Why Do Protein Shakes Give Me Diarrhea?

Why Do Protein Shakes Give Me Diarrhea?

Why Do Protein Shakes Give Me Diarrhea?
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Jody Braverman, CPT, FNS, RYT
Updated on July 31, 2025
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Protein shakes are supposed to help you build muscle, recover faster, and perform better. But for some, drinking a protein shake after a workout leads to diarrhea.

Here are some reasons why protein shakes might get you down, along with ways to possibly avoid digestive issues in the future.

Lactose Intolerance

Even if you don’t use dairy in your shakes, your protein powder might contain lactose. That’s because whey protein, a common ingredient for protein powder products, is actually a byproduct of cheese production.

In order to make cheese, manufacturers often add enzymes to milk. The enzymes curdle the milk and separate the liquid “whey” protein from the curds. This liquid contains lactose.

Symptoms of lactose intolerance include nausea, vomiting, swelling, colic gas, and diarrhea.

If you think lactose intolerance could be the culprit, look for “whey protein isolate,” which has a lower level of lactose. Or choose a different protein source, such as egg, pea, rice, or hemp protein. These may be better fits for a sensitive stomach.

 Also, make sure to seek medical care if your symptoms persist.

Sensitivity to Sweeteners

Look closely at the ingredients list of your protein powder of choice. Do you see any words ending in "-ol"? If so, it likely contains some sort of sugar substitute. Faux sweeteners can have a laxative effect and lead to bloating, gas, and diarrhea.

Even pure sugar can cause diarrhea.

 Sugar causes the gut to expel water and electrolytes, both of which loosen the bowels. And too much sugar in your protein shake — especially fructose, the kind found naturally in fruits such as pears and apples — can cause stomach trouble.

Carefully read the label on your chosen protein powder. Look for a product that contains low or no sugar, without artificial sweeteners.

Drinking Immediately After Exercise

If you start on your protein shake right after a workout, your stomach might not be ready for it.

Exercise impacts your digestive system in various ways. Small bowel transit is delayed, causing slower-than-normal movement of food through the intestine. And intestinal absorption may be compromised, meaning the intestines are not sufficiently taking up nutrients. This is especially the case following high-intensity exercise.

That’s why it's a good idea to wait a little while after your workout to let your system recover homeostasis before you introduce a heavy dose of protein.

Drinking Rapidly

Consuming anything too quickly can lead to an stomach upset. Because it's liquid, a protein shake may go down easily, but it's dense. That's a lot of material for your gut to process all at once, and your body may let you know in an unpleasant way.

Try sipping your shake more slowly, and see if that helps your tummy troubles.

The Takeaway

  • Many protein shakes contain lactose, which can cause digestive upset for people who have lactose intolerance. Also, some shakes contain artificial sweeteners and high amounts of sugar, both of which can lead to bloating, gas, and diarrhea.
  • Drinking a protein shake too soon after intense exercise can overwhelm your digestive system, which is still recovering and may have compromised nutrient absorption.
  • Drinking your protein shake too quickly also can lead to stomach discomfort. Sipping it slowly allows your gut more time to process the ingredients.
EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
  1. Is Whey Protein Good for You? Cleveland Clinic. January 4, 2021.
  2. Lactose Intolerance. Mayo Clinic. March 5, 2022.
  3. Artificial Sweeteners and Other Sugar Substitutes. Mayo Clinic. January 10, 2023.
  4. Is Something in Your Diet Causing Diarrhea? Harvard Health Publishing. August 8, 2023.
  5. Severo JS et al. Physical Exercise as a Therapeutic Approach in Gastrointestinal Diseases. Journal of Clinical Medicine. March 3, 2025.
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Roxana Ehsani, RDN

Roxana Ehsani, RD

Medical Reviewer

Roxana Ehsani, RD, is a Miami-based licensed dietitian-nutritionist, board-certified specialist in sports dietetics, and media spokesperson, consultant, and content creator for food and nutrition brands. She is an adjunct instructor for sports nutrition at Virginia Tech in Blacksburg.

Ehsani appears as a food and nutrition expert for television stations across the nation and in national publications, including Runner's World, Women's Health, Glamour, and more, and is a contributing writer for EatingWell. She has a strong background in sports nutrition and has worked with professional, Olympic, collegiate, and high school teams and individual athletes, whom she sees through her private practice. 

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Jody Braverman, CPT, FNS, RYT

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