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Does Cardio Burn Muscle?

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“Does cardio burn muscle” is one of the more frequent questions I get asked by those trying to put on muscle at the gym. Many of these people are worried that if they run or perform other forms of cardio, that they run the risk of losing their hard earned muscle. This leads many people, especially guys, to avoid cardio. From an overall health perspective, this can be counterproductive since cardio is extremely important for maintaining your overall health and burning fat.

That being said, there is some truth behind the bro-science that cardio can burn muscle, just probably not in the way you think. Furthermore, doing cardio at the wrong time or the wrong type, can be a detriment to gaining muscle. However, the exact mechanisms behind these claims are often misstated and overblown. There are actually certain aspects of cardio that can help you put on muscle and strength. In this article, we will be going over the science behind these claims, as well as how to fit cardio into your program to hit your goals and maximize your overall health.

When Does Cardio Burn Muscle

To answer this question, we first have to look at how the body utilizes the energy we provide it. When we eat, our body converts the food we eat into glucose. Our body typically converts most of the carbs we consume into glucose. Meanwhile, protein and fat also get converted into glucose, just in a lesser percent of their overall stored energy.

After digestion, this glucose enters the bloodstream. At this stage, it is referred to as blood sugar. In response to blood sugar levels increasing, the body releases insulin, which enables the storage of the blood sugar in the cells of our body. Inside the cells, the glucose is converted into glycogen, which the cells store to be used for energy. After the cells are full of glycogen, the remaining blood sugar will be brought to the liver for storage, where it will be released back into the blood to replenish the cells as they burn their glycogen stores. At this point, any extra glucose that cannot be stored in the liver is stored in our body’s fat cells. 

When our body performs work, such as walking, lifting, or running, our muscle cells are activating to provide motion. This requires the cells to use their stored glycogen. As the glycogen stores in the cells become depleted, the liver will release its stores of glucose back into the bloodstream to refill the cells. However, there will be a point when the liver runs out of glucose stores to refuel the cells. 

The body is capable of storing about 1500-2000 calories worth of energy between the glycogen stores in the cells and liver [1]. These glycogen stores are what the body uses during activity as its primary source of fuel. It also uses the glycogen for its normal functions, such as powering the organs, brain, and digestion, along with all of the other tasks our body performs. During normal activity, these glycogen stores can be replenished easily by eating food or by the body breaking down fat (fat stores 10,000+ calories worth of energy, even on lean individuals) [1]. In this state, the body has plenty of available energy and does not need to break down muscle protein for energy. However, during long sessions of moderate to intense cardio, we do run the risk of depleting the glycogen stores, requiring the body to seek other sources of energy.

To give you an idea of what a long cardio session is, the glycogen stores would typically be depleted around 90-120 minutes of vigorous exercise, or 150-200 minutes of moderate intensity exercise [1]. Marathon runners are aware of this moment, typically called “hitting the wall”, as their energy levels drop off significantly when this occurs. As we near this point, the body must convert other sources of fuel into glucose. This conversion process is called gluconeogenesis.

That big science word basically means that the body will now need to convert fat into glucose. This is the evolutionary reason our body stores fat, to be used as energy as our blood glucose becomes depleted, whether that be from cardio or a calorie deficit. 

You may be asking, if our body starts burning fat for energy during cardio, why would it burn muscle?

This is a question I had as well and the answer basically comes down to the fact that converting fat to glucose is a slow process [1]. Therefore, the body is not always capable of converting fat stores into glucose fast enough to keep up with the energy requirements of the body during certain intensities of exercise. 

During low intensity exercise, the body is capable of burning fat at a rate that meets the energy needs of the body. However, with moderate to intense forms of cardio, the body cannot burn fat fast enough to keep up with the energy needs. This is when the body could start having to break down protein in our muscles for extra energy.

Our body is able to supplement about 50% of our glucose needs with broken down fat during low to moderate intensity cardio. In the later stages of a long cardio session, once glycogen is nearing depletion, our body may have to start pulling up to 15% of its total energy expenditure from breaking down protein in the muscle [1]. Therefore, during long durations of moderate/intense cardio, we can run the risk of our body beginning to break down muscle to supplement its energy needs. 

Now, for the vast majority of people, they will not be doing long durations of cardio that put them at increased risk for burning off muscle. However, if you are someone who performs these long sessions of cardio, there are a few things you can do to minimize the risk of your body having to burn muscle for energy.

For one, it can help to consume enough carbs to fuel the body for your workouts. The exact amount can vary from individual to individual, depending on how you train and your body’s specific needs. Consider experimenting with carb intake ratio compared to the other macronutrients and see how your body responds. A good starting range would be somewhere between 40-65% of your daily calories from carbs. The longer duration and more frequent your cardio sessions are, the higher in that range you can aim for.

Furthermore, if you are doing cardio sessions over an hour in length, it is generally recommended that you consume about 30-60 grams of carbs per hour of cardio during the session. This should be consumed as a fast digesting carb, like Gu-energy packets or the energy chews. These products are basically just glorified sugar, which your body can quickly convert into energy. This will help keep your energy levels up and help protect your body from digesting muscle protein. 

This advice is meant for moderate to intense cardio sessions. If you are doing low intensity cardio, like walking, your body can convert fat into glucose fast enough to keep your energy levels up and prevent muscle protein being used as energy.

So, as long as you fuel your body properly and keep performing resistance training in your program, you should maintain your muscle mass. The risk of your muscle atrophying away is most prevalent in those who stop resistance training to focus on cardio. If you don’t use it, you lose it.

That being said, there are ways that cardio can hinder the rate at which you put on muscle (and also aid in gaining muscle). Which leads us to the question:

Should I Do Cardio if Trying To Build Muscle

Cardio and building muscle can go hand in hand for improving your overall health and appearance.

First off, here are just a few benefits of improved cardiovascular fitness.

  • Improved ability to build muscle
    • The better your cardiorespiratory system is, the better your body is at circulating blood and oxygen. This all leads to better recovery for your muscle cells, as they can get replenished faster due to the better blood flow. This means you recover better between sets, allowing you to perform better for each subsequent lift, maximizing potential strength and muscle gains. These improvements to the cardiovascular system have been linked to increases in strength gains in a few studies [2] [3]
  • Lower chance of all cause mortality
    • This is a linear relationship with no ceiling, meaning the better your cardiovascular fitness, the lower chance you have of dying from any cause. [4].
  • Natural energy boost
    • Better fitness means more energy. This means you can sustain a higher energy level throughout the day without having to rely on a 2nd, 3rd, and 4th cup of coffee. Trust me, it feels amazing to have what feels like an endless supply of energy. You can do so much more with your day and train that much harder at the gym
  • Strengthens the heart
    • The heart literally keeps you alive. A strong heart means a better you. With increased cardiovascular fitness, your heart can perform its job more efficiently, leaving you more energy for other tasks. Improving your cardio can literally make your heart larger and stronger so that it can pump more blood per pulse.

Secondly, cardio is a great way to burn more calories. This can help you keep a lower body fat percentage, meaning you can show off those hard earned muscles. Plus, you can eat more food because you are burning more calories!

So yes, you should do cardio even if you are trying to build muscle. But let’s get into a couple ways cardio could be hindering your muscle growth.

How to Gain Muscle and Do Cardio

For one, if you perform your cardio session before you lift, you are pre-exhausting your muscles. This means that your muscles will not be able to lift as much weight during your session. In terms of hypertrophy, muscle growth is maximized when you maximize your workout volume. 

Volume = weight x reps

What this means is if you cannot lift as much weight for as many reps as you are typically capable of because you did a cardio session before your resistance training, you are leaving potential growth on the table. To overcome this, consider performing your cardio session after your weight training session, or performing them as two different sessions with at least 6 hours between your cardio session and resistance training session [5].

Another way cardio could be limiting your muscle gains is hindering your recovery between resistance training sessions. The body needs time to rest and repair the damage that occurred to our muscles during weight training or a moderate/intense cardio session. If you are doing a moderate/intense cardio session the day after a hard workout at the gym, or vice versa, there is a chance you are not allowing your body to recover. By not allowing your body to recover, you may not be able to perform as well in your next session.

To overcome this, you need to listen to your body. If you are stringing together hard session after hard session and notice you feel overly fatigued or are noticing your progress plateau/regress, you may not be allowing adequate recovery. If this sounds like you, consider lowering the intensity of some of your sessions or taking extra rest days. When you train hard like this day in and day out, it is important to work time into your training schedule to allow your body to properly recover. Otherwise, you are at a higher risk of injury and may even notice your progress regressing.

A primary culprit for this could be adding HIIT sessions to your workouts. The high intensity of HIIT workouts means that you will be beating up your body more than you would in a typical low or moderate intensity cardio session. If you are training 5+ times a week, including HIIT workouts in all/most of your workouts could be hurting your progress in other areas of the gym. Again, just listen to how your body feels and notice if your progress starts to plateau or regress in other parts of the gym.

Conclusion

The relation between muscle growth and cardio can seem a bit complicated. However, with the info in this article, you can ultimately decide how you want to structure your workouts for your goals. If you want to absolutely maximize your muscle growth, minimizing cardio may deliver the fastest results. However, in my opinion, you are missing out on many extremely important health factors that come from cardio. 

In the end, how you structure your program, properly fueling your body, and the types of cardio you chose to do are the biggest factors in determining how cardio will interact with gaining muscle.

Personally, I still perform a lot of cardio and have put on plenty of muscle size during the process. But due to the factors listed in the article, my progress was most likely slower than if I had minimized cardio. What you do ultimately depends on what your goals are and what you want to do with the information in this article. There is no 100% correct way. Ultimately, how you go about your fitness journey is up to you and your unique goals.

If you have any further questions about anything in this article or would like help in structuring a program to meet your individual goals, please feel free to reach out to me HERE.

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[1] “The Body’s Fuel Sources,” Human Kinetics. [Online]. Available: https://us.humankinetics.com/blogs/excerpt/the-bodys-fuel-sources#:~:text=Provides%20energy%20in%20late%20stages,percent%20of%20the%20energy%20needed. [Accessed: 13-May-2021]. 

[2] Lundberg TR, Fernandez-Gonzalo R, Gustafsson T, Tesch PA. Aerobic exercise does not compromise muscle hypertrophy response to short-term resistance training. J Appl Physiol (1985). 2013 Jan 1;114(1):81-9. doi: 10.1152/japplphysiol.01013.2012. Epub 2012 Oct 25. PMID: 23104700.

[3] Borne R, Hausswirth C, Bieuzen F. Relationship Between Blood Flow and Performance Recovery: A Randomized, Placebo-Controlled Study. Int J Sports Physiol Perform. 2017 Feb;12(2):152-160. doi: 10.1123/ijspp.2015-0779. Epub 2016 Aug 24. PMID: 27139812.

[4] Bryant, Cedric X, and Daniel J. Green. Ace Personal Trainer Manual: The Ultimate Resource for Fitness Professionals. San Diego, CA: American Council on Exercise, 2010.

[5] Robineau J, Babault N, Piscione J, Lacome M, Bigard AX. Specific Training Effects of Concurrent Aerobic and Strength Exercises Depend on Recovery Duration. J Strength Cond Res. 2016 Mar;30(3):672-83. doi: 10.1519/JSC.0000000000000798. PMID: 25546450.