ON Optimum Nutrition Serious Mass Weight Gainer Review

A picture of Best Mass Gainer UK founder, personal trainer Andy Williams
Written By
Andy Williams
Last Updated on

If you’ve read any Optimum Nutrition Serious Mass reviews before, then you’ll probably know that this popular supplement has a reputation as one of the best mass gainers on the market.

Well, I’ve used Serious Mass on multiple occasions over the past 10 years to repeatedly test its effectiveness. And while Serious Mass is definitely a solid product that will help you pack on size, I think that better mass gainers have hit the market in recent times.

So, in my ON Serious Mass review, I’ll guide you through the pros and cons of this popular weight gainer and then talk about taste, mixability, side effects, and results. Stay tuned.

great for weight gain
Optimum Nutrition Serious Mass
A tub of Optimum Nutrition Serious Mass in chocolate flavour
Taste: 8/10
Mixability: 8/10
Ingredient Quality: 8.5/10
Mass-Building Potential: 9.5/10
Overall Rating: 8.5/10
  • Contains 1,263 calories to help hardgainers gain weight
  • Provides 50g of quality protein for muscle growth
  • Formulated with 251g of carbs to help you add mass
  • Includes 25 vitamins and minerals
  • Mixes really well in a blender
  • Has a proven track record of helping people gain weight
  • The flavours have very mixed reviews
  • Maltodextrin is the only carb source
  • It can make you feel really full and blunt your appetite

ON Serious Mass Nutrition Facts

Serious Mass by Optimum Nutrition definitely falls into the mega-calorie gainer category because it has over 1,200 calories per serving. No wonder these tasty shakes (more on the flavours later) have helped so many people gain weight!

The ON Serious Mass nutrition facts

Each 334g serving, which is equal to about 2 scoops (or should I say shovels), provides the following nutrition:

  • 1263 calories
  • 251g of carbs
  • 50g of protein
  • 5.5g of fat
  • 26g of sugar
  • 3.2g of fibre
  • 3g of creatine
  • A selection of 25 vitamins and minerals

As you can see, Optimum Serious Mass is nutrient-packed. However, in order to get all of this muscle-building goodness, you need to consume two giant scoops, which can be a bit of a chore for hardgainers, who don't always have the best appetites.

You can always check out my Gold Standard Gainer review if you’re looking for a weight gainer that’s lower in calories and easier to consume.

ON Serious Mass Ingredients

Here’s how each of the Serious mass ingredients can help (or hinder) you from a muscle-building perspective.

251g of Carbs

Carbohydrates and, more specifically, maltodextrin, is the main ingredient in ON Serious Mass Gainer. In total, there are 251g of carbs for the chocolate flavour.

Directions showing how to use Serious Mass for good results

Now, while I don’t mind maltodextrin in my mass gainer, I don’t like that it’s basically the only carb source in Serious Mass. If I wanted a 100% maltodextrin “mass gainer”, I could just buy a big bag of maltodextrin at a fraction of the cost of Serious Mass and then add some whey protein.

Obviously, ON Serious Mass is definitely not the only weight gainer to rely heavily on maltodextrin.

And to be honest, if you made your own gainer with maltodextrin and whey (or with other ingredients), it probably wouldn’t taste or mix as well as Serious Mass.

And to be fair to Optimum Nutrition, their Serious Mass formula has been producing results for over 10 years now—not many other brands can say that about their supplements!

Ultimately, while 251g of carbs might be excessive for some people, it could be just what a hardgainer needs to overcome a weight gain plateau.

50g of Protein

Protein is the second most abundant Serious Mass ingredient, and this time the quality is indisputable.

Serious Mass chocolate flavour

ON Nutrition Serious Mass uses a 50g protein blend consisting of whey and calcium caseinate. What this means for you is that you’ll experience superior gains because your muscles are getting a quick yet consistent supply of quality amino acids.

While the whey triggers a rapid spike in muscle protein synthesis, the slower-digesting calcium caseinate keeps providing your growing muscles with amino acids long after you’ve consumed your Serious Mass shake.

As a result of this consistent supply of amino acids, you’re less likely to miss out on muscle growth because your body will be in more of an anabolic state.

My only complaint with regard to the protein is that, although 50g is a lot of protein, it’s not that much to say there are over 1,200 calories. So, it would’ve been great if the protein-to-calorie ratio was a bit higher.

5.5g of Fat

Optimum Nutrition Serious Mass Weight Gainer Protein Powder contains just 5.5g per serving, which is quite low, even for a weight gainer.

A tub of Serious Mass about to be opened and reviewed

And while a check of the Serious Mass ingredient label does confirm the use of medium chain triglycerides (MCTs), the low-fat content is a dead giveaway that the product relies heavily on maltodextrin.

Other gainers that use more varied carb sources, such as oat flour, tend to have a higher fat content, which creates a more balanced macronutrient profile.

If you were to eat a 1,200-calorie meal, you’d certainly be consuming a lot more than 5g of fat (I hope).

Still, I can see why Optimum Nutrition has kept the fat content low. Having minimal fat means that the carbs and protein can be absorbed faster. It also means that the shake mixes a bit easier and ends up being more drinkable.

3g of Creatine

The results of filling the Optimum Nutrition Serious Mass scoop with powder

As many Serious Mass Protein Powder reviews will tell you, this formula does contain creatine, specifically 3 grams per serving.

A daily creatine dose of 3-5g is considered optimal, and some people take slightly more if they’re trying to “load” the creatine by saturating their muscles with it.

Obviously, 5 grams per serving would have been ideal to get those Serious Gainz. You could just take an extra couple of grams yourself, but many lifters understandably prefer to keep their supplements completely separate, so the fact that Serious Mass contains creatine isn’t necessarily a benefit.

25 Vitamins and Minerals

As I noted in the ON Serious Mass nutrition facts, each serving has 25 vitamins and minerals. In other words, you’re not just consuming empty calories or macronutrients; you’re drinking a nutritious mass gain shake.

Since Serious Mass is so calorie-dense, it might make up a fair amount of some people’s daily calorie intake. So if it didn’t contain all these vitamins and minerals, people would be getting inadequate nutrition because they’d be missing out on vital micronutrients.

And while I’m a big believer in getting most of your nutrition from healthy whole food sources, it’s good to see that Optimum Nutrition has equipped Serious Mass with an abundance of vitamins and minerals, including vitamin A, vitamin C, vitamin D, magnesium, iron, zinc, and many others.

Optimum Nutrition Serious Mass Taste

Over the years, I’ve tried every Serious Mass flavour available. And honestly, they’re all pretty good, which is quite rare for a weight gainer with so many calories.

A man pouring his Optimum Nutrition Serious Mass shake into a bottle to review it

I especially like the chocolate flavours, including Chocolate Peanut Butter, Chocolate Mint and, of course, regular Chocolate.

Since taste is subjective, you’ll naturally find some ON Serious Mass reviews that say the taste was awful.

Now, I’ve personally taste-tested over 60 mass gainers this year, and I can confidently say that Serious Mass tastes way better than average.

Yes, the shakes can be very thick, and you might not always enjoy drinking Serious Mass during a bulk. However, I find the drinks to be very much palatable, though I would say there are better-tasting options out there.

Optimum Nutrition Serious Mass Protein Powder Mixability

A person making a Serious Mass shake in a blender

If you want a lump-free Serious Mass shake, then you’ll need to use a blender.

It doesn’t matter how strong your arms are; if you try to mix a full serving of Serious Mass in your shaker bottle, then you will experience some clumping.

When you use a blender, Optimum Serious Mass mixes easily in around 700 ml of water and doesn’t leave any clumps.

Optimum Nutrition recommends blending your shake for 30-45 seconds in order to get a smooth consistency.

Also, the shake might foam a bit initially, but the foaming should die down if you leave it for a couple of minutes.

Optimum Nutrition Serious Mass Results

Serious Mass before and after making a shake

Read some Optimum Nutrition Serious Mass reviews, and you’ll see people saying how they gained weight and got great results from using Serious Mass.

I’ve heard plenty of people reporting substantial weight gain of between 2kg and 6kg in anywhere from 2-8 weeks.

Naturally, individual results will vary based on your starting weight, training experience, and consistency, as well as the rest of your diet and your training intensity.

Still, so many different people report excellent ON Serious Mass results in a relatively short space of time, so given the effort and consistency, this supplement can definitely trigger weight gain.

Optimum Nutrition Serious Mass Before And After Case Studies

Let’s go through a few real-life case studies to see some ON Serious Mass before and after results.

Case Study 1: A Jones Fitness

This case study from AJonesFitness shows that Serious Mass has a proven track record of delivering weight gain results.

In just 5 weeks, Aaron Jones gained 6 lbs, while increasing his bench press power by 30 lbs and his squat strength by 50 lbs.

While these results, especially the strength gains, might be on the upper end of what’s possible with a weight gainer, it shows that with hard work in the gym, Serious Mass really can produce results.

Obviously, the weaker and skinnier you are, the more pronounced your results will be because you have more size to gain and more room to progress.

Case Study 2: Tami The Creator

In her Optimum Nutrition Serious Mass Protein Powder review, Tami The Creator explained how she gained a very impressive 15 lbs in 2 weeks by drinking Serious Mass.

While she wasn't a fan of the taste, going as far as to call it disgusting, she did achieve substantial weight gain in a really short space of time.

Now, Tami started at 131.8 lbs, which is likely one reason why she was able to achieve such rapid weight gain.

If you’re 170 lbs, for example, then you won’t gain weight as fast when using Serious Mass because your calorie surplus won’t be as big as hers.

Still, this awesome case study shows that, yes, Serious mass can work just as well for women as it can for men.

Case Study 3: Kruobi

Kruobi used Serious Mass during his 90-day before and after transformation. And when I say transformation, I really mean it.

While he doesn’t give exact before and after weights, you can see that he got visibly bigger—much bigger.

Obviously, he trained hard and ate plenty of food as well, but Serious Mass really gave him that extra calorie boost that he was looking for.

So it just goes to show, that for any weight gainer to work, you need to supply the effort in the gym and have the discipline to drink Serious Mass in addition (not in replacement of) your regular meals.

How To Use On Serious Mass Gainer

The Serious Mass scoop size

Now that you’ve seen some real-life ON Serious Mass gainer reviews and case studies, it’s time to actually learn how to use Serious Mass for the best results.

To begin, fill your blender with around 700 - 750 ml of water and add 2 heaping scoops of Serious Mass powder. Then, blend for 30-45 seconds.

For even more calories, you can mix in whole milk, almond milk, nut butter, and even frozen berries.

Personally, I think Serious Mass tastes best by itself or just with milk. The calories are high enough, so I don't add in any extras, but you certainly can if you’re a real hardgainer.

If one serving is too big, then you can always split the 334g serving in half and have two smaller shakes per day (for example, one after your workout and the other before bed).

A blender jug and scoop beside a tub of Serious Mass by Optimum Nutrition

For a gainer that has smaller serving sizes, check out my Optimum Nutrition Pro Gainer review.

I don’t recommend having more than one shake per day because a) the tub/bag won’t last very long, and b) you still want to get the majority of your calories from whole food sources.

Should you take Serious Mass on rest days?

My vote is yes. While you might not be expending as much energy during your days off from the gym, this is prime muscle recovery time, so you should make the most of this crucial recovery period by drinking Serious Mass on rest days.

Optimum Nutrition Serious Mass Side Effects

The most obvious and likely Serious Mass side effect is fat gain.

A tub of chocolate Serious Mass next to an Optimum Nutrition shaker bottle

Of course, some fat gain is part and parcel of any successful bulking regime, but you don’t actually want to become fat.

Now, I will say that unwanted fat gain isn’t actually a direct result of Serious Mass. Rather, it stems from eating in an excessive calorie surplus for too long.

Additionally, since Serious Mass Gainer Protein contains 1263 calories per serving, the shakes can sit quite heavy on your stomach, which can make you feel pretty bloated.

To see how Serious Mass compares to a range of other weight gainers, you can also check out these supplement comparisons:

Serious Mass Weight Gainer FAQ

Since Serious Mass is so popular, people naturally have a lot of questions about it. After all, you want to make sure that it definitely works before spending your money on it!

Does Serious Mass Work?

Yes, Serious Mass works in the sense that it provides the necessary calories, carbs, and protein for you to gain weight. However, it doesn't do the work for you. You still need to eat your regular meals and train hard in the gym.

Is Serious Mass Safe?

Optimum Nutrition says that all of the ingredients in Serious Mass are considered safe. I haven’t lab-tested the formula myself or anything like that because I don’t have the facilities, but Serious Mass has been around for a while, so you could say that’s a good sign of its safety.

Can Serious Mass Make You Fat?

Yes, Serious Mass can make you fat if you drink too much of it. This is why I recommend sticking to one serving per day. Additionally, you may wish to monitor your overall calorie intake so that you don’t overeat.

Does Serious Mass Expire?

Yes, like any supplement, Serious Mass does expire. Looking at the tub beside me, I can see that my batch has around 9 months left, which is more than enough time to use it all up.

To keep your Serious Mass protein powder fresh, make sure to keep the lid on the tub or fasten the bag properly.

What Is The Serious Mass Scoop Size?

The Serious Mass scoop size is enormous! As I said earlier in my ON Serious Mass review, these aren’t scoops; they’re shovels! I’d estimate that each scoop is around 150g because 2 servings (2 heaped scoops) is equal to 334g of powder.

Does Serious Mass Contain Creatine?

Yes, each serving of Optimum Nutrition Serious Mass Weight Gainer contains 3g of creatine. This is important to know if you’re taking creatine separately.

What Is The Best Serious Mass Flavour?

I’ve tried every Serious Mass flavour, and for me, the best Serious Mass flavours are Chocolate and Chocolate Peanut Butter. Vanilla was my least favourite, but it honestly wasn’t that bad.

The Verdict: Is Serious Mass A Good Weight Gainer?

Back view of Serious Mass weight gain powder

All things considered, Optimum Nutrition Serious Mass Protein Powder is an undeniably effective weight gainer. It contains more than enough calories for a hardgainer to add muscle mass to their frame.

Additionally, the countless testimonials and Optimum Nutrition Serious Mass reviews show that this supplement can produce really good gains if you take it consistently.

Yet, Serious Mass isn’t without its flaws.

I don’t like the fact that maltodextrin is the only carb source. Maltodextrin is ok in moderation, but I really don’t like drinking 251g of the stuff!

Also, due to containing over 1200 calories per serving, Serious Mass can make you feel really full, which might affect your ability to eat your other meals, which could reduce your weight gain. 

The way around this is to choose a mass gainer with lower calories and higher-quality ingredients.

Also, Serious Mass has very mixed reviews when it comes to taste. I personally like the taste, but I know that isn’t the case for everyone.

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Two bags of Crazy Nutrition Mass Gainer positioned side by side
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  • Tastes amazing (to my surprise!) and mixes easily
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A picture of Best Mass Gainer UK founder, personal trainer Andy Williams
Andy Williams
Andy Williams is a UK-based personal trainer and bodybuilder who coaches gym-goers of all levels using proven evidence-based techniques in strength training, nutrition, and supplementation. He’s tested over 70 different mass gainers for their ingredient quality and muscle-building effectiveness and regularly shares his findings in his mass gainer reviews.
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