Optimum Nutrition Serious Mass review

Our take: it works, bluntly and expensively

Serious Mass is the product people reach for when they have been told to “eat more” and cannot face another plate. The tub makes one promise — muscle building and weight gain — and a single shake of around 1,250 kcal and 50 g of protein will certainly help with the weight part.

The question is not whether it works. Anything that adds 1,250 kcal a day will make almost anyone gain weight. The questions are what those calories are made of, whether you need that many, and whether you need to pay a brand price for them.

On all three, Serious Mass is weaker than its reputation. About four-fifths of the calories are carbohydrate, most of it maltodextrin, a cheap, fast-digesting starch. One full serving is a bigger surplus than the research suggests for gaining mostly muscle rather than fat. And the same calories and protein from milk, oats, bananas, peanut butter and a scoop of whey cost well under half as much.

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 tub, tested its contents or run a trial. We could not open the brand’s own GB product page while writing, so the next section says where each figure came from.

What exactly are you buying?

In the UK Serious Mass comes in a 2.73 kg tub (8 servings) and a 5.45 kg tub (16 servings). A serving is two heaping scoops, 334 g, blended with 650 to 750 ml of cold water or milk. That is a large drink: most people find it easier split into two or three smaller shakes across the day.

UK retailer listings give each 334 g serving as:

• About 1,250 kcal (one listing gives 1,263 kcal, 5,284 kJ)
• 50 g of protein, from a blend of whey, casein and egg protein
• About 250 g of carbohydrate, of which 20 to 26 g is sugars, depending on which version of the panel you read
• About 5 g of fat
• 3 g of creatine, and 25 added vitamins and minerals

Made up with 700 ml of whole milk instead of water, a serving comes to roughly 1,700 kcal and more than 70 g of protein. That is most of a day’s energy needs for many adults, in one glass.

The photo on this page shows the US tub, whose label figures differ slightly; the UK packs carry a UK-format panel. Check the one you receive.

What the calories are made of

The first ingredient is maltodextrin, a starch-derived carbohydrate that digests very quickly. At around 250 g of carbohydrate against 50 g of protein, a serving is about a 5:1 ratio of carbohydrate to protein. That is not dangerous for a healthy adult who trains hard, and fast carbohydrate is useful around heavy training. But it is not the “lean” profile the marketing implies: you are mainly buying cheap carbohydrate, with protein added.

The “no added sugar” wording on some listings is technically true and not very reassuring. Maltodextrin is broken down to glucose almost as fast as sugar is, and 20 to 26 g of sugars per serving is on top of that.

The 3 g of creatine is the one ingredient here with strong evidence of its own, and it is the right daily amount. Our Micronised Creatine review covers why 3 g a day is enough. It is also easy and cheap to take on its own.

What the research says about gaining weight well

Calories drive weight, protein and training drive muscle

Weight gain comes from eating more energy than you use. Whether that weight is mostly muscle or mostly fat depends on resistance training, enough protein and the size of the surplus. A 2019 review of off-season nutrition for bodybuilders suggested a modest surplus, aiming to gain around 0.25 to 0.5 per cent of body weight a week, precisely because larger surpluses add proportionally more fat.

For an 80 kg adult that is roughly 0.2 to 0.4 kg a week — usually a few hundred extra kilocalories a day, not 1,250. A full serving of Serious Mass on top of normal meals is, for most people, more than that range, which is why half a serving is often the more sensible starting point.

Protein has a ceiling too. A 2018 meta-analysis by Morton and colleagues found gains in fat-free mass levelled off at around 1.6 g of protein per kilogram of body weight a day. Our protein calculator works out your figure; many people already reach it before adding a gainer.

Why a shake helps some people

There is a genuine reason gainers exist. Liquid calories are less filling than solid food: in a 2000 study by DiMeglio and Mattes, people who drank 450 kcal a day as a soft drink did not eat less food to compensate, while those given the same calories as jelly beans did. For someone who genuinely struggles to eat enough, drinking calories is an effective tactic.

That argument supports a calorie-dense shake. It does not specifically support this one: a blended shake of milk, oats and peanut butter is just as liquid.

Third-party testing: what the brand does and does not claim

This matters mainly if you compete under anti-doping rules. As we set out in our Micronised Creatine review, Optimum Nutrition’s customer-service page on Informed Sport names only three products as Informed Sport registered: Gold Standard 100% Whey England Rugby, Gold Standard 100% Plant and the Optimum Bar. Serious Mass is not among them.

If you are drug-tested, buy a batch-tested product you can look up in the certifier’s database.

Price: work it out per 1,000 kcal

Because a gainer’s job is to supply calories, the fair comparison is the price per 1,000 kcal. Search listings for UK retailers on 28 September 2026 showed the 2.73 kg tub at £32.95 at one sports-nutrition retailer, £37.99 at another, £40.00 at Boots and £50.00 at Holland & Barrett. With eight servings of about 1,250 kcal, that is roughly £4.12 to £6.25 a serving, or about £3.30 to £5.00 per 1,000 kcal.

A home-made shake is far cheaper. Our estimate, using typical supermarket prices: 600 ml of whole milk, 100 g of oats, two bananas, 30 g of peanut butter and one 30 g scoop of whey protein come to about 1,250 kcal and 60 g of protein, for roughly £1.60 to £1.80 — about £1.30 to £1.45 per 1,000 kcal. That is well under half the price of Serious Mass at its cheapest, with more protein, more fibre and less maltodextrin.

We read these prices from search listings rather than taking an order through checkout, and the home-made figure is our own estimate, so check the prices where you shop.

Best alternatives

We compare Serious Mass with the options a UK buyer is realistically choosing between. 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, banana, peanut butter and a scoop of whey give about 1,250 kcal and 60 g of protein for roughly £1.60 to £1.80, and you can scale the calories to the surplus you actually need.
• Optimum Nutrition Gold Standard 100% Whey (reviewed) — if protein is what you are short of rather than calories, a whey shake with a normal meal is the better tool. 24 g of protein per scoop, with the calories coming from food of your choice.
• Myprotein Impact Whey (reviewed) — the cheaper whey most people compare against Gold Standard, and the base of the home-made shake above.
• Optimum Nutrition Micronised Creatine (reviewed) — the 3 g of creatine in Serious Mass is its best-evidenced ingredient; you can take it on its own for about 20p a day.
• Myprotein Impact Weight Gainer (reviewed) — a smaller-serving gainer: about 388 kcal and 31 g of protein per 100 g serving, listed at £32.78 to £37.99 for 2.5 kg. That works out at about £3.40 to £3.90 per 1,000 kcal, and its smaller serving is easier to fit to a modest surplus.
• Mutant Mass (reviewed) — the closest like-for-like rival: over 1,100 kcal and 56 g of protein per 280 g serving, listed at about £34.99 for 2.27 kg, or roughly £3.90 per 1,000 kcal.

Who should skip it, and what to watch for

Serious Mass contains milk and egg, so it is not suitable for anyone allergic to either, or for vegans. Check the tub’s allergen box for wheat and soy too, since some versions have included them.

Anyone with diabetes or prediabetes should not take a 250 g carbohydrate shake without talking to their GP or diabetes team; it will raise blood glucose sharply. The same caution applies to anyone with kidney disease increasing their protein intake.

Large, sweet, fast-digesting shakes commonly cause bloating, wind and loose stools, especially when first introduced. Start with half a serving, or a third, and build up. And watch your weight weekly: if you are gaining faster than about 0.5 per cent of body weight a week, reduce the serving, because the extra is mostly fat.

The added vitamins and minerals overlap with a multivitamin. If you also take one, such as Opti-Men, add up the labels for vitamin A, B6 and zinc. 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 calories you actually need to add — for most people it is a few hundred a day, not 1,250. Then check the pack size, the delivered price and the seller, and divide the price by the total calories in the tub to get the cost per 1,000 kcal. Compare that with a home-made shake. Counterfeit sports nutrition is a documented problem on marketplaces, and a verifiable seller is the simplest protection.

When the tub arrives, check that the seal is intact, that the label is in English with a GB or EU address for the responsible business, and that the batch code and best-before date are legible. Read the allergen box and the nutrition panel.

Our commercial link for Optimum Nutrition in the UK is our UK Optimum Nutrition link. It earns us a commission at no extra cost to you, and it has no influence on the score above — which is a 5.8, held back by a serving mostly made of maltodextrin, a surplus larger than most people need, and a cost well over double that of a home-made shake.

The fair summary is this. Serious Mass will help you gain weight, because it is a lot of calories. Most people will gain better, more cheaply, by eating a little more food and adding a scoop of whey.