Whey protein

What it is

Whey is the protein fraction of milk that stays in solution when milk is curdled; it is dried and sold as concentrate, isolate or hydrolysate. It is a complete protein with a relatively high leucine content, and acute studies show it stimulates muscle protein synthesis more than soy or casein in the hours after a dose. Whether that acute advantage translates into more muscle or strength over months of training is a separate question, and the long-term trials are what matter for the claims on the tub.

What the research says

What the trials support is protein supplementation in general alongside resistance training, not whey specifically. A meta-analysis of 49 trials found that added protein produced small extra gains in strength and fat-free mass - and also that the benefit disappeared once total protein intake passed about 1.62 g/kg/day, which means the supplement is doing nothing that the diet was not already doing above that point. Head-to-head, soy produced the same gains as whey, so the source appears to matter far less than the total amount and the training. In older people who are not training, an umbrella review of 33 meta-analyses found no increase in muscle mass, strength or physical performance from protein supplementation. On safety, higher protein intakes did not change kidney filtration in healthy adults, but a case-control study links whey supplement use to acne, and protein powders as a product category carry measurable heavy-metal content, with whey powders at the low end.

Evidence grade: Well established.

What goes wrong

A case-control study in male gym users found whey protein supplement use was more common in those with acne. (Source 1)

• Case-control study, Low certainty.
• Size: 201 males (100 with acne, 101 controls)
• Who: Male adolescents and young adults attending fitness centres in northern Jordan.
• How long: Not applicable - cross-sectional comparison.
• Result: 47% of the acne group versus 27.7% of controls used whey protein (p=0.0047); significance maintained after multivariate analysis adjusting for vitamin B12, corticosteroids and anabolic steroids.
• Funding: not stated.

considerably more participants in the acne group (47%) were taking whey protein supplements than in the control group (27.7%) (p=0.0047)

What the evidence supports

Protein supplementation during prolonged resistance training produced small additional gains in strength, fat-free mass and muscle fibre size. (Source 2)

• Meta-analysis, Moderate certainty.
• Size: 1863 participants across 49 studies.
• Who: Healthy adults undertaking resistance exercise training.
• How long: Prolonged resistance training (trials of at least 6 weeks)
• Result: One-repetition maximum +2.49 kg (0.64, 4.33); fat-free mass +0.30 kg (0.09, 0.52); muscle fibre cross-sectional area +310 um2 (51, 570); mid-femur CSA +7.2 mm2 (0.20, 14.30); all p<0.05.
• Funding: not stated.

Data from 49 studies with 1863 participants showed that dietary protein supplementation significantly (all p<0.05) increased changes (means (95% CI)) in: strength—one-repetition-maximum (2.49 kg (0.64, 4.33)), FFM (0.30 kg (0.09, 0.52))

What the evidence does not support

In the same meta-analysis, adding protein above a total intake of about 1.62 g/kg/day produced no further gains in fat-free mass. (Source 2)

• Meta-analysis, Moderate certainty.
• Size: 1863 participants across 49 studies (meta-regression)
• Who: Healthy adults undertaking resistance exercise training.
• How long: Prolonged resistance training.
• Result: Breakpoint at 1.62 g/kg/day total protein; effect on fat-free mass also fell with increasing age (-0.01 kg per year, 95% CI -0.02 to -0.00, p=0.002)
• Funding: not stated.

Protein supplementation beyond total protein intakes of 1.62 g/kg/day resulted in no further RET-induced gains in FFM.

Head-to-head trials found soy protein produced the same strength and lean body mass gains as whey and other animal proteins. (Source 3)

• Meta-analysis, Low certainty.
• Size: 266 participants across 9 studies (5 whey vs soy, 4 soy vs beef, milk or dairy protein)
• Who: Adults undertaking resistance exercise training.
• How long: Trials of 6 weeks or longer.
• Result: No between-group differences: bench press chi2 = 0.02, p = .90; squat chi2 = 0.22, p = .64; lean body mass chi2 = 0.00, p = .96.
• Funding: not stated; lead author has published extensively on soy.

Meta-analysis showed that supplementing RET with whey or soy protein resulted in significant increases in strength but found no difference between groups (bench press: χ2 = 0.02, p = .90; squat: χ2 = 0.22, p = .64).

An umbrella review of 33 meta-analyses found no benefit of protein supplementation for muscle mass, strength or physical performance in healthy older people. (Source 4)

• Review of reviews, Moderate certainty.
• Size: 33 systematic reviews with meta-analyses collating 441 unique studies.
• Who: Older people overall, healthy older people, older people with long-term conditions, and hospitalised hip-fracture patients.
• How long: Reviews published January 1990 to August 2024.
• Result: No increase in muscle mass, strength or physical performance in older people generally or in healthy older people; medium-certainty evidence of at least small increases in mass and strength in those with long-term conditions, more certain with concomitant exercise.
• Funding: not stated.

There was no increase in muscle mass, strength or physical performance from protein supplementation in older people in general, nor in healthy older people.

In healthy adults, higher-protein diets did not change kidney filtration from baseline. (Source 5)

• Meta-analysis, Moderate certainty.
• Size: 1358 participants across 28 trials.
• Who: Healthy adults without kidney disease.
• How long: Not stated in abstract.
• Result: Change in GFR did not differ between interventions (SMD 0.11; 95% CI -0.05, 0.27; P = 0.16), although post-intervention GFR was trivially higher after higher-protein intakes (SMD 0.19; 95% CI 0.07, 0.31; P = 0.002)
• Funding: not stated.

The change in GFR did not differ between interventions (SMD: 0.11; 95% CI: −0.05, 0.27; P = 0.16).

An independent laboratory analysis of 22 protein powders found no heavy metal content above regulatory limits. (Source 6)

• Survey study, Low certainty.
• Size: 22 commercially available protein powders (whey, vegan and beef based)
• Who: Products purchased online in Hungary.
• How long: Not applicable - laboratory analysis.
• Result: 16 elements analysed by LIBS and ICP-MS; none detected above the limits specified in regulations.
• Funding: not stated.

Neither LIBS nor ICP-MS measurements detected significant heavy metal content in the investigated samples above the limit specified in the regulations.

Where the evidence is mixed

Protein powders contain measurable arsenic, cadmium, mercury and lead, but a risk assessment found the modelled exposures below thresholds of concern, with whey powders lowest. (Source 7)

• Survey study, Low certainty.
• Size: 15 products (Consumer Reports data) and 133 products (Clean Label Project data) re-analysed.
• Who: Adult consumers of protein powder supplements, modelled at 1 or 3 servings/day.
• How long: Not applicable - exposure modelling.
• Result: All hazard indices below 1; highest HI, approaching 1, in mass-gain products; lowest in whey protein powders; all modelled blood lead levels below the CDC guidance value of 5 ug/dL. The same paper notes Consumer Reports found average heavy metals in three servings per day exceeded proposed US Pharmacopeia maximum limits, and that another study found 40% of 133 products had elevated levels.
• Funding: not stated.

Interestingly, the highest HI levels (which approached 1) were found in 'mass gain' type protein powder supplements, whereas the lowest calculated HI levels were in whey protein powders.

Where the research disagrees

Whether whey is better than other protein sources for building muscle

• Acute laboratory studies, as summarised by Messina et al. (2018), acute muscle protein synthesis studies: Acute studies indicate that whey protein, likely related to its higher leucine content, stimulates muscle protein synthesis to a greater extent than proteins such as soy and casein. (Source 3)
• Messina et al. meta-analysis of long-term trials (2018), meta-analysis of 9 trials of 6 weeks or longer, 266 participants: The results of this meta-analysis indicate that soy protein supplementation produces similar gains in strength and LBM in response to RET as whey protein. (Source 3)

Whether protein supplements help older adults

• Age and Ageing overview of meta-analyses (2025), umbrella review of 33 meta-analyses, 441 unique studies, AMSTAR-2 and GRADE applied: The evidence does not support its routine use in other groups of healthy older people. (Source 4)
• Age and Ageing overview of meta-analyses (2025), on people with long-term conditions, same umbrella review: There was medium-certainty evidence of at least small increases in muscle mass and strength in older people with LTCs, and the benefits of protein supplementation were more certain with concomitant exercise. (Source 4)

How much

• Reference intake: There is no reference intake for whey specifically; reference intakes are set for total protein. NIH ODS states that athletes require protein at 0.55 to 0.9 g/lb body weight (1.2 to 2.0 g/kg body weight) per day, which is above the general adult recommended intake. (Source 8)
• Upper limit: No tolerable upper intake level has been set for whey protein or for total protein by EFSA or the US Institute of Medicine. The closest evidence-based ceiling in the exercise literature is functional rather than regulatory: the Morton meta-analysis found no further gains in fat-free mass above a total intake of about 1.62 g/kg/day. (Source 2)
• Studied: The 49 trials pooled by Morton et al. supplemented protein alongside resistance training for at least 6 weeks; the meta-regression breakpoint was a total protein intake of 1.62 g/kg/day. (Source 2)
• Studied: Nine trials compared soy with whey, beef, milk or dairy protein supplements over 6 weeks or longer. (Source 3)

A common belief, and what the research shows

The belief: You need whey protein specifically, and more of it, to build muscle.

What the research shows: The pooled trial evidence points to total protein and training rather than the source or the extra scoop. Morton et al. report that Protein supplementation beyond total protein intakes of 1.62 g/kg/day resulted in no further RET-induced gains in FFM. On source, a meta-analysis of longer trials found that supplementing RET with whey or soy protein resulted in significant increases in strength but found no difference between groups (bench press: χ2 = 0.02, p = .90; squat: χ2 = 0.22, p = .64). And without training, in older people, There was no increase in muscle mass, strength or physical performance from protein supplementation in older people in general, nor in healthy older people.

Questions and answers

What is it?

Whey is one of the two main groups of proteins in milk. Whole milk protein is about 20% whey, which is digested quickly, and about 80% casein, which is digested much more slowly. (Source 9)

What does it do in the body?

Like other dietary protein, it supplies amino acids for building body tissue, including muscle. A 2017 sports-nutrition position stand says that in short laboratory studies whey raised muscle protein synthesis more than soy or casein. Those are short-term measures and do not show long-term muscle gain. (Source 10)

Is it good or bad for you?

Mixed. A 2025 meta-analysis of 25 trials in 1,454 older adults found whey supplements gave a modest gain in lower-body strength but did not improve body composition or heart and metabolic markers. Fasting insulin and insulin resistance actually rose. Case reports also link whey supplements to acne flares (see getRidOf). (Source 11)

How do you get more of it?

From dairy foods, or from whey protein powders and shakes. In one small trial, 16 healthy middle-aged men took 20 g of either milk protein or whey protein, and both raised muscle protein synthesis to a similar degree. (Source 12)

If it is harmful, what reduces it?

Whey is not generally treated as harmful, but acne linked to whey supplements is reported. In a case series of five teenage male athletes whose acne began after starting whey supplements, lesions fully cleared in four after they stopped, and one flared again when he restarted. Five cases cannot show that whey caused the acne. (Source 13)

Why might someone be low in it or missing it?

Whey is not an essential nutrient, so no one is deficient in whey as such. It is in the diet only through milk and dairy. The related concern in the literature is low total protein intake in older adults: a 2025 review notes that older adults tend to have less appetite and eat less food. (Source 14)

Which whole foods contain it or feed it?

Milk and foods made from milk, since whey makes up part of milk protein. In the small trial above, 20 g of whole milk protein raised muscle protein synthesis as much as 20 g of isolated whey. (Source 12)

What happens if you do not have it?

Nothing specific. Whey is one of several high-quality proteins, and the body's need is for protein and its amino acids, not for whey itself. The 2017 position stand notes that whey, soy and casein all score similarly on measures of protein quality. (Source 15)

How can you test for it?

No test measures whey in the body, and none is needed. The test that does exist concerns cow's milk allergy, which can involve whey proteins. A 2017 systematic review says a supervised oral food challenge is still the gold-standard test, though it is expensive, time-consuming and can carry a risk of severe allergic reactions. (Source 16)

References

1. Dermatology Research and Practice. The Effect of Whey Protein Supplements on Acne Vulgaris among Male Adolescents and Young Adults: A Case-Control Study from North of Jordan. 2024. PMID 38633058, DOI 10.1155/2024/2158229. Read the source
2. British Journal of Sports Medicine. 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. 2018. PMID 28698222, DOI 10.1136/bjsports-2017-097608. Read the source
3. International Journal of Sport Nutrition and Exercise Metabolism. No Difference Between the Effects of Supplementing With Soy Protein Versus Animal Protein on Gains in Muscle Mass and Strength in Response to Resistance Exercise. 2018. PMID 29722584, DOI 10.1123/ijsnem.2018-0071. Read the source
4. Age and Ageing (Oxford Academic). Effects of supplemental protein in older people: an overview of meta-analyses. 2025. DOI 10.1093/ageing/afaf351. Read the source
5. The Journal of Nutrition. 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. 2018. PMID 30383278, DOI 10.1093/jn/nxy197. Read the source
6. Journal of Nutritional Science. Analysis of heavy metal content in protein powders available on the Hungarian market: a reassuring snapshot, but not a reassuring quality guarantee. 2025. PMID 40703701, DOI 10.1017/jns.2025.10024. Read the source
7. Toxicology Reports. A human health risk assessment of heavy metal ingestion among consumers of protein powder supplements. 2020. PMID 33005567, DOI 10.1016/j.toxrep.2020.08.001. Read the source
8. National Institutes of Health, Office of Dietary Supplements. Dietary Supplements for Exercise and Athletic Performance - Health Professional Fact Sheet (protein intakes for athletes). 2025. Read the source
9. Nutrients. Consumption of Milk Protein or Whey Protein Results in a Similar Increase in Muscle Protein Synthesis in Middle Aged Men (Mitchell CJ et al.) - Introduction. 2015. DOI 10.3390/nu7105420. Read the source
10. Journal of the International Society of Sports Nutrition. International Society of Sports Nutrition Position Stand: protein and exercise (Jäger R et al.). 2017. DOI 10.1186/s12970-017-0177-8. Read the source
11. Healthcare. Effects of Whey Protein Supplementation on Body Composition, Muscular Strength, and Cardiometabolic Health in Older Adults: A Systematic Review with Pairwise Meta-Analysis (Khalafi M et al.). 2025. DOI 10.3390/healthcare13212814. Read the source
12. Nutrients. Consumption of Milk Protein or Whey Protein Results in a Similar Increase in Muscle Protein Synthesis in Middle Aged Men (Mitchell CJ et al.). 2015. DOI 10.3390/nu7105420. Read the source
13. Cutis. Whey Protein Precipitating Moderate to Severe Acne Flares in 5 Teenaged Athletes (Silverberg NB). 2012. PMID 22988649. Read the source
14. Healthcare. Effects of Whey Protein Supplementation on Body Composition, Muscular Strength, and Cardiometabolic Health in Older Adults: A Systematic Review with Pairwise Meta-Analysis (Khalafi M et al.) - Introduction. 2025. DOI 10.3390/healthcare13212814. Read the source
15. Journal of the International Society of Sports Nutrition. International Society of Sports Nutrition Position Stand: protein and exercise (Jäger R et al.). 2017. DOI 10.1186/s12970-017-0177-8. Read the source
16. Italian Journal of Pediatrics. Specific IgE and skin prick tests to diagnose allergy to fresh and baked cow's milk according to age: a systematic review (Cuomo B et al.). 2017. DOI 10.1186/s13052-017-0410-8. Read the source