Does Cardio Really Eat Muscle Mass? Of Course Not! Total Myth


Will Cardio Eat Into Your Gains?

This Commonly-Held Belief Is A Total Myth – We Explain Why!



Look on any fitness forum and you’re guaranteed to see somebody asking the following question: “Will adding cardio into my routine prevent me from making gains or eat into my existing muscle mass?”

You don’t have to look very hard to find different variations of this same question on practically every forum.

You also don’t have to look very hard to find people giving appalling advice: “Limit workouts to 30 minutes, no walking in the day or you’ll waste away, eat every 20 minutes or you’ll lose all your gains.”

The belief that any form of cardio will deplete muscle mass is surprisingly widespread in certain fitness communities. 



Generally speaking, this is a belief most commonly held by people who have been training for more than 6 months but less than 3 years. These people are dedicated enough to have read some articles on Flex, seasoned enough to believe they know everything about bodybuilding, but not experienced enough to have put any of the concepts they know to the test.

If you are ever unsure whether or not you should listen to someone, take a look at their physique, their strength and fitness, and ask yourself if that’s where you want to end up. Ask yourself if this person is really in a position to give advice that runs counter to the wisdom of the fitness industry professionals. And always, ask them where they got this information.

It should be pretty obvious that we think this idea is complete nonsense. 

Cardio is an absolute must for any strength and fitness program. 

Even if you are trying to gain a lot of weight in a short space of time, cardio can only help – assuming of course that it is programmed properly and you aren’t just looking to get fat!

In our opinion, it is glaringly obvious that cardio doesn’t eat away at muscle mass.

It pains us to see young guys being given terrible advice by people who have no business giving anybody fitness instruction. 

The proof is right in front of everybody’s eyes, and yet there is still a sizable number of people who preach the mantra of 45 minute workouts, no cardio, and the importance of staying in an “anabolic state”. 

To some people’s credit, there is some truth to the claim that too much cardio, relative to caloric intake and rest, can hold you back.

There is some truth to the claim that long distance running can inhibit recovery to some degree.

There is such a thing as over-training, although we’eve never seen anybody experience this first hand.

But the claim that cardio in and of itself is catabolic, that it “eats” muscle mass, is a total lie.

The claim that new trainees looking to gain muscle mass should avoid cardio altogether is not only a stupid lie, it is potentially causing unnecessary damage to young men’s health and preventing them from actually achieving the physiques they want. 

So where did this pernicious myth come from?

Why do so many people claim that cardio kills muscle mass gains?

Before we go down that rabbit hole, we first need to actually prove our case.

We’re going to look at the available scientific data on the subject to see whether cardio actually “eats” muscle mass. We’ll look at the evidence behind the idea that it inhibits muscle growth, as well as the evidence showing that it in fact helps with muscle growth, strength, and overall physical performance. 

We’ll look at some less scientific evidence too; the people who prove that cardio doesn’t deplete muscle mass through their actions and their incredible physiques. 

Finally, we will highlight the many health risks associated with ditching cardio altogether. Few guys actually go hell for leather in the gym, so the amount of intense aerobic exercise they get is very little indeed. 

Then we’ll give you our theory on where this myth came from, why it is so widespread, and why it is unlikely to go anywhere anytime soon. If you have any questions, please let us know in the comments section at the end. We’ll do our best to get back to you within 48 hours (unless you’re a crank, in which case we’ll get back to you right away).


Cardio & Catabolism – The Science

Let’s have a quick stroll through the available scientific data on cardio, catabolism, and muscle mass gain.

We’re going to try to stick to the clearest studies available. We’ll also try to avoid any studies with obvious problems; biases, confusing results, systemic design flaws, or statistical insignificance.

As always, we’re not going to try and cover every study on this subject. 

We’re not trying to write a sports science textbook here.

We are hopefully going to give you a good foothold to go off and do more of your own reading if you’re interested.

So, to kick things off, take a look at this article. It discusses a study which examined the weight loss of runners completing in the 2009 TransEurope-FootRace; an ultra-long distance race.

Researchers followed runners along the route and took scans of them at various points using a mobile MRI machine. 

The first finding noted in the article is that, “over the course of the event, runners lost an average of 5.4% total body volume, most of it during the first half of the race. Runners lost 50% of their total body fat during the race, but had rid themselves of 40% of total body fat by the midway point.”

The researchers found that fat seemed to be the first body tissue affected. This makes sense; it doesn’t take long to work through your blood sugar, and the next best fuel you have at your disposal is your body fat. Specifically, runners seemed to lose their visceral fat first. That’s the fat surrounding your organs. 

Overall, runners lost 70% of their visceral fat stores.

With such extreme fat loss, you might think there’d be a lot of muscle loss too.

Well, you’d be wrong: “the group also averaged 7% muscle volume loss in their legs”.

That should strike you as a pretty significant finding. 

The runners lost the majority of their visceral fat stores. They lost over 5% of their total body mass. Yet they only lost 7% of the muscle mass in their legs; the muscles working all day, every day, for 64 days without sufficient rest.



But evidence like this will only convince so many people. 

If we want to conclusively prove that cardio doesn’t waste muscle mass, we need to explore the mechanisms behind muscle mass catabolism. 

The first thing we need to establish is the fact that muscle mass is not your go-to tissue for fuel. 

As far as the body is concerned, muscle mass is a very valuable tissue.

It is extremely expensive to build.

It is extremely useful. 

It is also fairly difficult to break down; definitely more difficult to break down than fat tissue. 

No, your muscle mass is not the first thing your body will go to if it needs to tap its fuel reserves. 

If you burned your way through your muscle mass as soon as you ran out of ready glucose in the blood, wouldn’t that be a pretty serious adaptive advantage? We couldn’t imagine an animal with that metabolism to surviving too long. Evolution isn’t that kind.

Your body’s go-to fuel source when you run out of blood glucose is your fat stores.

Think about it: your fat stores are LITERALLY THERE TO SERVE THIS PURPOSE.

This is what fat is for.

It is worth going through the metabolic process here.

Your body usually runs on glucose; that is its favourite fuel.

When you consume carbohydrates, your body breaks it down into glucose and stores some of it in your cells as glycogen. Your muscles actually get stuffed full of glycogen; it is glycogen and water that actually gives bodybuilders’ muscles that full, bloated look. 

You will then slowly work your way through the glucose in your blood to fuel your bodily processes. Once you exhaust your blood glucose, you will then tap your glycogen stores and start working through them.

Only once your glycogen stores are depleted will your body turn to its reserves for fuel. 

And the reserves it turns to first are your fat stores.

But didn’t the long-distance runners in the study above lose some muscle mass, too?

Yes, but let’s think about why.

Aerobic exercise can induce muscle loss. That is not being contested here, and is in fact common knowledge.

When you put your muscles under severe stress, you effectively cause tears in the tissue. When we lift weights, that is exactly what we are trying to do; we’re trying to slightly break down the muscles. 

The idea behind bodybuilding is that you can build your muscles by breaking them down and then letting them rebuild, coming back bigger and stronger. This is the idea of adaptation to stress. 

But the runners weren’t allowing their leg muscles to recover. They were running long distances every day for over 2 months. 

Not only that, but they were deep into their fat stores.

They also only lost 7% of their leg muscles in these circumstances.

So, with that in mind, do you think the claim that a 20 minute jog will make your pecs disappear is reasonable?

Do you really think that your arms are going to waste away because you finish your workout with 15 minutes on the bike?

Do you think you are constantly glycogen depleted? Do you think you’ve ever actually been glycogen depleted?

Is it likely that your body will get rid of that hard earned back muscle because you walked home instead of driving?

The answer to all of these questions should be “obviously not”. 

But in case you’re still not convinced, let’s look at some real world examples of people who built incredible, powerful, bulky physiques while doing a lot more cardio than you will ever probably do. 


-Steady State Cardio vs. HIIT For Fat Loss-


Cardio Doesn’t Burn Muscle – Real World Proof

There are plenty of examples of people who did lots of cardio and still managed to build incredible physiques.

In fact, there are plenty of people that do much more cardio than you will ever attempt and have better physiques than you will ever probably achieve. 

Some of these examples are right in front of your eyes. You just might never think about how much long-distance running, walking, swimming etc that these people do. You might not realize just how much they do either. 

With others, you might not have ever known how much cardio they did.

So let’s start with some of the more famous examples: Arnold, Franco, and the golden era gods

Bodybuilders today do more cardio than you probably realize. If you watch any kind of ‘day in the life’ of Kai Green, Ronnie Coleman, or Jay Cutler, you’ll see that their days routinely started with 30 minutes of cardio (watch that and tell me cardio eats your muscles away with a straight face).

But the bodybuilders of the golden era did an awful lot more cardio than the bodybuilders of today, and they all had big, bulky, full physiques that most gym rats can only dream of today.



Take Arnold specifically. 

He regularly worked out for 2 hours at a time, twice a day, with great intensity.

He didn’t eat in the gym, he didn’t need to stop for a shake every 45 minutes, and he didn’t waste away in the gym.

And what did he do right after his workouts?

Him and Franco would regularly drive to the beach and go for a run along the shore before going to eat. 

So that is about 2 hours of intense weight training, with high volume and super-sets in play, followed by a run in the sand.

Clearly, all that work without food wasn’t turning Arnold, Franco et. al. into a bunch of skeletons. 



Another great example of a group that do lots of cardio but obviously don’t struggle to build muscle mass is rugby players

We’re sure we don’t need to explain that rugby players generally carry a lot of muscle mass.

In today’s game, even backs need to be powerful and bulky. 

But every rugby player also needs to have exceptional stamina and speed. That’s why their training includes so much cardio. 

A typical in-season training week for a professional rugby player might include 2 sessions of running through specific plays on the field, scrummaging, and lineout practice. They will likely have another specific cardio session, as well as 2 weight training sessions in the gym. Then, of course, they will have a game on Saturday, which involves running until exhaustion. 

Obviously training schedules differ between in-season, off-season, and pre-season. But the level of running will generally remain quite high, even in the off-season. 

So, with all that running, are rugby players turning into stick men towards the end of the season?

Do they start moving backwards in their lifts? Does their strength generally start faltering after 4 months of play? 

Of course not! 



Perhaps the best example here, though, is the boxer.

Boxers are living proof that cardio does not burn away muscle mass. 

Your typical boxer will spend far longer on aerobic capacity than things like strength training or even sparring practice. That’s because of the simple fact that, if your legs can’t carry you through 10+ rounds of ducking, dodging, dancing, and punching, you’re dead in the water. 

Yes, they obviously do spend a lot of time on punch bags, sparring, and so on. Some resistance training is obviously involved too (although Mike Tyson famously said that lifting weights was anathema to boxing training, apart from shrugs for trap power). 

But the bread and butter of every boxers training regime is running. Next is skipping. Next is more running. Then it’s high intensity circuits. 

Here is the training regime of Iron Mike:



We think any one of our readers would quite like to have the strength, power, speed, and look of Mike Tyson in his prime. 

Mike did train much more intensely than most other boxers, by virtue of the fact he never took rest days. But look at the schedule of any other boxer and you’ll see that they run for hours every day, spar for long periods, and then do resistance training on top of that. All without losing a noticeable amount of muscle mass. 

Some of our readers might point out that Mike Tyson was apparently using steroids while training like this. That is true of most of the people mentioned above. But if that means that a certain type of training will not work for you, then you also need to discount every training style you’ve ever seen any bodybuilder do, or any plan you’ve read in Flex.



We could go on and list military personnel, cyclists, and even sumo wrestlers (who only eat twice a day and start each day with a long, grueling cardio session, but who clearly don’t struggle to maintain mass). 

But we don’t need to.

Instead, we’ll just ask ourselves: what do all these people have in common?

Well, clearly, there is a metabolic explanation in play. The body simply prefers to get rid of fat tissue rather than muscle tissue if pressed for fuel. 

But for guys like Mike Tyson and Arnold, why were they able to do so much cardio and still retain so much muscle?

Why can professional rugby players do so much running and weight training and still make gains?

The answer is FOOD!

The answer is almost always in the food!

These athletes do lots of cardio, but they don’t eat into their muscle mass because they GIVE THEIR BODY SUFFICIENT FUEL TO WORK WITH!

They will generally never get completely depleted of glycogen. When they do (perhaps during a game), they will have plenty of visceral fat stores to work through before they get close to touching the muscle.

If you want to do more cardio but you don’t want to risk losing any muscle mass, just eat more food. 

If your body never gets rid of all its glycogen, then you wont ever even begin to touch your muscles. 

You wouldn’t get rid of any fat doing that either, but you certainly wont lose muscle mass.

If you do start to lose fat, you will wont lose an appreciable amount of muscle mass.

As we saw fro the study on long distance runners, you’d have to lose 70% of your visceral fat stores and work your muscles for 64 days in a row, non-stop, in order to see a mass decrease of 7%. 

To put it another way: doing a 30 minute run in the morning will not make you lose your gains. 


-Ab Training Explained-



Cardio Helps You Build Muscle Mass

Now that (we think) we’ve established that cardio doesn’t burn muscle mass away, we’re going to put forward a hypothesis: cardio can actually help you make gains. 

We believe that cardio improves your work capacity, which in turn improves your ability to train for long periods of time without rest. Since building muscle requires you to place your muscles under serious strain, this is key.

We believe that aerobic efficiency greatly improves recovery. 

Your muscles get damaged in the gym, and they repair (and grow) as you recover. Improving your ability to deliver oxygen and nutrients to your muscles during this recovery period would greatly increase your ability to efficiently repair and grow your muscles.

Instead of covering this in too much detail here, we will talk about this in a separate article (coming soon). 


Health Benefits Of Cardio

The people that tell you to stay away from cardio are not only doing so for no reason, nor are they just potentially holding back your progress.

People telling you to avoid cardio while building muscle are also probably harming your health. 

We don’t think we need to do much convincing here.

That aerobic exercise is good for your health is almost self evident at this point. 

But just to drive the point home, we’ll run through the main health benefits of doing cardio (or the health costs of avoiding it, depending on how you look at it).

This is a very brief but fairly comprehensive overview of the main benefits associated with cardio. We don’t think that article is particularly authoritative, but it does give you a brief primer. 

Let’s get more specific.

It is clear that running helps improve and protects cardiovascular health. Getting regular aerobic exercise significantly lowers the risk of developing a wide range of cardiovascular conditions and diseases. Specifically, aerobic exercise has been shown to reduce the risk of developing all types of heart disease, it reduces the incidence of heart attacks, and it reduces the incidence of stroke.

Regular aerobic exercise reduces systemic blood pressure, which in turn reduces your chances of having a heart attack or stroke. 

There is also a clear link between regular aerobic exercise and cancer incidence. Far from being fringe science or new age baloney, the link between regular exercise and reduced risk of cancer seems to be fairly well accepted.

There are lots of studies looking at running and cancer occurrence, like this one published in 2015. The conclusions the researchers drew here are pretty straightforward: “Running and walking may reduce incident kidney cancer risk independent of its other known risk factors.”

There is no reason to assume that regular aerobic exercise would only reduce the risk of kidney cancer. We think it is reasonable to assume here that running generally makes cancer less likely to grow.


Where Does This Myth Come From?

We think there are a number of things driving the “cardio burns muscle mass” myth. 

There isn’t just a single group of people perpetuating this idea.

But we believe that everybody who is trying their best to spread this myth is doing so for the same reason, and that is the same reason that lies are always spread: money. 

If someone is telling you something that is patently not true and you aren’t sure why, it’s safe to assume that it is money. 

So who is primarily behind this myth?

Well, for starters, we have fitness bloggers and bodybuilding YouTubers. They like to spread this idea for a number of reasons; for one thing, it lets them say something novel, that runs against the perceived wisdom of the fitness community, and therefore lets them offer a USP.

It’s also safe to assume that their subscribers want to be told that cardio is a waste of time. They can then regurgitate that info up in the gym and sound like real experts. Plus it lets them feel good about skipping the bike. This applies to personal trainers too; they need to justify their existence, so telling something that goes against their expectations, something “secret”, is a must. 

Then we have supplement companies

If you make a protein powder, then you need people to need food as often as possible. If you can convince people that they need to eat protein every 90 minutes or their arms will fall off, then unless they live at home they will come to rely on your product.

The idea that the slightest thing can start eating away at your muscle mass ties in nicely with the protein narrative. If just missing a meal means you basically turn back into you of 5 years ago, then imagine what a 30 minute run will do! – so the story goes. If you convince people that anything other than weight lifting and protein consumption will halt their progress, then you are guaranteed to see sales heading in the right direction. 

Finally we have gyms themselves. 

The business model of your typical corporate gym is to have your space oversubscribed by about 50%. Gyms get away with this because they know that most people join a gym and then never go. So they can get away with having twice as many members as the space realistically allows. 

But what happens when there are rushes and everyone comes at once?

Well, the “cardio kills gains” narrative takes care of that. 

Convincing people that they shouldn’t exercise for more than 45 minutes at a time is a good way to make sure that your gym never gets too crowded. 

Never mind that Arnold trained for nearly 3 hours at a time; the skinny guy wearing gloves for the Smith machine told you that anything more than 45 minutes will make you weak, so that’s that.

All of these interests intertwine and perpetuate each other’s interests. What we end up with is a serious catalog of industry “experts” all dancing to a tune set by supplement companies, ad schemes and referral programs. 

Don’t fall for it.

Train hard.

Lift with intensity.

Eat your food.

And for God’s sake, get your cardio done.

Your physique, your mind, and your health will all benefit. The only people who will lose out are the supplement companies and the gym owners.

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