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Research · Supplements

Protein powder

Two meta-analyses find protein supplements during resistance training add 0.3 to 0.7 kg of fat-free mass and some strength over control, with no further fat-free mass gain above about 1.6 g per kg a day of total protein. Plant and animal protein did not differ for strength. Tested powders contained lead, cadmium and arsenic, at levels modeled below health thresholds.

Add to Bio HackChecked 2026-11-09
Status
Dietary supplement or conventional food in the US, depending on how it is labeled.
Also called
whey protein, whey isolate, whey concentrate, casein, micellar casein, plant protein, pea protein, soy protein, rice protein, mass gainer, protein shake
Taken for
muscle gain, strength, recovery, weight loss, overnight recovery, meeting protein targets, vegan protein

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Shown to help5

Reported risks3

What people take it for, and what was found

Shown to help5

Strong

Muscle mass and strength during resistance training

Shown to add about 2.5 kg of strength and 0.3 kg of fat-free mass during resistance training, with no extra gain above 1.62 g per kg of total protein.

The numbers and sources
Effect
Across 49 RCTs (1,863 participants), protein supplements increased 1RM strength by 2.49 kg (95% CI 0.64 to 4.33) and fat-free mass by 0.30 kg (95% CI 0.09 to 0.52) over control. Gains in fat-free mass did not rise further once total protein intake passed 1.62 g per kg a day. The effect fell with age and was larger in trained lifters (0.75 kg, 95% CI 0.09 to 1.40) [1]. An earlier meta-analysis of 22 RCTs (680 people) found 0.69 kg more fat-free mass (95% CI 0.47 to 0.91) and 13.5 kg more leg press 1RM (95% CI 6.4 to 20.7) [2]. In 12 studies of adults aged 18 to 30 (261 participants), whey raised bench press by 8.87 kg (95% CI 5.95 to 11.79) and squat by 9.60 kg (95% CI 5.61 to 13.60), and neither whey nor soy significantly changed lean body mass [3].
Population
Healthy adults in resistance training programs of 6 weeks or longer [1][2]. Young adults [3].
Sources
[1], [2], [3]
Caveats
The supplement effect depends on how much protein the diet already provides; the 1.62 g per kg breakpoint is total intake from food plus powder [1]. Trials mix whey, casein, soy and blends. The smaller 2025 review of young adults did not find the lean mass gain the two larger reviews report [3].
Moderate

Muscle and strength in older adults

Shown to raise lower-body strength slightly in older adults, with lean mass gains only when combined with resistance training.

The numbers and sources
Effect
In 25 trials (1,454 adults aged 64 to 84), whey raised lower-body strength (SMD 0.16, 95% CI 0.04 to 0.28, 19 trials) and did not change upper-body strength or body composition overall. Combined with resistance training, whey raised lean body mass [4].
Population
Older adults, with or without resistance training [4].
Sources
[4]
Caveats
One meta-analysis. The lean mass finding comes from a subgroup [4].
Moderate

Recovery of muscle function after training

Shown to speed recovery of muscle function modestly in the 4 days after hard resistance training in young adults, with half of studies reporting benefit.

The numbers and sources
Effect
Across 13 RCTs from 8 studies, whey protein produced effect sizes of 0.4 to 0.7 for restoring contractile function from under 24 to 96 hours after strenuous resistance training. Half of the individual studies reported a benefit [7].
Population
Young, healthy adults [7].
Sources
[7]
Caveats
One meta-analysis of a small pool of trials. Funded by Food Health Ireland, a dairy research partnership [7].
Limited

Pre-sleep casein

Limited evidence: pre-sleep casein raised overnight muscle protein synthesis, with gains in size and strength in young men but not older men.

The numbers and sources
Effect
20 to 40 g of casein about 30 minutes before sleep raised overnight protein synthesis rates in young and older men. Over 10 to 12 weeks of training, pre-sleep protein increased fiber size, strength and muscle mass in young men but not in older men [8]. In 24 trained soccer players, 30 g of casein before sleep and the same dose right after training both improved next-day anaerobic performance against no casein, and the two timings did not differ [9]. After sleeve gastrectomy, casein-enriched milk before sleep or during the day did not change body composition, strength or physical performance over 12 weeks compared with no supplement [10]. In 9 trained adults, 40 g of casein before sleep did not change sleep measures against a carbohydrate control, while the same dose of whey shortened sleep onset and lengthened sleep [11].
Population
Nine studies, mostly men [8]. 24 male soccer players [9]. 45 adults after bariatric surgery [10]. 9 young adults, crossover [11].
Sources
[8], [9], [10], [11]
Caveats
Reference [8] is a systematic review without pooling. Its authors note uneven total protein intake between groups, so the timing effect cannot be separated from extra protein [8]. The three newer trials are small [9][10][11].
Limited

Weight and fat loss

Limited evidence: one review found whey favored small reductions in body weight and fat mass in adults with overweight or obesity.

The numbers and sources
Effect
In 9 RCTs in people with overweight or obesity, whey favored reductions in body weight (mean difference 0.56, 95% CI 0.30 to 0.81) and fat mass (1.12, 95% CI 0.77 to 1.47), with lower blood pressure, glucose and total cholesterol [12].
Population
Adults with overweight or obesity [12].
Sources
[12]
Caveats
The abstract does not state units and lists lean mass (0.77) among the reductions favoring whey, which is hard to read. One meta-analysis of 9 trials.

Not shown to help1

Strong

Plant protein compared with animal protein

Not shown to differ in strength by protein source. Soy matched milk protein for muscle, while non-soy plant proteins trailed animal protein in 5 trials.

The numbers and sources
Effect
In 16 RCTs, protein source did not change absolute lean mass or strength; adults under 50 gained 0.41 kg more lean mass (95% CI 0.08 to 0.74) on animal protein [5]. In 30 RCTs, plant protein produced lower muscle mass than animal protein (SMD -0.20, 95% CI -0.37 to -0.03). Soy and milk protein did not differ (SMD -0.02, 17 RCTs). Non-soy plant proteins such as rice, oat and potato trailed animal protein (SMD -0.58, 5 RCTs). Strength (14 RCTs) and physical performance (5 RCTs) did not differ [6].
Population
Adults of all ages, most with protein intakes above the RDA [5][6].
Sources
[5], [6]
Caveats
Few trials tested pea, rice or other non-soy powders, so the non-soy estimate rests on 5 trials [6].

Doses studied

Supplement trials added protein to diets during 6 weeks or more of resistance training; in pooled data, gains in fat-free mass stopped rising once total daily intake reached about 1.62 g per kg [1]. Pre-sleep trials used 20 to 40 g of casein about 30 minutes before sleep [8], or 30 g of micellar casein 30 to 60 minutes before bed [9]. The acne trial used 30 g of whey a day for 6 months [13]. Contamination models assumed 1 to 3 servings, or 30 to 90 g, a day [14][15].

Risks

Reported risks3

ModerateHow often is not known

Heavy metals in tested products

Product tests found heavy metals in many powders, with modeled exposure mostly below the hazard threshold, highest in mass gainers and plant-based products.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
A risk assessment built on prior product testing (including a report that 40% of 133 products had elevated heavy metals) found hazard indexes below 1 for arsenic, cadmium, mercury and lead at 1 to 3 servings a day. The highest values, approaching 1, were in mass gainer products; the lowest were in whey [14]. In 30 products sold in China, every sample contained at least one target metal. Plant-derived products had higher mean lead (0.18 mg/kg) and cadmium (0.05 mg/kg) than dairy products (0.06 and 0.01 mg/kg). A brown rice and cocoa product reached 0.60 mg/kg lead. Modeled hazard index was 0.45 at 30 g a day and 0.90 at 90 g a day, with a 95th percentile of 1.28 under high use. Microplastics were found in every sample, 2 to 19 particles per 30 g serving [15].
Population
Product testing and exposure modeling, not people [14][15].
Sources
[14], [15]
Caveats
A hazard index below 1 is a modeled threshold, not a measured health outcome. Products differ by country and batch.
LimitedHow often is not known

Mislabeled protein content and undeclared ingredients

Limited evidence: in Indian market testing, most protein supplements missed their labeled protein, and some contained aflatoxins, pesticides, lead or herbal extracts linked to liver injury.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In popular protein supplements sold in India, most products did not meet their labeled protein content; some exceeded it, which the authors flag as possible amino acid spiking. Major brands contained aflatoxins and pesticide residues, many contained lead and arsenic, and some contained herbal extracts linked to liver injury, such as green tea extract and Garcinia cambogia [16].
Population
Product testing in one market [16].
Sources
[16]
Caveats
Self-funded and limited to the Indian market. The abstract reports no counts or percentages.
ModerateHow often is not known

Insulin resistance markers in older adults

Shown to raise fasting insulin and insulin resistance markers in older adults on whey, with no change in fasting glucose or lipids.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In a meta-analysis of 25 trials in older adults, whey supplementation raised fasting insulin and HOMA insulin resistance compared with control. Fasting glucose and lipids did not change [4].
Population
1,454 adults aged 64 to 84 [4].
Sources
[4]
Caveats
One review. The abstract does not give the size of the change [4].

Checked, no increase found2

Moderate

Kidney function

Kidney filtration change did not differ between higher-protein and normal or lower protein diets in adults without kidney disease.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Across 28 RCTs (1,358 participants), higher-protein diets (1.5 g per kg or more, 20% of energy or more, or 100 g a day or more) gave a slightly higher filtration rate after the intervention (SMD 0.19, 95% CI 0.07 to 0.31), but the change in filtration rate did not differ from normal or lower protein (SMD 0.11, 95% CI -0.05 to 0.27) [17].
Population
Adults without kidney disease [17].
Sources
[17]
Caveats
Diets, not powders specifically. People with kidney disease were excluded. Selection bias risk was unclear in most trials [17].
LimitedStudied in men

Acne

One trial found no change in acne lesions on 30 g of whey a day for 6 months in young men with existing acne.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
In a 6-month RCT of 49 men aged about 20 with mild to moderate acne, 30 g a day of whey did not change total facial or truncal acne lesions compared with a non-whey supplement (facial mean difference -5.99, 95% CI -13.18 to 1.19) [13].
Population
Young men with existing acne [13].
Sources
[13]
Caveats
One small trial in men. No trial in women or adolescents.

Claims tested and not found

  1. That more protein powder keeps adding muscle. Pooled trials found no added fat-free mass gain once total intake passed about 1.62 g per kg a day [1].
  2. That taking casein before sleep beats other timing. Pre-sleep and post-exercise casein did not differ on next-day performance [9], and pre-sleep and daytime casein milk did not differ after bariatric surgery [10].
  3. That plant protein builds less strength. Strength did not differ between plant and animal protein in 14 to 16 RCTs [5][6].
  4. That high-protein intake reduces kidney function in healthy adults. The change in filtration rate did not differ from normal intake in 28 RCTs [17].

Sources

  1. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine. 2017. https://doi.org/10.1136/bjsports-2017-097608
  2. Cermak NM, Res PT, de Groot LC, et al. Protein supplementation augments the adaptive response of skeletal muscle to resistance-type exercise training: a meta-analysis. American Journal of Clinical Nutrition. 2012. https://doi.org/10.3945/ajcn.112.037556
  3. Davis BE, Young I, Giglotti JC, et al. The Impact of Whey and Soy Protein Supplementation on Resistance Training in Young Adults: A Systematic Review and Meta-Analysis. Journal of Dietary Supplements. 2025. https://doi.org/10.1080/19390211.2025.2604679
  4. Khalafi M, Fatolahi S, Jafari R, et al. Effects of Whey Protein Supplementation on Body Composition, Muscular Strength, and Cardiometabolic Health in Older Adults: A Systematic Review with Pairwise Meta-Analysis. Healthcare. 2025. https://doi.org/10.3390/healthcare13212814
  5. Lim MT, Pan BJ, Toh DWK, et al. Animal Protein versus Plant Protein in Supporting Lean Mass and Muscle Strength: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients. 2021. https://doi.org/10.3390/nu13020661
  6. Reid-McCann RJ, Brennan SF, Ward NA, et al. Effect of Plant Versus Animal Protein on Muscle Mass, Strength, Physical Performance, and Sarcopenia: A Systematic Review and Meta-analysis of Randomized Controlled Trials. Nutrition Reviews. 2025. https://doi.org/10.1093/nutrit/nuae200
  7. Davies R, Carson B, Jakeman P. The Effect of Whey Protein Supplementation on the Temporal Recovery of Muscle Function Following Resistance Training: A Systematic Review and Meta-Analysis. Nutrients. 2018. https://doi.org/10.3390/nu10020221
  8. Reis CE, Loureiro LM, Roschel H, et al. Effects of pre-sleep protein consumption on muscle-related outcomes, a systematic review. Journal of Science and Medicine in Sport. 2021. https://doi.org/10.1016/j.jsams.2020.07.016
  9. Bayrakdaroğlu S, Ateş ZH, Ceylan Hİ, et al. Casein Supplementation Timing and Exercise Performance in Soccer Players: Pre-Sleep vs. Post-Exercise Intake, A Randomized Controlled Trial. Nutrients. 2025. https://doi.org/10.3390/nu17243938
  10. Yıldız N, Coşkun H, Tanal M, et al. Presleep vs. Daytime Consumption of Casein-Enriched Milk: Effects on Muscle Function and Metabolic Health After Sleeve Gastrectomy. Nutrients. 2025. https://doi.org/10.3390/nu17172750
  11. Aussieker T, Kredig N, Wasserfurth P, et al. Presleep Whey but Not Casein Protein Ingestion Improves Sleep Quality After Resistance Exercise When Compared With an Isocaloric Control in Resistance Trained Individuals. International Journal of Sport Nutrition and Exercise Metabolism. 2026. https://doi.org/10.1123/ijsnem.2025-0205
  12. Wirunsawanya K, Upala S, Jaruvongvanich V, et al. Whey Protein Supplementation Improves Body Composition and Cardiovascular Risk Factors in Overweight and Obese Patients: A Systematic Review and Meta-Analysis. Journal of the American College of Nutrition. 2017. https://doi.org/10.1080/07315724.2017.1344591
  13. Sompochpruetikul K, Khongcharoensombat T, Chongpison Y, et al. Whey protein and male acne: A double-blind, randomized controlled trial. Journal of Dermatology. 2024. https://doi.org/10.1111/1346-8138.17109
  14. Bandara SB, Towle KM, Monnot AD. A human health risk assessment of heavy metal ingestion among consumers of protein powder supplements. Toxicology Reports. 2020. https://doi.org/10.1016/j.toxrep.2020.08.001
  15. Chen M, Dong R, Lai W. Heavy metal and Microplastic exposure from sports protein Supplements: Integrated health risk modeling and scenario analysis. Environmental Research. 2026. https://doi.org/10.1016/j.envres.2026.123988
  16. Philips CA, Theruvath AH, Ravindran R, et al. Citizens protein project: A self-funded, transparent, and concerning report on analysis of popular protein supplements sold in the Indian market. Medicine. 2024. https://doi.org/10.1097/md.0000000000037724
  17. Devries MC, Sithamparapillai A, Brimble KS, et al. Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis. Journal of Nutrition. 2018. https://doi.org/10.1093/jn/nxy197

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