Are Mass Gainers Bad for Your Liver?

Mass gainers promise an easy way to add calories for people who struggle to put on weight. Online, they are sometimes blamed for liver damage, while others insist they are completely harmless. Our editorial team looks at the real relationship between gainers and liver health, which turns out to depend on how and by whom they are used.

What is in a mass gainer

A typical mass gainer is a powder combining carbohydrates and protein, often in a ratio of roughly three or four parts carbohydrate to one part protein. A single serving can provide anywhere from 500 to over 1,000 kilocalories, depending on the brand and the amount of powder used.

The carbohydrate component is usually maltodextrin, sometimes with dextrose, oat flour or added sugars. Protein typically comes from whey concentrate, milk protein or a blend. Some products add creatine, vitamins, fats such as MCT oil, or digestive enzymes.

From a nutritional standpoint, a gainer is simply a convenient, concentrated source of energy. It contains nothing that is inherently toxic to the liver. Problems, when they arise, relate to the total amount consumed, the type of sugars and the overall diet and lifestyle.

This distinction matters, because the liver is affected not by the label "gainer" but by what happens to the excess energy and sugar in the body.

How excess calories affect the liver

The liver is a central hub of energy metabolism. When we consume more energy than we use, particularly from refined carbohydrates and sugars, the liver converts some of the surplus into fat through a process called de novo lipogenesis. Over time, fat can accumulate in liver cells.

This is the basis of metabolic dysfunction-associated steatotic liver disease, abbreviated MASLD and previously known as NAFLD. The name was changed in 2023 by an international consensus (Rinella et al.) to emphasise its link with metabolic factors such as obesity, insulin resistance and type 2 diabetes.

MASLD is very common, affecting roughly a quarter of adults worldwide according to a large meta-analysis (Younossi et al., 2016). In most people it does not progress, but in some it leads to inflammation, scarring and eventually cirrhosis.

Fructose deserves special mention. Unlike glucose, it is metabolised almost entirely in the liver, and high intakes are strongly linked with liver fat accumulation (Jensen et al., 2018). Gainers containing large amounts of added sugar or fructose may therefore contribute more to liver fat than those based mainly on maltodextrin and oats.

Who is unlikely to be harmed

For a lean, active person who trains regularly and genuinely struggles to eat enough, a gainer used to reach energy needs is unlikely to harm the liver. Exercise increases muscle glucose uptake and insulin sensitivity, so much of the incoming carbohydrate is used for glycogen and fuel rather than being converted to fat.

Protein, the other main component, is not toxic to a healthy liver. The International Society of Sports Nutrition position stand (Jäger et al., 2017) concluded that intakes of 1.4 to 2.0 g per kilogram per day are safe and appropriate for most exercising individuals.

Persistent myths that protein "overloads" the liver confuse healthy people with those who have advanced liver disease. In severe liver failure, protein handling can be impaired, but this is a medical situation managed by specialists, not a reason for healthy people to fear protein.

The same applies to creatine, often included in gainers: in healthy people at standard doses it has not been shown to damage the liver.

Who should be cautious

The picture changes for people who are already overweight, sedentary or have metabolic risk factors. Adding hundreds of extra calories a day, largely from fast-digesting carbohydrates, can promote weight gain, insulin resistance and liver fat.

Alcohol and excess sugar combine to increase liver fat, so people who drink regularly should be particularly careful about large calorie surpluses.

Another real but less discussed risk is contamination. Reviews of supplement-associated liver injury (Navarro et al., 2017) have mostly implicated bodybuilding products adulterated with anabolic steroids, and some multi-ingredient fat burners, rather than plain gainers. Choosing reputable, batch-tested products reduces this risk.

Using a gainer sensibly

For those who need extra calories, a few principles help protect both liver and overall health. The aim is a moderate, controlled surplus that supports muscle growth rather than rapid fat gain.

• Aim for a modest surplus, often suggested as about 250 to 500 kcal per day above maintenance.
• Choose products with lower added sugar and no high-fructose syrups.
• Consider half servings or mixing with milk and oats rather than using the full scoop.
• Pair gainer use with consistent resistance training.
• Track weight and waist circumference; a rapid increase in waist size suggests fat gain.
• Limit alcohol during periods of calorie surplus.

Home-made shakes with oats, milk, banana, peanut butter and whey can provide similar energy with more fibre and micronutrients.

People with a family history of diabetes, elevated liver enzymes or known fatty liver should speak to their GP before using gainers. NICE guidance on non-alcoholic fatty liver disease focuses on lifestyle measures, with weight management at their core.

Editorial conclusions

Mass gainers do not contain substances that are toxic to a healthy liver, and protein itself is safe at common sports nutrition intakes.

The real risk comes from chronic calorie and sugar surplus, especially in inactive or overweight people, which can promote fatty liver disease.

Used in moderation by active people, with attention to sugar content and product quality, a gainer is unlikely to harm the liver.

For further reading, our editorial team suggests articles on how to calculate a calorie surplus for lean gains, protein and kidney myths, and liver function tests for people who train.

References

1. Rinella ME, Lazarus JV, Ratziu V, et al. A multisociety Delphi consensus statement on new fatty liver disease nomenclature. J Hepatol. 2023;79(6):1542–1556.
2. Younossi ZM, Koenig AB, Abdelatif D, et al. Global epidemiology of nonalcoholic fatty liver disease: meta-analytic assessment of prevalence, incidence, and outcomes. Hepatology. 2016;64(1):73–84.
3. Jensen T, Abdelmalek MF, Sullivan S, et al. Fructose and sugar: a major mediator of non-alcoholic fatty liver disease. J Hepatol. 2018;68(5):1063–1075.
4. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition position stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20.
5. Navarro VJ, Khan I, Björnsson E, et al. Liver injury from herbal and dietary supplements. Hepatology. 2017;65(1):363–373.
6. National Institute for Health and Care Excellence. Non-alcoholic fatty liver disease (NAFLD): assessment and management (NG49). London: NICE; 2016.