Myprotein Impact Weight Gainer review

Our take: the right size, not the cheapest route

Most weight gainers are built around one enormous serving. Myprotein Impact Weight Gainer is not. A 100 g serving gives about 388 kcal and 31 g of protein, which is close to the modest daily surplus the research suggests for gaining mostly muscle rather than fat. That alone makes it a more sensible product than the 1,250 kcal gainers it sits beside.

It also has a better balance. Our Serious Mass review found roughly five grams of carbohydrate for every gram of protein; here it is about 1.6 to one. You are still buying mostly carbohydrate, but a much larger share of each scoop is protein.

What it cannot escape is the arithmetic every gainer faces. Its main ingredients are whey protein, maltodextrin and oat flour, and you can buy all three — or their supermarket equivalents — for far less than a blended pouch. It is a convenience product, and it is priced like one.

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 Myprotein’s own product page while writing, so the next section says where each figure came from.

What exactly are you buying?

Impact Weight Gainer is sold in 1 kg, 2.5 kg and 5 kg packs, in several flavours and unflavoured. The 2.5 kg pack is listed at 25 servings of 100 g. UK retailer listings give each serving as about:

• 388 kcal
• 31 g of protein, from whey protein concentrate
• About 50 g of carbohydrate, from maltodextrin and oat flour
• About 6 g of fat

Myprotein notes that the figures vary slightly by flavour, with protein up to about 31 g and carbohydrate up to about 54 g. The listings we read did not give the sugars figure or the split between maltodextrin and oats, which is the part of the label we would most like to see: maltodextrin digests almost as fast as sugar, while oat flour is slower and brings some fibre.

Mixed with water, a serving is 388 kcal. Mixed with 400 ml of semi-skimmed milk it comes to roughly 580 kcal and more than 45 g of protein, and two servings a day with milk approach the 1,250 kcal of a single Serious Mass shake — with more protein.

What the research says about gaining weight well

Weight gain comes from eating more energy than you use. Whether that weight is muscle or fat depends on resistance training, enough 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 — for an 80 kg adult, roughly 0.2 to 0.4 kg — because larger surpluses add proportionally more fat.

That usually means a few hundred extra kilocalories a day, and one serving of Impact Weight Gainer fits inside it. That is its main advantage over the bigger gainers: you can start at the right dose, rather than having to divide a tub by three.

Protein matters, but only up to a point. 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.

And there is a real reason to drink rather than eat the extra calories. Liquid calories are less filling: 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 argument supports any calorie-dense shake — including one you make yourself.

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. In listings read on 28 September 2026, the 2.5 kg pack was £28.29 at one retailer for the unflavoured version, with a price comparison site listing it from £32.78 to £37.99. At about 9,700 kcal a pack, that is roughly £2.90 to £3.90 per 1,000 kcal, or £1.13 to £1.52 a serving.

That is cheaper than Serious Mass, which we priced at about £3.30 to £5.00 per 1,000 kcal. It is still well over double a home-made shake: our estimate, using typical supermarket prices, is that milk, oats, banana, peanut butter and a scoop of whey come to roughly £1.30 to £1.45 per 1,000 kcal.

Two cautions. The 5 kg listing we saw was £104.49 — more than twice the 2.5 kg price for twice the powder — so check the price per kilo before assuming the bigger pack is cheaper. And Myprotein runs frequent site-wide offers, often 25 per cent or more; at full price, it is rarely worth buying. 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, with more fibre and less maltodextrin, for well under half the price per 1,000 kcal.
• Myprotein Impact Whey (reviewed) — if protein is what you are short of rather than calories, whey with an ordinary meal is the cheaper and more flexible tool, and the base of the home-made shake above.
• Optimum Nutrition Serious Mass (reviewed) — only if you need a very large surplus and struggle to eat. About 1,250 kcal and 50 g of protein per serving, but roughly five grams of carbohydrate for every gram of protein, at a higher price per 1,000 kcal.
• Mutant Mass (reviewed) — another big-serving gainer, over 1,100 kcal and 56 g of protein per 280 g serving, at about £3.90 per 1,000 kcal on the listing we saw.
• Myprotein Impact Creatine (reviewed) — not a gainer, but if you are trying to build muscle it is the supplement with the strongest evidence, at around 10p a day.

Who should skip it, and what to watch for

Impact Weight Gainer is made with whey protein, so it contains milk and is not suitable for anyone with a milk allergy, or for vegans. Oat flour may carry gluten, so anyone with coeliac disease should check the allergen box on the pack.

Anyone with diabetes or prediabetes should talk to their GP or diabetes team before adding a maltodextrin-based shake, and anyone with kidney disease should get advice before increasing protein. Sweetened shakes can cause bloating and loose stools when first introduced; starting with half a serving helps.

Weigh yourself weekly. If you are gaining faster than about 0.5 per cent of body weight a week, cut back: the extra is mostly fat. 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 one serving a day is plenty to start. Check the flavour, the delivered price and the seller, and divide the price by the total calories in the pack to get the cost per 1,000 kcal. Compare that with a home-made shake, and with the pack sizes — bigger is not always cheaper here.

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

Our commercial link for this product is our Myprotein Impact Weight Gainer link. It earns us a commission at no extra cost to you, and it has no influence on the score above — which is a 6.8, the higher of the two gainers we have reviewed, held back by a label that does not give sugars and a price a home-made shake undercuts.

The fair summary is this. If you want a gainer, this is a better-designed one than the big-serving tubs: a sensible dose with a decent share of protein. If you are happy to use a blender, you can make the same thing for less.