Protein Shakes & Diarrhea: Causes + Fixes Explained

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Protein shakes can be a convenient way to meet your daily protein requirements, but not if you end up experiencing diarrhea.

Key Takeaways

🥛 Lactose Intolerance: Up to 65% of people globally are lactose intolerant, which can cause diarrhea when consuming whey protein, especially if it contains lactose. Opting for lactose-free whey protein powders may alleviate symptoms.

🍬 Sugar Alcohol Sensitivity: Sugar alcohols like sorbitol in protein shakes can cause digestive issues for some individuals. Choosing a protein powder sweetened with alternative sweeteners or no sweeteners may prevent diarrhea.

🚀 Rapid Consumption: Drinking protein shakes too quickly can draw excess water into the small intestine, potentially causing diarrhea. Slow, mindful consumption of shakes, taking at least 5-10 minutes per serving, can mitigate this issue.

6 Reasons Whey Protein Shakes Can Cause Diarrhea

Here are the six reasons why whey protein shakes can cause diarrhea:

1. You Have A Lactose Intolerance

Up to 65% of the world’s population is intolerant to lactose, the naturally occurring form of sugar in dairy milk from cows and goats.  

People who are lactose intolerant do not make the enzyme lactase in their bodies. 

This means they have a more challenging time digesting lactose, and symptoms can include pain, bloating, diarrhea, gas, and nausea.  

While whey protein isolate has little or no lactose, whey protein concentrate can have several grams of lactose per serving.  

Depending on the severity of the lactose intolerance, even a tiny amount of lactose in a scoop of whey protein isolate could cause problems.

How to Fix

If you are lactose intolerant, switch to a lactose-free whey protein powder. 

Note that even pure whey isolate can still have a small amount of lactose remaining, so if you are extremely sensitive, it is essential to look for “lactose-free” on the label. 

I recommend Transparent Labs’ 100% Grass Fed Whey Protein.

While it has a higher cost per serving, Transparent Labs contains 93% protein, higher than any other protein I’ve tested.  

It also has completely transparent labels, so you know the exact amount of each ingredient in the product (most supplement companies don’t do this).

When buying whey protein, you need to check the ingredients carefully for whey protein concentrate; this version is less costly to manufacture, so supplement companies often make whey protein blends with both isolate and concentrate to keep their costs down.

You can expect to pay more for pure whey protein isolate than a whey protein blend, but having a shake that doesn’t give you diarrhea is well worth it.

Transparent Labs

The Transparent Labs Grass-Fed Whey Isolate is the #1 protein that recommends.

2. The Sugar Alcohol Content Is Too High

Sugar alcohols such as sorbitol or mannitol are commonly found in protein shakes because they provide sweetness for a fraction of the calories of sugar.  

The downside is that they can cause digestive upset in sensitive individuals.  

How to Fix

If you are sensitive to sugar alcohols, look for unsweetened protein powders or protein powders sweetened with sugar or high-intensity sweeteners such as Sucralose, acesulfame potassium, or stevia instead.

A good bet is to buy an unflavored version, which you then have the option to flavor yourself using sweeteners you know don’t upset your stomach.

Check out our round-up of the Best Unflavored Protein Powders.

3. There Are Additives You’re Not Used To Digesting

Food additives like gums, thickeners, and emulsifiers are common in protein powders and ready-made protein shakes to give them a smooth, creamy texture.  

Typical examples are acacia, arabic, carob bean, guar, gellan or xanthan gums, carrageenan, and soy and sunflower lecithin.

In sensitive individuals, any one or more of the ingredients above can result in intestinal discomfort, including diarrhea.

How to Fix

If you suspect that additives in your protein shake are causing diarrhea, read labels carefully and switch to a brand with little to no added ingredients beyond protein powder. 

4. The Fiber Content Is Too High 

Both too much and too little fiber have been linked to diarrhea. 

Fiber absorbs water and adds bulk to stool, slowing digestion time and keeping food from moving through the intestines too quickly.  

For people who do not consume enough fiber in their daily diet, missing out on this bulk can mean diarrhea caused by food moving too quickly through the digestive tract. 

If their protein shake also does not contain fiber, the problem can be even worse because it adds to the liquids consumed.

On the other hand, too much fiber can result in too soft stools, which also move too quickly through the digestive tract. 

For people facing this problem, a protein shake with added fiber would exacerbate this issue.

How to Fix

If you suspect fiber intake is causing diarrhea, aim for a total daily intake of 25g-28g of fiber for women and 31-34g of fiber for men

Make sure your protein shake helps you achieve this target, either by adding fiber to help you reach the minimum or looking for a brand with no added fiber to avoid exceeding the maximum recommended amounts.

5. You Are Drinking The Protein Shake Too Quickly

Drinking a protein shake too quickly can cause digestive upset, including diarrhea, by causing your body to send a lot of liquid to the stomach and small intestine to start digesting a large amount of ingested nutrients.  

Drawing water into the small intestine can soften stool to the point of diarrhea. 

How to Fix

If you’re downing your protein shake, slow down and take a break between each mouthful. 

Aim for it to take at least 5 minutes for each cup (250mL) of shake, e.g. a 500mL shake should take about 10 minutes to drink.

6. You Have A Milk Allergy

A milk allergy differs from lactose intolerance: the allergy is an adverse reaction to one or more whey proteins in dairy milk. 

Only about 0.4% of the adult population in the US has a milk allergy, making it much less common than lactose intolerance.  

How to Fix

If you are allergic to milk, avoid all forms of whey protein and switch to alternative protein powders instead, such as egg white or plant-based protein powders (e.g. rice, pea or hemp).

I recommend starting with our Pea vs. Whey Protein article to understand the differences between these two types of protein powders.

6 Simple Steps for Choosing A Whey Protein That Doesn’t Cause Diarrhea

6 simple steps for choosing a whey protein that doesn’t cause diarrhea

Follow the six simple steps below to choose a whey protein that doesn’t cause diarrhea.

Step 1: Mix With Water

First, try mixing your protein powder with just water and no other ingredients to ensure that your reaction is to the whey protein powder and not to the liquid it is mixed with or any other added ingredients.

Step 2: Drink Your Shake Slowly

Next, drink your protein shake slowly, taking time between each small sip versus chugging it down in big gulps after your workout. 

Slowing down the consumption of your shake could be enough to alleviate your symptoms.

Step 3: Switch to Whey Protein Isolate

If you’re still experiencing issues, the problem could be that you are lactose intolerant.  

Whey protein isolate contains little to no lactose, but whey protein concentrate and protein powder blends containing whey protein concentrate will still have lactose.  

Switch to 100% whey protein isolate or look for brands that are explicitly lactose-free.

Step 4: Skip Sugar Alcohols

Be sure your protein powder is unsweetened or sweetened only with high-intensity sweeteners such as sucralose, acesulfame potassium or stevia instead of sugar alcohols.  

High-intensity sweeteners are so sweet that only a few milligrams are needed to achieve the same sweetness as several grams of sugar or sugar alcohol.  

We’ve compiled a list of the Best Protein Powders That Only Use Stevia.

Step 5: Avoid Additives

Beyond sweeteners, if your whey protein powder is still causing issues, steer clear of any additives such as acacia gum, carrageenan, soy or sunflower lecithin, xanthan gum, etc.  This will let you know if it’s an additive that is causing the problem.

Step 6: Try Plant-Based Protein Powder

Although true allergies to dairy milk (an allergy to the whey proteins in milk versus lactose intolerance, the sugar in milk) are infrequent in adults, if the first five steps do not improve your diarrhea, then you could try a plant-based protein powder such as hemp, rice, or pea protein powder.

What To Do If You Still Get Diarrhea After Implementing These Tips

stomach ache

If you still get diarrhea after implementing these tips, take a break from protein powder and focus on getting your protein from whole foods instead. 

It’s also wise to consult with your healthcare provider to rule out other causes of your diarrhea.

Frequently Asked Questions

Is It Common For People To Get Diarrhea After Drinking Whey Protein?

Yes, diarrhea is a common issue after drinking whey protein for people who are lactose-intolerant or allergic to whey protein. 

Diarrhea is also common for people who are sensitive to sugar alcohols and/or other food additives such as thickeners or emulsifiers.  Diarrhea also occurs when drinking whey protein too fast.

If I Have Diarrhea After Drinking Whey Protein Does The Protein Go To Waste?

No, not all of the protein goes to waste. The amount of protein that is lost due to diarrhea depends on the type of diarrhea as well as its frequency, severity, and duration. 

The effects vary greatly from person to person but rarely does diarrhea cause a complete loss of the calories and nutrients consumed.

Can Drinking Too Much Protein At Once Cause Diarrhea?

Yes, drinking too much protein at once can cause diarrhea, especially if the protein powder contains lactose (common in whey protein concentrate), has been sweetened with sugar alcohols, contains other additives, or is consumed too quickly. 

A large serving will increase the amount of offending ingredients.

Why Do Meal Replacement Shakes Cause Diarrhea?

Meal replacement shakes can cause diarrhea for a variety of reasons, depending on their ingredients.  Common culprits include lactose, sugar alcohols, liquid oils, fiber, and food additives such as thickeners and emulsifiers. 

The effects are greater for sensitive individuals with allergies or intolerances, or IBS. 

Can You Drink Protein Shakes When You Have Diarrhea?

Yes, you can drink protein shakes when you have diarrhea.  If you have a stomach bug that is causing diarrhea, chances are that you could be experiencing nausea too, which can make it hard to eat solid food and keep it down, and both cause dehydration. 

Protein shakes can help provide needed nutrients and hydration.

Other Whey Protein Resources


Adrienne Lenhart , William D Chey, A Systematic Review of the Effects of Polyols on Gastrointestinal Health and Irritable Bowel Syndrome, Advances in Nutrition, Volume 8, Issue 4, July 2017, Pages 587–596,

About The Author

Lauren Graham

Lauren Graham is a Precision Nutrition Level 1 certified nutrition coach. She focuses on helping busy professionals balance healthy eating and purposeful movement.  Lauren has a background in competitive swimming and is currently competing as a CrossFit athlete.  She has a passion for training, teaching, and writing. 

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