A mass gainer is not a stronger protein powder. It is protein plus a large dose of carbohydrate — sometimes 400–600 kcal a serving — aimed at people who cannot eat enough to gain weight. If you already eat a surplus, a gainer just makes you fatter, faster.
This 2026 UK guide covers when a mass gainer is worth it, how to read the calorie line, how it compares with whey plus food, and which tubs GymStack actually stocks. If you only need protein, go back to the protein powder guide and buy whey instead.
In this guide
- Who a mass gainer is actually for
- Mass gainer vs whey protein
- Calories, protein and the maltodextrin question
- How to use a gainer without wrecking digestion
- GymStack mass gainer picks
- FAQs
Who a mass gainer is actually for
Hard gainers: people with a fast metabolism, a physically heavy job, or an appetite that dies after a normal dinner. If three meals plus a whey shake already put you in a surplus, you do not need a 6 kg bag of powder.
A mass gainer earns its place when:
- You have tried eating more food and still cannot hit calories
- You train with progressive overload and you are tracking body weight weekly
- You would rather drink 500 kcal than force a fourth plate of rice
It is not a shortcut around training. Food supplements do not replace a varied diet — the NHS line still applies.
Mass gainer vs whey protein
| Whey protein | Mass gainer | |
|---|---|---|
| Job | Add 20–30 g protein | Add protein and a lot of calories |
| Typical serving | ~100–130 kcal | 400–600+ kcal |
| Carbs | Low | High (often maltodextrin, oats, or both) |
| Price per gram of protein | Usually better | Worse — you are paying for carbs too |
| Buy if | You eat enough food | You cannot |
A cheaper homemade version is whey, oats, banana and milk. A tub wins when you will not do that three times a week. Shop the category at mass gainer.
Calories, protein and the maltodextrin question
Ignore “extreme mass” on the front. Read:
- kcal per serving — and the serving size in grams. A 150 g serving at 560 kcal is a meal, not a “scoop”.
- Protein per serving vs per 100 g — gainers look weak per 100 g because carbs dilute the percentage. That is the design.
- Carbohydrate source — maltodextrin is common and fine for most people; oat or ground-rice blends sit a bit heavier and slower. Neither is magic.
- Fat and sugars — some “premium” gainers add oils or sugars you could have eaten as food.
- Allergens — almost all popular UK gainers are milk-based. Plant eaters should not assume the tub is vegan.
- Informed Sport — tested athletes still need a batch on the Informed Sport register.
How to use a mass gainer without wrecking digestion
- Start with half a serving for a week. Full servings blow a lot of people out.
- Do not slam it immediately after a huge meal. Use it as a meal replacement you were going to skip, or a mid-morning drink.
- More water or milk than the label suggests if it sits like concrete.
- Track body weight 2–3 times a week, average it. If you are gaining faster than ~0.25–0.5 kg a week and you feel sloppy, drop the extra scoop — not the training.
- Keep protein in the 1.6–2.2 g/kg/day range from food plus powder, consistent with the ISSN protein position stand. Extra gainer on top of that is just extra calories.
Creatine still belongs in a size-focused stack more than a second gainer. See creatine monohydrate.
GymStack mass gainer picks
In-stock examples — not a paid league table. Check the page for flavour and size.
- Applied Nutrition Critical Mass Professional 6 kg Chocolate — large bag if you already know you tolerate gainers.
- Mutant Mass Triple Chocolate 2.27 kg — smaller bag to test digestion.
- Optimum Nutrition Serious Mass 5.45 kg
If 6 kg feels like a commitment, buy 2–3 kg first. Free UK delivery over £75.
Need protein without the carbs? Buy whey. Need plant protein instead of a milk gainer? Use a vegan protein shake and add food calories.
Mass gainer FAQs
What is the best mass gainer in the UK?
The one you can drink daily whose calories actually put you in a surplus. “Best mass gainer UK” lists that ignore serving size are selling bags, not results. Start with a 2 kg bag, not 6 kg.
Will a mass gainer make me bulky?
It will make you heavier if you eat in a surplus. Fat and muscle both weigh something. Training quality decides the split. There is no powder that only adds muscle.
Can I use a mass gainer to lose weight?
No. That is the wrong product. Use whey or food and a calorie deficit.
Is mass gainer just sugar?
Many use maltodextrin or similar starch, not table sugar. It still counts as carbohydrate calories. If the ingredients list is sugar first, pick another tub or make whey + oats at home.
When should I take a mass gainer?
When you would otherwise miss calories — not stacked on top of three already-large meals unless you are genuinely under-eating. Post-workout is convenient. Bedtime only if it does not wreck sleep or sit heavy.
The final say
If food is the bottleneck, a mass gainer is a tool. If protein is the bottleneck, buy whey. Do not treat a 600 kcal shake as a free extra.
Disclaimer: This article is for information only and should not replace medical care. Check with your GP or pharmacist before trying new supplements, especially if you are pregnant, breastfeeding, take prescribed medicines, or have a health condition. Food supplements must not be used as a substitute for a varied, balanced diet and a healthy lifestyle.