Mass Gainer Side Effects: Is Mass Gainer Bad For You?

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Andy Williams
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Are mass gainers bad for you? Is weight gainer safe? These are just a couple of the common questions that people have about the side effects of mass gainer.

Popular with hardgainers and skinny guys, mass gainers are known for reliably increasing your body weight and muscle mass. But do these gains come at a cost? And, if so, is the cost worth the gains?

Let’s take a look at some common (and not so common) mass gainer side effects to see just how safe (or how dangerous) weight gainers really are.

Is Mass Gainer Bad For You?

Mass gainer scoops of various sizes lined up side by side

Are mass gainers made for you? It depends on your current health status. If you’re in good health and have no existing conditions, then drinking weight gainers in moderation should be okay and may even enhance your overall health if you’re underweight.

However, it’s important that you select a supplement from a reputable brand that has, ideally, been batch-tested by a third party for purity and banned substances.

In excess, though, mass gainers can be bad for you because the sheer amount of calories and protein can overload your body and cause you to feel bloated and nauseous

As such, it’s best to start with a smaller serving size or a supplement with a more moderate amount of calories.

So, while there are undeniable benefits of weight gainer, mass gainers do have their side effects as well, depending on your current health status and how you consume them.

Possible Mass Gainer Side Effects

These mass gainer side effects don’t necessarily apply to everyone. But you should keep them in mind before you start taking a weight gainer.

Fat Gain

A man pinching his belly fat

Does mass gainer make you fat? If taking a weight gainer causes your overall calorie surplus to become excessive, then you’re very likely to gain unwanted and unnecessary body fat.

Although some fat gain is to be expected when you’re bulking, you don’t actually want to become fat because obesity is linked to a variety of chronic diseases, including cancer, diabetes, and heart disease.

So, while you can certainly take mass gainers to meet your daily calorie and macronutrient targets, you shouldn’t consume them in excess because more is definitely not better when it comes to weight gain.

Instead, exercise your patience. While you can gain weight quite fast if you’re currently very skinny, you need to be patient if you want to experience major changes in your physique.

Nausea and Vomiting

A nauseas man being sick in the toilet

Because most commercial mass gainers contain whey and/or casein protein, they naturally have significant amounts of lactose, which not everyone can digest.

I actually wrote an article on how to find a lactose free mass gainer in the UK, which might be useful if your weight gainer is causing you stomach problems.

Other reasons why your weight gainer might make you feel nauseous include drinking it too quickly, drinking it too soon after a workout, and drinking mass gainers that are loaded with maltodextrin.

While likely fine in small quantities (i.e., if it’s used as a bulking agent or thickener in a mass gainer formula), research strongly suggests maltodextrin can impair your intestinal functioning and health, which could lead to disease. [1]

A good fix in this regard is to get a mass gainer that uses oats for carbohydrates rather than potentially harmful maltodextrin.


A man showing his tummy

There are three main reasons why mass gainers can make you bloated.

The first is lactose intolerance. If your body can't break down the lactose from the milk proteins, then you might feel bloated and nauseous, which can affect your quality of life. [2]

Unsurprisingly, maltodextrin is another ingredient that can make you bloated. [3] Unfortunately, most commercial weight gainers are absolutely full to the brim with maltodextrin, with servings of 200+ grams being quite common.

Also, the sheer amount of calories from your mass gainer can cause you to feel bloated. After all, whether you drink a large amount of calories or have them as whole foods, you’re probably going to feel slightly bloated at the very least.

See Also:

Gas and Constipation

A constipated man holding his stomach

Gas and constipation are two other side effects of mass gainer that you should be aware of. Excessive protein consumption has long been known to increase flatulence, especially among bodybuilders who opt for a low-carbohydrate diet.

But since mass gainers are loaded with carbs, why would flatulence be a problem?

Well, your typical mass gainer has basically no fibre, which may impair your body’s ability to digest all the protein and carbohydrates that you’re consuming. 

Again, this is why getting an oat-based weight gainer is important. Not only will cutting out the maltodextrin soothe your stomach, but you’ll also improve your digestion and constipation by getting enough healthy fibre.

After all, in the context of a high-calorie meal, it would be pretty unusual to have, say, 800+ calories and basically no fibre. So don’t fall into the trap of drinking a bunch of sugar and/or maltodextrin just because it might help you gain weight. There are far healthier options out there!


A man holding a roll of toilet paper

If you’re lactose intolerant or consume too much maltodextrin, then diarrhoea could be one of the unpleasant weight gainer side effects that you experience.

If your small intestine can’t produce enough lactase to break down the lactose, then you might experience stomach discomfort and, eventually, diarrhoea.

See my article titled does mass gainer give you diarrhoea? for more information.

Kidney Damage

A doctor holding a kidney

Don’t be too alarmed; drinking a mass gainer to get a bit of extra protein isn’t going to destroy your kidneys unless you have an underlying kidney disease.

Although opinions differ on this topic, I wanted to include it anyway, as some research does suggest that long-term high-protein diets can negatively affect a person’s kidney function.

On the other hand, other research shows that a high-protein diet, at least over the period of a year, doesn’t impair kidney function in any way. [4]

The way I look at it is that many professional athletes consume fairly high protein diets and yet, enjoy great health and live long lives. 

But I also think that many brands encourage a needlessly high protein intake so that you have to spend more money on their supplements. Honestly, you may not need as much protein to build muscle as you think you do.

What I like about mass gainers is that they have more carbs than protein. This way, you’re very unlikely to overconsume protein just so that you can meet your calorie requirements.

Impaired Gut Function

A woman holding her stomach in discomfort

Due to the high amounts of maltodextrin contained within many weight gainer formulas, there’s a chance that mass gainer consumption could harm your gut microbiome. [4]

However, this isn’t an area that’s been extensively studied. But considering the ridiculous amount of maltodextrin that many big-name brands use in their weight gain formulas, it wouldn’t be at all surprising if your average mass gainer negatively affected your gut health in some way.

Learn More About The Side Effects of Mass Gainer:

Conclusion: Is Mass Gainer Worth It?

The side effects of trying lots of mass gainers; weight gainers everywhere

Considering the possible mass gainer side effects, is drinking weight gainer still worth it? If you’re skinny and use mass gainer sensibly, then I’d say yes!

Remember, mass gainers are a supplement. And if you pick a good one, then you’re unlikely to experience any bad weight gainer side effects if you drink it responsibly, which is to say as a supplement to your wholefood meals.

Given the fact that many protein supplements are contaminated with dangerous toxins and heavy metals, it’s more important than ever to select a brand that uses quality, natural ingredients and has strict manufacturing processes.

While some weight gainers can be pretty expensive, they enable hardgainers to effectively achieve a calorie surplus without force-feeding themselves with whole foods.


  1. Zangara, M. T., Ponti, A. K., Miller, N. D., Engelhart, M. J., Ahern, P. P., Sangwan, N., & McDonald, C. (2022). Maltodextrin consumption impairs the intestinal mucus barrier and accelerates colitis through direct actions on the epithelium. Frontiers in Immunology, 13.
  2. Lactose intolerance - Symptoms & causes - Mayo Clinic. (2022, March 5). Mayo Clinic.
  3. Ko, G. J., Rhee, C. M., Kalantar-Zadeh, K., & Joshi, S. (2020). The effects of High-Protein diets on kidney health and longevity. Journal of the American Society of Nephrology, 31(8), 1667–1679.
  4. António, J., Ellerbroek, A., Silver, T., Vargas, L., Tamayo, A., Buehn, R., & Peacock, C. A. (2016). A high protein diet has no harmful effects: A One-Year crossover study in Resistance-Trained males. Journal of Nutrition and Metabolism, 2016, 1–5.
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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