Mutant Mass review

Our take: the best of the big gainers, with the big gainers’ problem

Mutant Mass is the main rival to Optimum Nutrition’s Serious Mass: one enormous shake meant to add a meal’s worth of calories or more. Compared side by side, Mutant Mass is the better-built product. It has more protein per serving, a lower share of maltodextrin, some fat from oils and seeds, and in its 6.8 kg bag it is the cheapest gainer we have priced.

It shares their central problem, though. A full serving is about 1,050 to 1,100 kcal, which for most people is far more surplus than the research suggests for gaining mostly muscle. And its ingredient list, while complete, is long, organised into blends without individual amounts, and full of allergens.

This is a desk review. We have read the published label details, UK retailer listings and the research on weight gain and protein. We have not used this product, tested its contents or run a trial. We could not open Mutant’s own product page while writing, so the next section says where each figure came from.

What exactly are you buying?

Mutant Mass is sold in a 2.27 kg bag (8 servings) and a 6.8 kg bag. A serving is 280 g — four scoops — and UK listings give it as:

• About 1,050 to 1,100 kcal (listings give both figures for the same serving)
• 56 g of protein
• About 192 g of carbohydrate
• About 12 g of fat

That is roughly 3.4 g of carbohydrate for every gram of protein — better than Serious Mass at about five to one, but a long way from Myprotein Impact Weight Gainer at about 1.6 to one.

The three blends

The ingredients are grouped into named blends, listed in order of weight:

• Carbohydrate blend: maltodextrin, waxy maize (corn) starch, barley starch, sweet potato and rolled oats.
• Protein blend: whey concentrate, hydrolysed whey, whey isolate, micellar casein, milk protein concentrate and milk protein isolate — all milk proteins.
• Lipid blend: MCT oil from coconut, avocado, flaxseed, pumpkin seed and sunflower oil.

After these come thickeners (including defatted soybean, pea and guar gum), flavourings, sucralose, sunflower lecithin, lactase and protease enzymes, a cinnamon extract and tart cherry.

Two points on that list. First, the blend format tells you the order but not the amounts, so there is no way to know how much sweet potato or oat is in a serving; maltodextrin is listed first, so it is the largest carbohydrate. Second, the cinnamon extract and tart cherry are almost certainly present in small amounts, and there is no reason to buy a gainer for them. The added lactase is a genuinely useful touch for people who struggle with dairy.

What the research says about gaining weight well

Weight gain comes from eating more energy than you use; whether it is muscle or fat depends on resistance training, protein and the size of the surplus. A 2019 review of off-season nutrition for bodybuilders suggested aiming to gain around 0.25 to 0.5 per cent of body weight a week — roughly 0.2 to 0.4 kg for an 80 kg adult — because larger surpluses add proportionally more fat. For most people that is a few hundred extra kilocalories a day, so a full serving of Mutant Mass on top of normal meals is usually too much. Half a serving, about 530 kcal and 28 g of protein, is a better starting point.

Protein gains level off too: a 2018 meta-analysis by Morton and colleagues found gains in fat-free mass plateaued at around 1.6 g per kilogram of body weight a day. Our protein calculator works out your figure.

The case for drinking calories is real. In a 2000 study by DiMeglio and Mattes, people who drank an extra 450 kcal a day did not eat less food to compensate, while those given the same calories as jelly beans did. That supports any calorie-dense shake, including one you make yourself.

Price: work it out per 1,000 kcal

In listings read on 28 September 2026, the 2.27 kg bag ranged from £24.99 to £36.99 at mainstream UK retailers. With eight servings of about 1,075 kcal, that is roughly £2.90 to £4.30 per 1,000 kcal, or £3.12 to £4.62 a serving.

The 6.8 kg bag is where Mutant Mass pulls ahead. Mainstream listings ranged from £54.95 to £69.99, with a price comparison site showing £64. At about 24 servings, that is roughly £2.10 to £2.70 per 1,000 kcal — the cheapest gainer we have priced, ahead of Myprotein Impact Weight Gainer at about £2.90 to £3.90 and Serious Mass at about £3.30 to £5.00. We also saw listings as low as £40.56, but at least one was marked as past its best-before date, so check dates on any unusually cheap bag.

A home-made shake still wins: our estimate, using typical supermarket prices, is that milk, oats, a banana, peanut butter and a scoop of whey come to roughly £1.30 to £1.45 per 1,000 kcal. We read these prices from listings rather than taking an order through checkout.

Best alternatives

The ones we have fully reviewed are linked, and appear in the comparison cards below this article. The others we name for comparison only: we have not reviewed them in full, so do not read their inclusion as a recommendation.

• A home-made gainer shake — our first choice for most people. Milk, oats, a banana, peanut butter and a scoop of whey let you set the calories to the surplus you need, for well under the price per 1,000 kcal of any gainer.
• Myprotein Impact Weight Gainer (reviewed) — our higher-scoring gainer: a 388 kcal serving that fits a modest surplus, with a much higher share of protein, though dearer per 1,000 kcal than the 6.8 kg Mutant Mass.
• Optimum Nutrition Serious Mass (reviewed) — the closest like-for-like rival. Mutant Mass has more protein, a lower share of maltodextrin and a lower price per 1,000 kcal; Serious Mass adds 3 g of creatine.
• Myprotein Impact Whey (reviewed) — if protein is what you need rather than calories, whey with ordinary meals is the cheaper, more flexible tool.

Who should skip it, and what to watch for

The allergen list is long. UK listings show milk (several forms), soy, barley, coconut and corn in the ingredients, with may-contain warnings for wheat and fish. That rules it out for anyone with a milk or soy allergy, anyone with coeliac disease (barley contains gluten), and vegans. Check the allergen box for your flavour, since ingredients vary slightly.

Anyone with diabetes or prediabetes should talk to their GP or diabetes team before adding a shake with nearly 200 g of carbohydrate, and anyone with kidney disease should get advice before increasing protein. Big, sweet shakes commonly cause bloating and loose stools when first introduced; start with half a serving.

Weigh yourself weekly, and cut back if you are gaining faster than about 0.5 per cent of body weight a week. The listings we read made no banned-substance testing claim, so drug-tested athletes should look elsewhere. Food supplements sold in Great Britain are not approved by a regulator for safety or effectiveness before sale, so the label is the manufacturer’s responsibility rather than a regulator’s endorsement.

Sources and shopping: our UK buying checklist

Before you order, decide how many extra calories you need; for most people it is a few hundred a day, not a full serving. Check the bag size, the delivered price, the seller and the best-before date — heavily discounted bags are often short-dated. Divide the price by the total calories to get the cost per 1,000 kcal, and compare it with a home-made shake.

When the bag arrives, check the seal, the batch code and the best-before date, and read the allergen box for your flavour.

Our commercial link for this product is our Mutant Mass link. It earns us a commission at no extra cost to you, and it has no influence on the score above — which is a 6.4, ahead of Serious Mass and behind Myprotein Impact Weight Gainer, held back by an oversized serving, blends without amounts and a long allergen list.

The fair summary is this. If you want a big gainer, Mutant Mass is the better-built and, in the large bag, cheaper choice. Most people will still do better with half a serving — or with a blender and a bag of oats.