Side Effects of Whey Protein: What Is Real and What Is Myth

Whey protein is one of the most researched and widely used supplements in the world. It is generally safe for healthy adults, but it is not free of side effects, and some long-standing fears turn out to be unfounded. Our editors separate real issues, such as digestive discomfort and acne flares, from common myths about kidneys and liver.

Whey in brief

Whey is the liquid fraction of milk left after cheese-making. After filtration and drying, it becomes a powder rich in high-quality protein, with all essential amino acids and a particularly high leucine content.

The main forms are whey concentrate, which typically contains 70–80% protein along with some lactose and fat; whey isolate, with around 90% protein and very little lactose; and hydrolysed whey, in which proteins are partly broken down for faster digestion.

Research consistently shows that whey, combined with resistance training, supports gains in muscle mass and strength. Devries and Phillips described its advantages in terms of digestibility and amino acid profile, and the ISSN considers it a valuable protein source.

As with any food, however, individual reactions vary. Most side effects are linked to digestion, allergies, product quality or the overall diet rather than whey itself.

Digestive symptoms and lactose

The most common complaints are bloating, wind, stomach cramps and loose stools. Lactose is often the cause, especially with whey concentrate. Many adults have reduced lactase activity, and lactose malabsorption is common in many populations.

Switching to whey isolate, which contains much less lactose, often solves the problem. Hydrolysed whey or lactose-free products are other options. Taking a smaller serving can also help people identify their tolerance level.

Sweeteners and additives may contribute. Sugar alcohols, such as sorbitol, and some thickeners can cause digestive symptoms in sensitive people. Checking the ingredient list helps identify possible triggers.

Drinking a large shake very quickly or on an empty stomach can increase discomfort. Mixing with water rather than milk, spreading intake through the day and including it with meals may improve tolerance.

Skin: the acne question

Several case reports and small studies have linked whey protein to acne. Silverberg described patients whose moderate to severe acne flared after starting whey and improved after stopping. A study in Brazil by Pontes and colleagues observed new or worsened acne in young adults using protein supplements.

A plausible mechanism is that whey increases insulin and insulin-like growth factor 1 (IGF-1), both of which can stimulate sebum production and skin cell growth. Dairy intake in general has been linked to acne in some research.

Not everyone is affected, and the evidence is based mainly on observational data rather than controlled trials. People with a history of acne seem more likely to experience flares.

If acne appears or worsens after starting whey, options include reducing the dose, trying a plant-based protein or getting more protein from whole foods. A GP or dermatologist can help if the problem persists.

Kidneys, liver and bones

A persistent belief is that high protein intake damages the kidneys. In healthy people, evidence does not support this. A meta-analysis by Devries and colleagues in 2018 found that higher protein intake did not significantly affect kidney function compared with lower intakes in adults without kidney disease.

The situation is different for people with existing chronic kidney disease. They may need to limit protein intake under medical supervision, and whey supplements should not be used without advice.

Concerns about the liver are similar. There is no good evidence that whey harms the liver in healthy individuals. Cases of liver injury associated with "protein supplements" often involve products containing other ingredients, such as anabolic steroids or herbal extracts.

Bones have also been a subject of debate. Earlier theories suggested that high protein intake leaches calcium from bones. Current evidence suggests adequate protein supports bone health, especially when calcium intake is sufficient. Studies by Antonio and colleagues on very high protein diets in trained adults have not found harmful effects on routine health markers over months.

Quality, contaminants and how to minimise problems

Product quality matters. Analyses have sometimes found protein powders with less protein than stated, a practice known as "amino spiking", where cheap amino acids are added to inflate protein readings. Contamination with heavy metals has also been reported in some products.

For athletes in tested sport, there is the risk of contamination with prohibited substances. Using batch-tested products, for example under the Informed Sport programme, reduces this risk.

People with a cow's milk protein allergy must avoid whey completely, as it can cause severe reactions. This is different from lactose intolerance, which concerns sugar rather than protein.

Practical steps to minimise side effects:

1. choose isolate or lactose-free whey if dairy causes digestive symptoms;
2. keep serving sizes reasonable, typically 20–40 g of protein per serving;
3. check the ingredient list for sweeteners or additives that may cause problems;
4. select brands that publish independent test results;
5. use supplements to complement, not replace, a balanced diet.

Conclusions of the editorial team

Whey protein is safe for most healthy adults and one of the best-supported sports supplements, but side effects such as digestive discomfort and acne flares can occur.

Fears about kidney and liver damage are not supported in healthy people, though those with existing kidney disease need medical guidance.

Choosing the right form, sensible doses and tested products addresses most issues. For anyone with persistent symptoms, alternative proteins and whole foods remain excellent options.

We also suggest our articles on plant protein for athletes, on protein needs during fat loss, and on reading protein powder labels.

References

1. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition position stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20.
2. Devries MC, Phillips SM. Supplemental protein in support of muscle mass and health: advantage whey. J Food Sci. 2015;80(Suppl 1):A8–A15.
3. 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. J Nutr. 2018;148(11):1760–1775.
4. Silverberg NB. Whey protein precipitating moderate to severe acne flares in 5 teenaged athletes. Cutis. 2012;90(2):70–72.
5. Pontes TC, Fernandes Filho GM, Trindade AS, et al. Incidence of acne vulgaris in young adult users of protein-calorie supplements in the city of João Pessoa—PB. An Bras Dermatol. 2013;88(6):907–912.
6. Antonio J, Ellerbroek A, Silver T, et al. A high protein diet (3.4 g/kg/d) combined with a heavy resistance training program improves body composition in healthy trained men and women—a follow-up investigation. J Int Soc Sports Nutr. 2015;12:39.
7. Misselwitz B, Pohl D, Frühauf H, et al. Lactose malabsorption and intolerance: pathogenesis, diagnosis and treatment. United European Gastroenterol J. 2013;1(3):151–159.