Does whey protein cause constipation?

We found no evidence that it does, and almost no trial that tested it. Two meta-analyses of diet and constipation found no trial of protein or whey at all. The one randomized trial of a whey product in constipated people went the other way: in 100 adults over 60, 15 g a day of a traditional dried whey for 4 weeks cut the number reporting hard stools from 48 to 0, against 46 to 13 on placebo. That product is not a protein isolate, and the placebo itself contained lactose.

Unsettled. The honest answer is that we found no trial of whey protein powder with constipation as the outcome. What exists is one trial of a traditional whey food, tolerance data from protein drinks, and a national survey in which high protein intake went with more constipation in women and less in men.

What the trials found

Reviews of food and constipation

A meta-analysis of 23 studies in 1,714 adults with chronic constipation covered kiwifruit, mineral water, prunes, rye bread, mango, fig, cereal, oat bran, yogurt, water and high-fiber or no-fiber diets. None tested protein. A 2026 network meta-analysis of 19 randomized trials ranked fruit-based foods first for constipation severity, ahead of fiber supplements and placebo, with moderate certainty. No protein option was among the ones it ranked. The absence of protein trials is the finding.

The one whey trial

The trial in 100 older adults in Iran with functional constipation used Ma'aljobon, a traditional dried whey, at 15 g a day for 4 weeks, against a placebo of lactose, calcium carbonate, maltodextrin and sugar. All constipation scores improved more than on placebo, and 94 people finished. It was a single center, and the product differs from the whey protein powder sold for training.

Protein drinks in other trials

In 67 nursing home residents, two high-protein drinks with 18 to 20 g of protein a day for 8 weeks were well tolerated, and they differed only in flatulence. There was no protein-free comparison, so the trial cannot say whether the protein changed bowel habits. In 46 people aged 65 or over with gastric cancer on chemotherapy, constipation worsened over 8 weeks in the group without high-protein drinks and not in the group taking them. Those drinks were a mixed supplement, not whey alone, given during cancer treatment.

In observational data

In NHANES data on 14,048 US adults, women in the top fifth of total protein intake had twice the odds of constipation of women in the bottom fifth (OR 2.00, 95% CI 1.09 to 3.68). Men in the top fifth had half the odds (OR 0.50, 95% CI 0.28 to 0.91). The study does not give the absolute rates behind those ratios. This is all dietary protein, not whey, and a single survey that cannot show cause.

Who should be careful

Not for everyone. Whey can carry lactose. The US NIDDK lists whey among the label words that mean a product contains lactose, which matters if you are lactose intolerant. Whey isolate usually carries less lactose than concentrate, but the NIDDK page does not separate them.

Other gut effects in trials were mild. In the whey trial in 100 older adults, nobody had a serious side effect, and 2 people on whey stopped because of mild nausea. In a crossover trial in 13 runners, a whey shake of 0.4 g per kg of body weight an hour before a 10 km run raised bloating severity (1.23 against 0.54) compared with 0.15 g per kg, and no other gut symptom changed. That was during hard exercise, and bloating is not constipation.

On kidneys, a systematic review found only 11 documents on whey harms, many in animals, and most tied kidney and liver effects to chronic heavy use. In the nursing home trial, protein drinks for 8 weeks did not change kidney function, including in people with stage 3 chronic kidney disease. That trial was short and did not include advanced kidney disease. We found no trial evidence on whey and constipation in pregnancy, in children or alongside medicines.

What expert bodies say

For constipation, the US NIDDK advises eating enough fiber, 22 to 34 g a day for adults depending on age and sex, and drinking plenty of liquids. It also advises limiting low-fiber foods such as meat and processed foods. The page does not mention whey or protein powders. The EU register of health claims holds 25 claims naming whey and authorizes none of them, among them a gut health and prebiotic claim for fermented whey.

How we searched

What would change this answer

• A randomized trial of whey protein powder at the doses people use for training, against a protein-free drink, with stool frequency and consistency measured as the main outcome.
• A comparison of whey isolate and whey concentrate in people with and without lactose intolerance.
• Survey or cohort data that separate protein supplements from protein in food, and that explain why women and men point in opposite directions.

What whey does for strength and muscle, and what the evidence says about it, is on the whey page. Fiber, which the NIDDK advises for constipation, has its own guide.

Sources

1. ev-wpc-01 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs (17) and uncontrolled trials (6) · n = 1,714
   We included 23 studies (17 RCTs, 6 uncontrolled; 1714 participants): kiwifruit (n = 7), high-mineral water (n = 4), prunes (n = 2), rye bread (n = 2), mango, fig, cereal, oat bran, yoghurt, water supplementation, prune juice, high-fibre diet, no-fibre diet (n = 1).
   
   Who: adults with chronic constipation in intervention trials of foods, drinks and diets
   Effect: studies covered kiwifruit, mineral water, prunes, rye bread, mango, fig, cereal, oat bran, yogurt, water, prune juice, high-fiber and no-fiber diets; none tested protein
   Certainty: the absence of protein trials is the finding; search to July 2023
   Aliment Pharmacol Ther, 2024 · checked 2026-10-07 · we read the abstract
2. ev-wpc-02 · Meta-analysis or systematic review · systematic review and Bayesian network meta-analysis of RCTs · n = 19 trials
   Regarding the severity of constipation, fruit-based foods (category 1) ranked first, superior to fiber supplements (moderate certainty) and placebo (moderate certainty).
   
   Who: people with functional constipation in randomized trials of dietary interventions
   Effect: fruit-based foods superior to fiber supplements and placebo for severity (moderate certainty); no protein intervention among the ranked options
   Certainty: answers what to eat for constipation; does not test whey
   Am J Clin Nutr, 2026 · checked 2026-10-07 · we read the abstract
3. ev-wpc-03 · Randomized controlled trial(s) · randomized double-blind placebo-controlled parallel trial, 4 weeks · n = 100
   Notably, the number of participants reporting hard stools (types 1 and 2) decreased from 48 to 0 in the intervention group, compared to a decrease from 46 to 13 in the control group.
   
   Who: adults aged 60 and over with functional constipation (Rome III), Iran, 15 g a day Ma'aljobon whey powder vs placebo of lactose, calcium carbonate, maltodextrin and sugar; 94 completed
   Effect: hard stools (Bristol types 1 and 2): 48 to 0 with whey vs 46 to 13 with placebo; all constipation scores better than placebo (p < 0.001)
   Certainty: traditional whey product, not a protein isolate; placebo itself contained lactose; single center
   Health Sci Rep. 2026. Effect of Ma'aljobon (Whey) Consumption on Functional Constipation in Older Adults: A Randomized Double-Blind Controlled Trial · checked 2026-10-07 · we read the fulltext
4. ev-wpc-04 · Randomized controlled trial(s) · randomized double-blind placebo-controlled parallel trial, 4 weeks · n = 100
   Of the six participants who withdrew, two from the intervention group cited mild nausea as the reason for discontinuation.
   
   Who: adults aged 60 and over with functional constipation, 15 g a day dried whey vs placebo
   Effect: no serious adverse events; 2 withdrawals for mild nausea on whey, 1 for abdominal pain on placebo
   Certainty: short trial; safety reported as withdrawals only
   Health Sci Rep. 2026. Effect of Ma'aljobon (Whey) Consumption on Functional Constipation in Older Adults: A Randomized Double-Blind Controlled Trial · checked 2026-10-07 · we read the fulltext
5. ev-wpc-05 · Randomized controlled trial(s) · single-blind randomized crossover trial · n = 13
   A significant increase in bloating severity was observed following the moderate-protein shake as compared to the low-protein shake during the run (0.54 vs 1.23; p = 0.03), but no other symptoms assessed were significantly impacted by the shake composition.
   
   Who: recreational runners (8 women, 5 men), whey shake 60 minutes before a 10 km treadmill run at 85% race pace
   Effect: bloating severity 1.23 vs 0.54 (p = 0.03); total symptoms not affected by protein dose (p = 0.85)
   Certainty: acute, during exercise, tiny sample; bloating is not constipation
   J Int Soc Sports Nutr, 2026 · checked 2026-10-07 · we read the abstract
6. ev-wpc-06 · Randomized controlled trial(s) · randomized controlled trial, 8 weeks · n = 46
   Additionally, constipation symptoms worsened significantly in the non-ONS group by week 8 (p = 0.008).
   
   Who: patients aged 65 or older with locally advanced gastric cancer on neoadjuvant chemotherapy; high-protein oral supplement plus standard care vs standard care
   Effect: constipation symptoms worsened significantly in the non-supplement group by week 8 (p = 0.008)
   Certainty: mixed oral nutritional supplement, not whey alone; cancer and chemotherapy context
   Support Care Cancer, 2025 · checked 2026-10-07 · we read the abstract
7. ev-wpc-07 · Randomized controlled trial(s) · randomized single-blind parallel-group multi-country trial · n = 67
   No clinically relevant and, except for flatulence, no statistically significant differences in gastro-intestinal tolerance were observed between groups.
   
   Who: nursing home residents needing oral nutritional supplements; 6.8 fl oz (200 ml) (20 g protein) vs 4.2 fl oz (125 ml) (18 g protein) high-protein drink daily for 8 weeks
   Effect: stool frequency, consistency and gut symptoms: no clinically relevant differences between versions, except flatulence
   Certainty: two protein drinks compared, no protein-free control
   J Nutr Health Aging, 2016 · checked 2026-10-07 · we read the abstract
8. ev-wpc-08 · Randomized controlled trial(s) · randomized single-blind parallel-group multi-country trial · n = 67
   High protein ONS seems to be well-tolerated and safe; there is no indication that it affects renal function in nursing home residents, including patients with stage 3 chronic kidney disease, under the conditions tested.
   
   Who: nursing home residents, including stage 3 chronic kidney disease, 18 to 20 g protein drink daily for 8 weeks
   Effect: no difference in eGFR or urinary albumin/creatinine ratio between groups or over time
   Certainty: 8 weeks only; not advanced kidney disease
   J Nutr Health Aging, 2016 · checked 2026-10-07 · we read the abstract
9. ev-wpc-09 · Observational data · cross-sectional analysis of a national survey · n = 14,048
   After comprehensive adjustment for covariates in model 3, women in Q5 had a significantly higher risk of constipation compared to Q1 (OR = 2.00, 95% CI: 1.09, 3.68, p = 0.0247), with a significant effect observed also in Q4 (OR = 1.54, 95% CI: 1.02, 2.34, p = 0.0423).
   
   Who: US women aged 20 and over, NHANES 2005-2010, constipation by Bristol stool types 1 and 2
   Effect: women Q5 vs Q1 OR 2.00 (95% CI 1.09 to 3.68); Q4 OR 1.54 (1.02 to 2.34)
   Certainty: all dietary protein, not whey; cross-sectional; OR is odds, not risk
   Front Nutr. 2024;11:1393596. Gender differences in the association between dietary protein intake and constipation: findings from NHANES · checked 2026-10-07 · we read the fulltext
10. ev-wpc-10 · Observational data · cross-sectional analysis of a national survey · n = 14,048
    In the fully adjusted model 3, the ORs of constipation in Q5 remained 0.50 times that of Q1 (95% CI: 0.28, 0.91, p = 0.0235).
    
    Who: US men aged 20 and over, NHANES 2005-2010
    Effect: men Q5 vs Q1 OR 0.50 (95% CI 0.28 to 0.91), fully adjusted
    Certainty: all dietary protein, not whey; opposite direction to women
    Front Nutr. 2024;11:1393596. Gender differences in the association between dietary protein intake and constipation: findings from NHANES · checked 2026-10-07 · we read the fulltext
11. ev-wpc-11 · Observational data · systematic review · n = 11 documents
    The majority of the papers associated the damaging effect with the chronic and abusive use of whey protein, with the kidneys and liver being the main organs affected.
    
    Who: experimental and randomized studies of whey protein supplementation
    Effect: harms linked to chronic, excessive use: kidney and liver, also acne and microbiota changes
    Certainty: few and mixed studies, many in animals
    Appl Physiol Nutr Metab, 2021 · checked 2026-10-07 · we read the abstract
12. ev-wpc-12 · Position of an expert body · agency patient guidance · n = —
    Eat enough fiber. Drink plenty of liquids to help the fiber work better.
    
    Who: people with constipation
    Effect: fiber 22 to 34 g a day for adults, depending on age and sex
    Certainty: general advice; whey and protein powders not mentioned
    NIDDK (NIH), Eating, Diet, & Nutrition for Constipation · checked 2026-10-07 · we read the section
13. ev-wpc-13 · Position of an expert body · agency patient guidance · n = —
    The following words mean that the product contains lactose: milk, lactose, whey, curds, milk by-products, dry milk solids, nonfat dry milk powder
    
    Who: people with lactose intolerance reading food labels
    Effect: whey named as a lactose-containing ingredient
    Certainty: whey isolate carries far less lactose than concentrate; the page does not split them
    NIDDK (NIH), Eating, Diet, & Nutrition for Lactose Intolerance · checked 2026-10-07 · we read the section
14. whe-dr-06 · Position of an expert body · EU Register on nutrition and health claims, claim POL-HC-7090, snapshot of 2026-09-21 · n = —
    POL-HC-7090 Fermented whey For stomach health. Maintains a healthy gut bacteria population and aids the metabolism. The (L+) lactic acid resulting from fermentation is a natural prebiotic and makes the metabolism more effective. Non-authorised
    
    Who: —
    Effect: 25 claims naming whey are in the register and 25 carry the status Non-authorized, with none authorized; the example row is a gut health and prebiotic claim for fermented whey
    Certainty: a refusal means the submitted dossier did not establish the relationship to the regulator's satisfaction, and not that the substance was shown to do nothing. The register is indexed by substance, and claims about protein in general, which cover whey in practice, were assessed separately. The conditions of use column is missing from this snapshot
    EU Register on nutrition and health claims, claim POL-HC-7090, snapshot 2026-09-21 · checked 2026-09-22 · we read the row