Does Mass Gainer Give You Diarrhoea?

A picture of Best Mass Gainer UK founder, personal trainer Andy Williams
Written By
Andy Williams
Last Updated on

For gaining weight and building muscle—and doing so quickly—there are few better supplements than mass gainers. But, like any bodybuilding supplement, mass gainers aren’t without their drawbacks.

Are mass gainers bad?

No, not for most people. However, some people report feeling nauseous and having diarrhoea (that’s “diarrhea”, for my North American readers) after drinking their mass gainer. So, with that in mind, this article will explore some of the reasons for your mass gainer diarrhoea and give you some advice for avoiding it in the future.

Can Mass Gainers Cause Diarrhoea?

A weight gainer bag positioned on the toilet seat to illustrate the fact that mass gainer can cause diarrhoea

Yes, mass gainers can cause diarrhoea for a few reasons. One possible cause of mass gainer-induced diarrhoea is lactose intolerance. Since weight gainers contain the milk proteins whey and casein, it can cause diarrhoea for those whose bodies are unable to process this sugar.

Also, the artificial sweeteners that many mass gainer manufacturers use can be another trigger for diarrhoea. [1] So you may want to choose a weight gainer that opts for natural flavourings rather than artificial ones, even if that means sacrificing on taste a bit.

There may also be a specific food additive that your body doesn’t react well with. This is one reason why I always advocate choosing a mass gainer with a simple, understandable ingredient list so that you know exactly what you’re putting inside your body.

Another possibility is that you consumed your mass gainer too soon after your workout. After training, your digestive system may not be able to absorb the food and nutrients that you consume as effectively, which scientists refer to as exercise-induced gastrointestinal syndrome. [2

Similarly, another reason for mass gainer causing diarrhoea could be downing your shake post-workout. if you drink your mass gainer as fast as possible, the immediate influx of hundreds of calories could cause diarrhoea.

Is The Maltodextrin To Blame?

Some maltodextrin powder

Most commercial mass gainers are absolutely loaded with maltodextrin. So if protein shakes don’t give you diarrhoea, but mass gainers do, then perhaps this overused carbohydrate is to blame?

Health experts note that maltodextrin can cause the proliferation of bad bacteria in the gut while simultaneously decreasing the amount of good bacteria. [3]

Furthermore, the absurd quantities that maltodextrin is used in many mass gainer formulas (often 200+ grams) can easily overload your stomach and cause diarrhoea. 

So, while maltodextrin in your mass gainer might be okay if it’s used as a bulking agent, you should try to find a weight gainer that uses oats as the primary carbohydrate source.

Not only do oats digest much slower than maltodextrin, giving you more consistent energy, but they also contain natural, healthy fibre that promotes proper digestion rather than bloating and diarrhoea.

How To Stop Mass Gainer From Giving You Diarrhoea

A man holding a roll of toilet paper

The first step to stopping mass gainers from giving you diarrhoea is to find the cause of your problem in the first place. I’ve given you some possible causes, but you might need to consult with a gastroenterologist to get further clarity on why weight gainers are giving you diarrhoea.

One thing that you can do is try to find the cause by a process of elimination. So, you could start by switching to a supplement that only uses natural sweeteners. And if that doesn’t work, you could try drinking your mass gainer more slowly and away from your workout.

A good option in this regard is to have a whey protein drink after training to take advantage of the post-workout muscle protein synthesis spike and then save your mass gainer for later in the day (or have it earlier in the day, depending on when you work out).

A lot of bodybuilders won’t want to hear this, but you could always try switching to a plant-based gainer instead. At the very least, you could do this temporarily to rule out the lactose as a cause for your mass gainer diarrhoea.

You could also buy some protein powder, mix it with milk, and then add some oats and nut butter. This way, you’re making your own high-calorie shake while keeping it free from fillers and other unnecessary ingredients (not that all commercial mass gainers have fillers).

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Conclusion: Why Do Weight Gainers Cause Diarrhoea?

A constipated man holding his stomach

Some people can drink two massive mass gainers every day without experiencing any problems. On the other hand, other gym-goers can have diarrhoea from a small serving of weight gainer. So, what’s the deal?

Well, you might have lactose intolerance without realising it. Many people cut out lactose from their diet and report an improvement in their stomach health.

Of course, you might just be overconsuming protein, calories, or mass gainers in general, which could overload your stomach and lead to diarrhoea.

You may also have diarrhoea due to a range of factors, which are best explored and understood with the help of a gastroenterologist. This way, you’ll be able to see how much of a problem mass gainers really are, or if there are other health conditions or lifestyle factors causing your diarrhoea.


  1. Harvard Health. (2023, August 8). Is something in your diet causing diarrhea?
  2. Costa, R. J. S., Snipe, R. M., Kitic, C. M., & Gibson, P. R. (2017). Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 46(3), 246–265.
  3. WebMD Editorial Contributors. (2021, April 20). What is maltodextrin? WebMD.
A picture of Best Mass Gainer UK founder, personal trainer Andy Williams
Andy Williams
Andy Williams is a UK-based personal trainer and bodybuilder who coaches gym-goers of all levels using proven evidence-based techniques in strength training, nutrition, and supplementation. He’s tested over 70 different mass gainers for their ingredient quality and muscle-building effectiveness and regularly shares his findings in his mass gainer reviews.
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