Timing around workouts changes muscle gains — Mixed results, evidence grade C (Limited). Meta-analyses disagree: one of five trials finds no timing effect on lean mass while a 116-trial network finds an after-exercise benefit; downgraded for inconsistency and small numbers.
Animal protein builds more muscle than plant protein — Mixed results, evidence grade C (Limited). Meta-analysis of 16 trials found no difference in absolute gains and a small favour for animal protein on percent lean mass; a 2026 review found only a small, uncertain difference in muscle protein synthesis; downgraded for inconsistency and few trials.
Soy protein works as well as whey — No effect, evidence grade C (Limited). Meta-analysis of nine trials with 266 people; downgraded for small sample size.
Helps older adults build muscle — Mixed results, evidence grade C (Limited). Meta-analyses disagree; downgraded for inconsistency and risk of bias, and the frail-group evidence is rated very low.
Harms kidney function in healthy adults — No effect, evidence grade C (Limited). Meta-analysis of 28 RCTs (1,358 people); downgraded for unclear selection bias and because filtration rate is a surrogate for kidney disease.
Very low protein diets slow kidney disease — Benefit, evidence grade B (Moderate). Cochrane review of 17 RCTs (2,996 people); very low vs low protein probably reduced kidney failure (moderate certainty); downgraded once for open-label designs with high performance bias.

What it is

Protein supplements come as whey, casein, soy, pea, rice, milk or blends [1].

What the research says

A 2018 meta-analysis of 49 trials with 1,863 adults doing resistance training for at least six weeks found protein supplements added about 0.30 kg of fat-free mass and 2.49 kg to one-rep-max strength [2]. A 2020 review found protein improved lean body mass by 0.62 kg in adults and 0.46 kg in older adults, with no clear effect of timing [3]. Soy and whey gave similar gains in one meta-analysis of nine small trials [4]. Animal and plant protein did not differ in absolute lean mass or strength changes in a meta-analysis of 16 trials, though animal protein favoured percent lean mass [5]. A 2026 network meta-analysis ranked whey first for lean mass gains [1]. Timing evidence conflicts: a 2025 meta-analysis of five trials found no effect on lean mass [6], while a 2024 network meta-analysis of 116 trials found protein after exercise gave the largest fat-free mass gain versus placebo [7]. In older adults, a 2018 review of nonfrail people found no significant effect on lean mass, strength or physical performance [8], while a 2022 review found small gains with resistance exercise only [9].

Doses used in studies

The 2018 meta-analysis found no further fat-free mass gain once total protein intake went beyond about 1.62 g per kg of body weight a day, including supplements [2]. A 2026 network meta-analysis estimated a plateau near 30 to 35 g of whey a day, which its authors describe as exploratory [1]. Trials in these reviews ran for at least six weeks of resistance training [2].

Safety and interactions

In 13 trials of people aged 65 and over with sarcopenia or frailty, no serious adverse events were reported [10]. In healthy adults, changes in kidney filtration rate did not differ between higher-protein and normal-protein diets, but that review looked at diets, not powders, and included only trials longer than four days [11]. In adults with non-diabetic chronic kidney disease, very low protein diets (0.3 to 0.4 g per kg a day) probably reduce progression to kidney failure compared with low protein diets. Low compared with normal protein may make little or no difference [12]. In a 2025 test of 22 powders, none had heavy metal levels above regulatory limits [13]; a 2020 risk assessment estimated typical use posed no adverse effect, with a hazard index below 1 [14]. Not covered by the searches behind this page: common side effects, upper intake limits, medicine interactions, pregnancy and breastfeeding, and liver disease or bleeding disorders. These need a separate search before this section is complete.

Caveats and open questions

Older-adult results conflict [8][9]. Timing results conflict [6][7]. The whey dose estimate is exploratory [1]. Heavy-metal evidence is one small test of 22 products and a risk model that used earlier reported concentrations [13][14]. Kidney data in healthy adults come from higher-protein diets rather than powders [11]. Author SMP (Phillips SM) of the 2018 meta-analysis has received grant support, travel expenses and honoraria from the US National Dairy Council, which supported trials in that review [2]. The view that total daily protein matters more than the product is our reading of that review and [5] together, not a finding of either study.