Two of the most trusted mass gainers in the UK. One decision. This comparison gives you the honest verdict so you can choose the right one for your goals — and stop wasting money on the wrong product.

Applied Nutrition Critical Mass and Optimum Nutrition Serious Mass are consistently two of the most searched and best-selling mass gainers in the UK. Both are from globally respected brands. Both deliver serious calories. Both are trusted by thousands of UK gym-goers.

But they are not the same product — and choosing the wrong one for your specific goal could slow your progress significantly.

At Muscle Products Store we stock both products and hear from our customers about them every day. This comparison is built on real product knowledge, genuine ingredient analysis, and honest feedback from UK buyers — not brand copy or marketing claims.

Quick Verdict — Which One Should You Buy?

Overall winner for most people: Applied Nutrition Critical Mass Professional

Overall winner for maximum calorie support: Optimum Nutrition Serious Mass

The Products at a Glance

Applied Nutrition Critical Mass Professional

Applied Nutrition Critical Mass is one of the UK’s most trusted mass gainers — and the Professional formula is their most advanced version. A revamped lower sugar formula built around a premium multi-source protein blend, dual creatine matrix, HMB, AstraGin® for absorption, DigeZyme™ digestive enzymes, and LactoSpore™ probiotics. This is a formula that goes significantly beyond just delivering calories.

• Price: £59.99 for 6kg (25 servings)
• Protein per serving: 42g
• Calories per serving: Approximately 550–600 kcal (with water)
• Carbohydrates per serving: 74g
• Fat per serving: 10g
• Creatine: Dual matrix — Creatine Monohydrate 3g + Kre-Alkalyn® 2g
• Certifications: Halal certified

Optimum Nutrition Serious Mass

Optimum Nutrition is the world’s most trusted sports nutrition brand — the name behind Gold Standard Whey — and Serious Mass is their flagship weight gainer. A high-calorie nutrient-dense formula delivering the highest calorie count of any mainstream mass gainer in the UK market alongside 50g of whey protein, 251g of carbohydrates, creatine, and 25 essential vitamins and minerals.

• Protein per serving: 50g
• Calories per serving: 1,259 kcal (with water)
• Carbohydrates per serving: 251g
• Fat per serving: 6.2g
• Sugar per serving: 12g
• Creatine: 3g per serving
• Vitamins and minerals: 25 essential vitamins and minerals

Head to Head — Full Comparison

Calories

Optimum Nutrition Serious Mass: 1,259 kcal per serving
Applied Nutrition Critical Mass Professional: approximately 550–600 kcal per serving

This is the most significant difference between the two products and the most important factor for many buyers. Serious Mass delivers more than double the calories of Critical Mass Professional per serving — making it the clear winner for anyone who needs maximum calorie support to overcome a very fast metabolism or extremely low appetite.

For genuine hard-gainers who have tried everything and still cannot move the scale, the sheer calorie volume of Serious Mass is often the difference maker.

Winner: Optimum Nutrition Serious Mass — by a significant margin

Protein

Optimum Nutrition Serious Mass: 50g per serving
Applied Nutrition Critical Mass Professional: 42g per serving

Both deliver a substantial protein dose per serving. Serious Mass edges Critical Mass Professional on raw protein volume at 50g vs 42g. However the quality of the protein blend in Critical Mass Professional is more sophisticated — combining ISO-XP™ 95% Whey Protein Isolate, MPI 90 Milk Protein Isolate, and WPC 80 Whey Protein Concentrate for multi-phase absorption.

Winner: Optimum Nutrition Serious Mass on volume — Applied Nutrition Critical Mass Professional on quality

Carbohydrates

Optimum Nutrition Serious Mass: 251g per serving
Applied Nutrition Critical Mass Professional: 74g per serving

Serious Mass delivers significantly more carbohydrates per serving — over three times the carbohydrate content of Critical Mass Professional. For hard-gainers who need maximum glycogen replenishment and calorie support this is a meaningful advantage. For those who want more controlled carbohydrate intake or are following a leaner approach to bulking, Critical Mass Professional’s 74g carbohydrate content is far more manageable.

Winner: Optimum Nutrition Serious Mass for maximum calorie and carb support — Applied Nutrition Critical Mass Professional for controlled intake

Creatine

Optimum Nutrition Serious Mass: 3g Creatine per serving
Applied Nutrition Critical Mass Professional: 5g Dual Creatine Matrix (3g Creatine Monohydrate + 2g Kre-Alkalyn® Buffered Creatine)

Both products include creatine — but Critical Mass Professional goes further with a dual creatine matrix combining standard monohydrate with Kre-Alkalyn® buffered creatine for improved stability and absorption. Serious Mass includes a straightforward 3g creatine monohydrate dose which is effective but less sophisticated.

Winner: Applied Nutrition Critical Mass Professional

Advanced Performance Ingredients

Optimum Nutrition Serious Mass: 25 vitamins and minerals, creatine, whey protein blend

Applied Nutrition Critical Mass Professional: HMB, L-Leucine, Glutamine Peptides, HICA, AstraGin™, DigeZyme™ enzyme complex (5 digestive enzymes), LactoSpore™ probiotics, CLA, MCT, ALA, Chromium Picolinate, Pterostilbene, Vitamin and Mineral profile

This is where the two products are most different. Serious Mass focuses on delivering maximum macronutrients with a comprehensive vitamin and mineral profile. Critical Mass Professional adds a significantly more advanced performance and recovery ingredient stack.

HMB — reduces muscle protein breakdown during intense training
AstraGin™ — clinically proven to improve absorption of all other ingredients
DigeZyme™ — five enzyme digestive complex for improved comfort and nutrient uptake
LactoSpore™ probiotics — gut health support with every serving
Glutamine Peptides and L-Leucine — muscle recovery and protein synthesis support

Winner: Applied Nutrition Critical Mass Professional — significantly more advanced formula

Digestive Comfort

Optimum Nutrition Serious Mass: Standard formula — no specific digestive support

Applied Nutrition Critical Mass Professional: DigeZyme™ five enzyme complex + LactoSpore™ probiotics

High-calorie mass gainers can cause digestive discomfort, bloating, and gas — particularly for those new to high-calorie supplementation. Critical Mass Professional specifically addresses this with DigeZyme™ and LactoSpore™ probiotics. Serious Mass at over 1,250 calories per serving is significantly more likely to cause digestive discomfort for some users, particularly when starting out.

Winner: Applied Nutrition Critical Mass Professional

Vitamins and Minerals

Optimum Nutrition Serious Mass: 25 essential vitamins and minerals
Applied Nutrition Critical Mass Professional: Comprehensive vitamin and mineral profile

Serious Mass specifically highlights its 25 essential vitamins and minerals as a key feature — making it one of the most nutritionally complete mass gainers available for overall micronutrient support during a calorie surplus.

Winner: Optimum Nutrition Serious Mass on micronutrient breadth

Taste and Mixability

Both products receive consistently positive customer feedback on taste. At 1,259 calories per serving, Serious Mass produces a very thick filling shake — particularly with milk. Some users find this extremely satisfying while others find it overwhelming. Critical Mass Professional at approximately 550–600 calories produces a more manageable shake that is significantly easier to consume daily without feeling overwhelmed.

Winner: Personal preference — but Critical Mass Professional is easier to drink daily

Value for Money

If you measure value purely in calories per pound spent, Serious Mass wins. If you measure value in ingredient quality, formula sophistication, and performance support, Critical Mass Professional is the better investment.

Winner: Depends on what you value — calories per pound or formula quality

Who Should Buy Optimum Nutrition Serious Mass?

Very fast metabolisms who genuinely cannot gain weight — 1,259 calories per serving is one of the most aggressive calorie deliveries available. If you eat enormous amounts and still cannot move the scale, Serious Mass is built for you.

Extreme hard-gainers and ectomorphs — the combination of high calories, 50g protein, and 251g carbohydrates creates the most substantial calorie surplus of any mainstream mass gainer in the UK.

Anyone who wants a simplified high-calorie formula — Serious Mass does not overcomplicate things. Calories, protein, carbohydrates, vitamins, minerals, creatine. Maximum nutrition without an advanced ingredient stack.

Budget-conscious buyers who need maximum calories — exceptional value per calorie makes this one of the most cost-effective ways to hit a very high daily calorie target.

Optimum Nutrition brand loyalists — if you already use Gold Standard Whey and trust the brand, Serious Mass delivers the same quality assurance and consistency you expect.

Who Should Buy Applied Nutrition Critical Mass Professional?

Anyone who wants lean controlled muscle growth — the lower calorie count and more balanced macros support quality muscle development over rapid scale weight increase.

Gym-goers who want a more advanced formula — the dual creatine matrix, HMB, AstraGin™, DigeZyme™, and LactoSpore™ probiotics put Critical Mass Professional in a different performance category to standard mass gainers.

Anyone who has experienced digestive discomfort with high-calorie shakes — DigeZyme™ and LactoSpore™ make this one of the most gut-friendly mass gainers available.

Muslim athletes — Halal certification makes Critical Mass Professional one of the few formally certified premium mass gainers in the UK.

Experienced lifters in a structured training phase — the performance ingredients add meaningful value for those training consistently at meaningful intensity.

Anyone who wants a manageable daily shake — at approximately 550–600 calories per serving, Critical Mass Professional is significantly easier to consume daily without feeling overwhelmed.

Can You Use Both Together?

Yes — and for some people this combination makes sense.

Use Serious Mass post-workout when your body needs maximum calorie and carbohydrate replenishment immediately after training.

Use Critical Mass Professional between meals or before bed when you want the advanced ingredient stack — particularly the DigeZyme™ for digestive comfort and the slower-releasing milk protein isolate for overnight recovery support.

However for most people one product is sufficient. Choose based on whether maximum calories or formula quality is your primary priority.

The Honest Summary

Choose Optimum Nutrition Serious Mass if:
Your absolute priority is maximum calories per serving and you need the most aggressive calorie support available to overcome a very fast metabolism or extremely low appetite.

Choose Applied Nutrition Critical Mass Professional if:
You want a more advanced complete formula with superior digestive support, a dual creatine matrix, HMB, and AstraGin™ — and you prefer a more manageable daily calorie intake that supports lean quality muscle growth.

Both products are excellent — they simply serve different needs. The biggest mistake would be buying Serious Mass when you actually need controlled lean gains, or buying Critical Mass Professional when you genuinely need 1,200+ calories per serving to gain any weight at all.

Ready to Order? Shop Both at Muscle Products Store

Still deciding? Use this as your final guide:

🏆 Want the most advanced formula? → Shop Applied Nutrition Critical Mass Professional — £59.99
🔥 Need maximum calories? → Shop Optimum Nutrition Serious Mass
💪 Want to see all options? → Shop All Mass Gainers UK

Fast UK delivery on every order. Buy now pay later with Clearpay. Earn Muscle Points on every purchase.

Frequently Asked Questions

Which has more calories — Critical Mass or Serious Mass?
Optimum Nutrition Serious Mass delivers significantly more calories at 1,259 kcal per serving compared to Applied Nutrition Critical Mass Professional at approximately 550–600 kcal per serving. If maximum calorie support is your priority, Serious Mass is the clear choice.

Which has more protein — Critical Mass or Serious Mass?
Optimum Nutrition Serious Mass delivers 50g of protein per serving compared to 42g in Applied Nutrition Critical Mass Professional. However Critical Mass Professional uses a more advanced multi-source protein blend including ISO-XP™ 95% Whey Protein Isolate as the primary source.

Is Applied Nutrition Critical Mass better than Serious Mass?
It depends on your goal. Critical Mass Professional has a more sophisticated ingredient formula with dual creatine, HMB, DigeZyme™, and AstraGin™. Serious Mass delivers more calories, more protein, and 25 vitamins and minerals. Neither is objectively better — the right choice depends on whether formula quality or calorie volume is your priority.

Is Serious Mass good for skinny guys?
Yes — Serious Mass is one of the best options for genuine hard-gainers and very fast metabolisms. At 1,259 calories per serving it provides the most aggressive calorie support of any mainstream mass gainer in the UK and is specifically designed for people who struggle to consume enough calories through food alone.

Does Critical Mass Professional have creatine?
Yes — Applied Nutrition Critical Mass Professional contains a dual creatine matrix with 3g of Creatine Monohydrate and 2g of Kre-Alkalyn® Buffered Creatine per serving — a more advanced creatine inclusion than Serious Mass which contains 3g of standard creatine monohydrate.

Is Optimum Nutrition Serious Mass Halal?
Serious Mass is not specifically listed as Halal certified. Applied Nutrition Critical Mass Professional holds formal Halal certification making it the better choice for Muslim athletes specifically.

Can I mix Serious Mass with milk?
Yes — mixing with milk significantly increases the calorie, protein, and carbohydrate content of each serving. At 1,259 calories with water, adding full fat milk will push each serving well above 1,500 calories — ideal for extreme hard-gainers but worth being mindful of for those wanting more controlled intake.

Which is better for beginners — Critical Mass or Serious Mass?
For beginners who are naturally lean and struggle to gain weight, Serious Mass is a strong starting point due to its high calorie content. For beginners who want a more controlled approach to lean muscle building, Critical Mass Professional’s more manageable calorie count and advanced digestive support make it a better daily supplement.

Related Guides and Products

• Best Mass Gainer UK 2026 — Top 5 Ranked and Reviewed
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• Applied Nutrition Critical Mass Professional Review UK 2026
• Best Creatine UK 2026 — Stack With Your Mass Gainer
• How to Gain Weight Fast for Skinny Guys — Complete Guide
• Shop Applied Nutrition Critical Mass Professional at Muscle Products Store
• Shop Optimum Nutrition Serious Mass at Muscle Products Store
• Shop All Mass Gainers at Muscle Products Store