Do Cardio Workouts Cause Muscle Loss?


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


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