Does protein powder cause constipation?

There is no clear sign that it does, but nobody has tested it directly. We found no randomized trial of protein powder with constipation as the outcome. The closest randomized evidence comes from 15 trials with 5,277 critically ill adults fed by tube, where more protein (about 1.05 against 0.47 g per kg a day) did not change the risk of constipation (RR 1.02, 95% CI 0.92 to 1.13). The abstract gives only the ratio, so we cannot say how many people in a hundred were affected on either side. Tube feeding in intensive care is far from a shake at home, which is why this page answers with indirect evidence and says so.

Unsettled. The direct question is untested. Higher-protein diets do bring more stomach and bowel complaints overall, but the indirect data do not single out constipation, and in one survey harder stools went with protein only when carbohydrates were low. One trial that compares beef and whey protein on a constipation scale is registered, and its results are not yet posted.

What the trials found

More protein and the gut in general

A review of 74 randomized trials comparing higher- and lower-protein diets in adults found stomach and bowel side effects were more common on the higher-protein diets. The abstract does not separate constipation from other complaints and gives no rates. In the tube-feeding trials above, the extra protein raised vomiting by 25% (RR 1.25, 95% CI 1.11 to 1.42) while diarrhea and constipation did not change.

Supplements specifically

Two small randomized trials come closest to a protein supplement in healthy people. In a 10-week pilot in 24 endurance runners, a whey and beef protein supplement did not change stool water content or pH compared with maltodextrin. Bowel habits were not measured. In a 7-day crossover feeding trial in 10 healthy adults, one person had constipation on a diet with 25% of energy from protein, against none reported on a 10% protein diet. Tolerance was a secondary outcome, and one case in ten people over a week cannot carry a conclusion.

What people eat around the protein

Among 13,941 US adults in a national survey, eating 10 g more protein a day was not linked to constipation, judged by stool frequency (OR 0.94, 95% CI 0.54 to 1.62) or by stool consistency (OR 1.02, 95% CI 0.75 to 1.39). In people eating few carbohydrates, each extra 10 g of protein went with 8% higher odds of hard stools (OR 1.08, 95% CI 1.02 to 1.14). In people eating moderate carbohydrates the odds went down slightly (OR 0.94, 95% CI 0.89 to 0.99). That is a subgroup finding from a one-time diet recall, and low-carbohydrate diets are usually also low in fiber, which this analysis cannot separate.

Fiber is the measure with trial support. In 16 randomized trials with 1,251 adults with chronic constipation, 66% responded to fiber against 41% on control (RR 1.48, 95% CI 1.17 to 1.88), with psyllium and pectin showing significant effects. The trials varied considerably. They were in adults with chronic constipation, not in people taking protein supplements.

Who should be careful

Not for everyone. Very low-carbohydrate eating is where constipation shows up most. Across 36 studies of ketogenic diets, constipation was the most common single side effect, reported in 24% of participants. Those diets are high in fat rather than protein and many of the participants had epilepsy, so the number describes the diet pattern and does not measure protein powder.

In 35 randomized trials with 3,701 people with cancer taking protein supplements of at least 10 g per serving, no serious adverse effects were reported. Minor complaints such as constipation are not reported in that abstract, and many of the trials were at high risk of bias.

Young children are the one group where a protein clearly matters, and it is the type of protein rather than the amount. A review of 52 trials with 4,668 children with functional constipation found a cow's milk exclusion diet may help, based on 2 trials in children whose constipation may reflect cow's milk allergy. In a double-blind crossover trial in 65 children aged 11 to 72 months with stubborn constipation, 44 of them (68%) improved on soy milk and none improved while drinking cow's milk. That was a selected clinic group, often with signs of allergy, and it tells us nothing about adults or about protein powder. In these studies the milk change was made in children seen in specialist clinics, with the allergy confirmed by food challenge.

What expert bodies say

The European Academy of Allergy and Clinical Immunology states that in studies of children whose cow's milk allergy was confirmed by food challenge, between 28% and 78% improve on a cow's milk free diet. That is a consensus position, written because the data were too sparse for a systematic review.

The US National Institute of Diabetes and Digestive and Kidney Diseases advises people with constipation to avoid foods with little or no fiber, and names meat, chips, fast food and processed foods among them. It does not mention protein powder. We found no position on protein powder and constipation from the NIH Office of Dietary Supplements, EFSA or the FDA.

How we searched

What would change this answer

• Results from a registered trial comparing beef and whey protein on a standard constipation scale in 100 participants (NCT07477223). Its completion date was June 2026 and no results are posted yet. It would be the first direct test of the question on this page.
• A recruiting trial of fermented dairy protein with added prebiotic fiber (NCT06946017), with primary completion in August 2026, which could show whether adding fiber to a protein drink changes bowel habits.
• A placebo-controlled trial of an ordinary protein powder in healthy adults with bowel habits as the main outcome. Nothing like it turned up.

For the products themselves, see the protein supplement and whey pages, and the protein page for the nutrient.

Sources

1. ev-pcon-01 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 74 trials
   Adverse gastrointestinal events were more common with high-protein diets.
   
   Who: adults in randomized controlled trials comparing higher- and lower-protein diets
   Effect: adverse gastrointestinal events more frequent with high-protein diets; type and rate not given in the abstract
   Certainty: Constipation is not separated from other GI events in the abstract; full text not in PMC.
   Eur J Clin Nutr, 2012 · checked 2026-10-06 · we read the abstract
2. ev-pcon-02 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized trials · n = 5277
   The higher-protein group had an increased risk of vomiting (RR, 1.25; 95% CI, 1.11-1.42; P = 0.0004), with no evidence of heterogeneity (I2 = 0%; P = 0.58) and no influence on gastric residual volumes (GRVs) ≥300 ml (RR, 1.18; 95% CI, 0.69-2.00; P = 0.55), prokinetic administration (RR, 1.24; 95% CI, 0.98-1.56; P = 0.07), abdominal distension (RR, 0.67; 95% CI, 0.40-1.13; P = 0.14), diarrhea (RR, 0.99; 95% CI, 0.81-1.20; P = 0.90), or constipation (RR, 1.02; 95% CI, 0.92-1.13; P = 0.68).
   
   Who: critically ill adults on enteral nutrition in parallel-group randomized trials with different protein delivery
   Effect: constipation RR 1.02 (95% CI 0.92 to 1.13); vomiting RR 1.25 (1.11 to 1.42); diarrhea RR 0.99 (0.81 to 1.20)
   Certainty: Tube feeding in intensive care is far from a shake at home; this is the best randomized data on protein dose and constipation, but indirect.
   JPEN J Parenter Enteral Nutr, 2026 · checked 2026-10-06 · we read the abstract
3. ev-pcon-03 · Observational data · cross-sectional study · n = 13941
   After adjusting for covariates, an increase in dietary protein intake of 10 g was not associated with constipation, as defined by stool frequency (OR = 0.94, 95% CI = 0.54, 1.62) or stool consistency (OR = 1.02, 95% CI = 0.75, 1.39).
   
   Who: 13,941 adults from the National Health and Nutrition Examination Survey 2005-2010
   Effect: per 10 g/day protein: OR 0.94 (95% CI 0.54 to 1.62) for frequency-defined and OR 1.02 (0.75 to 1.39) for consistency-defined constipation
   Certainty: Cross-sectional, one-time diet recall; OR is odds, not risk.
   Neurogastroenterol Motil, 2024 · checked 2026-10-06 · we read the abstract
4. ev-pcon-04 · Observational data · cross-sectional study · n = 13941
   Subgroup analyses revealed that dietary protein intake was associated with an increase in constipation defined by stool consistency risk in participants who consumed a low amount of carbohydrates (OR = 1.08, 95% CI = 1.02-1.14 for every 10-g increase in protein intake), but a decrease in risk in participants in the moderate-carbohydrate group (OR = 0.94, 95% CI = 0.89-0.99 for every 10-g increase in protein intake), suggesting a significant interaction (p = 0.001).
   
   Who: US adults in NHANES 2005-2010, stratified by carbohydrate intake
   Effect: per 10 g/day protein: OR 1.08 (1.02 to 1.14) in the low-carbohydrate group; OR 0.94 (0.89 to 0.99) in the moderate-carbohydrate group; interaction p = 0.001
   Certainty: Subgroup finding; low-carbohydrate diets are usually also low in fiber, which this analysis cannot separate.
   Neurogastroenterol Motil, 2024 · checked 2026-10-06 · we read the abstract
5. ev-pcon-05 · Randomized controlled trial(s) · randomized, controlled, double-blind pilot study · n = 24
   Fecal pH, water content, ammonia, and SCFA concentrations did not change, indicating that protein supplementation did not increase the presence of these fermentation-derived metabolites.
   
   Who: cross-country runners, protein supplement (whey isolate and beef hydrolysate) n = 12 vs maltodextrin n = 12
   Effect: fecal pH, water content, ammonia and SCFA unchanged over 10 weeks; some health-related gut bacteria decreased
   Certainty: Pilot of 24 people; bowel habits and constipation were not measured directly.
   Nutrients, 2018 · checked 2026-10-06 · we read the abstract
6. ev-pcon-06 · Randomized controlled trial(s) · randomized controlled crossover feeding study · n = 10
   Minor dietary tolerance symptoms included constipation in one participant on the higher-protein diet, mild discomfort in three participants, and fatigue in seven participants on the lower-protein diet.
   
   Who: ten healthy adults aged 22-34 in a controlled crossover feeding trial of 10% vs 25% energy from protein
   Effect: constipation in 1 of 10 on the higher-protein diet; mild discomfort in 3 and fatigue in 7 on the lower-protein diet
   Certainty: Very small and short; tolerance was a secondary outcome.
   Sci Rep, 2026 · checked 2026-10-06 · we read the abstract
7. ev-pcon-07 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 4668
   Meta-analyses for treatment success and/or defecation frequency, including 20 RCTs, showed abdominal electrical stimulation (n = 3), Cassia Fistula emulsion (n = 2), and a cow's milk exclusion diet (n = 2 in a subpopulation with constipation as a possible manifestation of cow's milk allergy) may be effective.
   
   Who: children aged 2 weeks to 18 years with functional constipation in 52 RCTs
   Effect: cow's milk exclusion diet may be effective (2 RCTs in a subpopulation with possible cow's milk allergy); high risk of bias overall
   Certainty: Two small trials in a selected subgroup; this is about milk protein allergy, not protein amount.
   J Pediatr, 2022 · checked 2026-10-06 · we read the abstract
8. ev-pcon-08 · Randomized controlled trial(s) · double-blind crossover randomized controlled trial · n = 65
   Forty-four of the 65 children (68 percent) had a response while receiving soy milk.
   
   Who: 65 children aged 11 to 72 months with chronic constipation referred to a pediatric gastroenterology clinic
   Effect: response (8 or more bowel movements in 2 weeks) in 44 of 65 on soy milk vs 0 on cow's milk; confirmed by double-blind cow's milk challenge
   Certainty: Selected clinic population with laxative-resistant constipation and often allergy signs; not about adults or protein powder.
   N Engl J Med, 1998 · checked 2026-10-06 · we read the abstract
9. ev-pcon-09 · Position of an expert body · position paper using Delphi consensus · n = —
   However, many studies in children report challenge proven cow's milk allergy and constipation as a primary symptom and have found that between 28% and 78% of children improve on a cow's milk elimination diet.
   
   Who: young children with constipation and cow's milk allergy confirmed by food challenge
   Effect: 28% to 78% improve on cow's milk elimination diet; constipation listed in all English-language cow's milk allergy guidelines
   Certainty: Consensus because data were too sparse for a systematic review.
   Pediatr Allergy Immunol, 2024 · checked 2026-10-06 · we read the abstract
10. ev-pcon-10 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 3701
    No serious adverse effects were reported.
    
    Who: adult patients undergoing cancer therapy in randomized controlled trials of supplements with at least 10 g protein per serving
    Effect: no serious adverse effects reported
    Certainty: Minor GI effects such as constipation are not reported in the abstract; high risk of bias in many trials.
    Am J Clin Nutr, 2024 · checked 2026-10-06 · we read the abstract
11. ev-pcon-11 · Observational data · systematic review of prospective interventional studies · n = 36 studies
    Constipation (24%) was the most commonly reported individual adverse event.
    
    Who: participants of prospective interventional studies of ketogenic diets (classic, modified Atkins, MCT, low glycemic index) published 2019-2024
    Effect: constipation 24%, the most common adverse event; GI events 40% of all adverse events; stricter diets had more events
    Certainty: Mixed designs, many epilepsy patients, inconsistent adverse event reporting; ketogenic diets are high fat rather than high protein.
    BMC Nutr, 2026 · checked 2026-10-06 · we read the abstract
12. ev-pcon-12 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 1251
    Overall, 311 of 473 (66%) participants responded to fiber treatment and 134 of 329 (41%) responded to control treatment [RR: 1.48 (95% CI: 1.17, 1.88; P = 0.001); I2 = 57% (P = 0.007)], with psyllium and pectin having significant effects.
    
    Who: adults with chronic constipation in randomized controlled trials of fiber supplementation
    Effect: response RR 1.48 (95% CI 1.17 to 1.88); best with psyllium, doses above 10 g/d, at least 4 weeks; more flatulence
    Certainty: Considerable heterogeneity; this is about fiber, the nutrient protein shakes usually lack, not about protein itself.
    Am J Clin Nutr, 2022 · checked 2026-10-06 · we read the abstract
13. ev-pcon-13 · Position of an expert body · agency patient guidance · n = —
    To help prevent or relieve constipation, avoid foods with little to no fiber, such as chips, fast food, meat, prepared foods, such as some frozen meals and snack foods, processed foods, such as hot dogs or some microwavable dinners
    
    Who: general public with constipation
    Effect: avoid low-fiber foods (chips, fast food, meat, prepared and processed foods)
    Certainty: Patient advice without its own analysis; protein powder is not named.
    NIDDK, Eating, Diet, & Nutrition for Constipation (NIH) · checked 2026-10-06 · we read the section