Do Cardio Workouts Cause Muscle Loss?

Do Cardio Workouts Cause Muscle Loss?

(Last Updated On: March 19, 2019)


Cardiovascular exercise boosts endurance and heart health while burning calories and fat. But some fitness gurus will tell you to limit the amount of cardiovascular training you do if you’re trying to build muscle strength and lean body mass. They believe doing too much cardio breaks down muscle tissue, which can work against you if you’re trying to build muscle definition. Is there any truth to this idea?

What Fuel Does the Body Use to Power Cardiovascular Exercise?

Your body relies on fat, carbs, and protein as its three sources of fuel for energy, albeit in different ratios depending on your activity intensity and duration. During low-intensity cardiovascular exercise such as walking at a moderate pace, your body uses more fat as fuel. If you pump up the intensity of your workout and break into a run, your body uses a higher percentage of carbohydrates as fuel as you activate more “fast-twitch” muscle fibers and levels of the hormone epinephrine rise. What about protein? Normally, the body doesn’t turn to protein as a fuel source except under unusual circumstances. You should always remember that when exercising your body will always prefer to burn Carbohydrates (both stored and from foods you have eaten) first, then Fats (from foods you have eaten and from your stored fat cells) and as only a last resort Proteins (amino acids from muscle tissue).

So why would doing too much cardio break down muscle when the body doesn’t like using protein as fuel? Fitness gurus will tell you the answer lies with cortisol, a hormone produced by an organ called the adrenal cortex that’s located just above your kidney. One effect of cortisol is to break down protein in muscle tissues to amino acids. These amino acids can be shuttled to the liver and used to produce glucose through a process called gluconeogenesis. This occurs during times of stress to help maintain blood sugar levels.

Under normal circumstances, the amount of muscle broken down to use as fuel during exercise accounts for less than 5% of your total energy supply, depending upon the duration and intensity of the cardiovascular exercise. Your body will normally not use a significant amount of protein for fuel unless your workout lasts more than 90 minutes and you are nutritionally depriving yourself by limiting calories, protein, carbohydrates and fat in your diet. If you’re eating a nutritionally sound diet that contains enough calories and sufficient amounts of all three dietary components, your body isn’t going to break down significant amounts of muscle tissue to use as fuel even if you do long cardiovascular sessions. Increased cortisol secretion becomes a problem mainly when glycogen levels, stored carbohydrates in the body, drop. A National Research study backs this up by saying that restricting calories by 50% can raise cortisol by 38%. So getting enough carbohydrates in your diet to maintain glycogen stores puts a brake on muscle breakdown due to the effects of cortisol.

In addition, if you’re weight training and getting enough protein in your diet, you’ll compensate for the very small amount of muscle loss that comes from doing cardiovascular exercise. You could run into problems if you’re doing long cardiovascular training sessions without taking in enough calories, protein, fat, and carbohydrates. In this case, your body senses starvation and starts pumping out more cortisol to break down muscle tissue to try to send to the liver for conversion to glucose.

Cardiovascular Can Help in the Quest for More Defined Muscles

If your goal is to have more defined muscles, you need resistance training to break down muscle fibers and stimulate them to repair and grow. But even if you work hard building muscle at the gym, it won’t turn any heads if it’s hidden by a layer of fat tissue. Cardiovascular exercise helps to remove that layer of fat so the lean body mass you’ve worked so hard to build becomes visible. To build body definition requires a three-pronged approach – strength training, cardiovascular exercise, and good nutrition. Skimp on any one of them, and you may not get the results you’re looking for. The key is balance. Too much strength training can work against you too. If you overtrain with weights and don’t give your body a chance to rest and recover, this will elevate cortisol levels and make it more difficult to build muscle.

What Does This Mean in Terms of Your Workout?

You’ll maximize your gains if you strike a balance between cardiovascular exercise and strength training. They’re both important for building a lean, defined physique. One builds strength and muscle mass, while the other lowers body fat so muscle tissue becomes more visible. Plus, cardiovascular exercise improves the health of the most important muscle in your body, your heart.

In your quest to get fit and defined, make sure you’re getting adequate calories in your diet based on your size and activity level. Use an online calculator, like our free Workout Manager to see exactly how many calories you need a day. Even if you’re trying to lose weight, don’t restrict calories by more than 500 a day, and never go below 1,200 calories daily. The recommended daily intake of protein is 0.8 grams per kilogram per day, but if you’re strength training and doing aerobic exercise, you may need up to 2.0 grams per kilogram of body weight, depending upon how hard you’re working out.

Make sure you’re eating enough carbohydrates to maintain adequate glycogen stores. Complex carbohydrates like those in whole grains, fruits and vegetables are best, because they don’t cause rapid fluctuations in glucose and insulin levels. Good sources of complex carbohydrates for athletes include oatmeal, beans, lentils, whole grain bread, and fresh fruits and vegetables. Complex carbohydrates are the best choice prior to your workout, although it’s tricky since the high fiber content of some complex carbs can cause diarrhea and flatulence. If this is a problem for you, eat your complex carbs two hours before a workout.

After a workout, it’s important to replenish glycogen stores quickly to avoid a post-workout surge in cortisol. There’s some evidence that simple carbohydrates after a workout are best for recovery. They also cause a more rapid insulin surge, which sends more amino acids into cells for building lean body mass. Combine this with a source of lean protein to give muscles the building blocks they need for growth after a workout.

The Bottom Line?

Don’t be afraid that doing cardiovascular exercise will limit your muscle growth. If you’re eating a nutritionally sound diet and replenishing your glycogen stores after a workout, you’ll build a lean, defined body through a balanced training routine of strength training and cardio.


Exercise Physiology. Fifth Edition. 2001. Lippincott, Williams and Wilkins


Related Articles By Cathe:

Does Cardiovascular Exercise Interfere With Muscle Growth From Resistance Training?

5 Hormones That Impact Muscle Growth and How They Work

Is Muscle Damage Necessary for Muscle Growth?

A Different Approach to Training: Integrated Concurrent Training

Can You Build Muscle Size Through Aerobic Exercise?

13 thoughts on “Do Cardio Workouts Cause Muscle Loss?

  1. Wow Cathe! Thank you so much for this information. I have never replied on your website before, but this article got me so excited! It was so informative, and it makes a lot of sense. You answered a question for me which I have had for many many years! Now I can continue what I have been doing and do it with confidence! I really look forward to future articles in your newsletters. Thanks again!

  2. Great article! I have been trying to lose some stubburn body fat and I have heard that I should do more weight training and limit my cardio. Thanks for spelling it out and making it clear. This was very helpful.

  3. This was a great article. I’ve heard this before, and never really understood if/when cardio could cause muscle loss. I love these articles in the newsletter!!!!

  4. Another great article Cathe. I read about fitness and nutrition on many websites, but you are the only one I truly trust, and follow!. Thanks again!

  5. Thanks Cathe!! I loved this article, me and my husband have been trying to figure this out! We both love to strength train, and I love to run also and am trying to convince him to go with me and now I think he is convinced after reading this 😀 And I agree, you are the only one that I trust and follow too! P.S. Thanks for the new newsletters, I love the article and all you put in them now.

  6. Cathe, Thanks but iam still confused!!! HELP!!!! see I do alot of muscle training , eat the amt of calories needed including carbs, fats & proteins and i do alot of cardio (Love love doing all Your kickboxing cardio dvds) but how do i know whats too much?? Example, If i do an sts workout strength training dvd shoiuld i do a cardio one too on the same day or should i only do the cardio on the day off of my sts strength training??? Help cus i am gaining weight on the scale not seeing any any muscle defination nor inches loss Iam getting diacouraged and very very depressed, i feel as though Iam killing my self day in and day out with NO RESULTS!!! Help…. what should i do??? Thanks I hope to hear from u !! A big big fan, Jen (You are my idol…u amaze me:)) )

  7. Great article. I think a combination of cardio and strength training is a great approach to hitting all the different muscle groups.

  8. Look at shoulders, triceps, and body size in the picture above. One man might have bigger bis and forearms, but that man is also proportionately larger than the figure on the left, who has much better defined shoulders and triceps. See the problem through an artist’s eyes. While I’m not telling you to be content with what you have, I ~am~ telling you that there are some who might try to convince you that you have a God-given right to be a certain shape or size, which simply isn’t true. Exercise all the time, definitely – but find your own perfect shape.

  9. P.S.: My gut sticks out quite a bit. For a living, I deliver household appliances. Fitness folks generally laugh at me until I tell them that I’m a distance runner, or that my body fat is lower than 15%. Now… take a look at the “World’s Strongest Man” competitions, and see how many lean machines exist there. Don’t make fun of working folks that might seem a little “stout around the tum”. Odds are, we can hold our own.

  10. I do cardio classes at the gym 5-6 days a week. I do strenth training 2-3 days a week. Everything looks nice and proportional except my big belly. I’ve tried crunches, sit ups, leg lifts. Is there anything else that can be done for results? I do not eat junk food–no cake, candy, cookies, ice cream, etc for the past 8 months. I’ve cut back on fresh fruit because of the sugar. I have not lost any weight and have not lost the belly. On the positive side, I feel great, have good strength and stamina and good BP. Any suggestions?

  11. Thx. Ur article was really helpful. But I do hav 2 ask u something. After yrs of my weight yo-yo-ing I read a book last yr that really changed my perception on weightloss. I now focus more on beibg fit. Yet after losing 12kgs in 4 months approx with only aerobics classes my weight stopped dropping. After feeling stuck for a few months n my health also not being upto par I quit da class. I now cycle but wasn’t regular due to frequent trips I took. I had done weight training a few yrs ago but it didn’t help at all n then I started to feel a poking needle feeling in my heart. After seeing a doc I was told not to do weights anymore. I was 16 then n im 24 now. But weightloss is not happening as easily as it used to n im not that old that i should feel exhausted da way i do after a workout. After someone pointed out that weights r imp I started reading about it. Would u suggest I start weights again or just stick to cycling until at least I don’t hav a layer of fat on me? My weight was 85kgs n now is 73kgs.

  12. What about if you’re in Ketosis? What about if you’re doing Intermittent Fasting? What if you’re doing Keto & Intermittent Fasting at the same time? How does that affect cortisol levels?

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