I hear from a lot of people that they think doing cardio “burns” muscle.

And as a result, avoid adding cardio to their workout regimens because they would rather add size and strength.

But does this logic hold up?

In this article, I go over cardiovascular exercise’s effect on muscle gain and if there is any merit to those who decide to omit cardio altogether from their workout regimens.

An overview of how your body breaks down energy

Let me give you an overview on your body’s sources of energy during exercise to make it clearer to understand.

When you exercise, your body uses carbohydrates and fat as primary fuel sources. Fat is the main source of energy during low-intensity exercise, such as walking or cycling, where the duration of exercise is greater.

Carbohydrates are the main source of fuel during short, intense bouts of energy.

Your body uses carbohydrates in the form of glycogen, which is the storage form of glucose in the body.

This even includes your brain.

The more intense your exercise, the more glycogen you use. Thus, to improve your performance, it is best to consume carbohydrates before high-volume training.

Your body uses fat as a constant source of energy and is used in all forms of exercise.

To achieve a state of burning fuel as a primary energy source, carbohydrate intake should be less. This is the idea behind low carbohydrate diets, also known as ketosis.

Ketosis, a popular new diet, is the burning of fat molecules called ketones.

Although ketosis will allow for greater burning of fat stores, your energy will be less and your performance may suffer.

How Would Cardio Burn Muscle?

Your body will only burn muscle if you are not consuming a sufficient amount of calories.

So the idea that cardio “burns” muscle is based on the number of calories you are consuming.

As a result, if you don’t consume enough calories, your body will start to eat your muscles for energy.

Your fat stores are your body’s last resort for energy, but if your body fat stores are really low and you are not consuming enough calories, your body will resort to cannibalizing your muscles.

What are objective measurements of low fat stores for men and women?

Normally, for guys, this would be about 3-4% and for women, anything under 10%.

If these people with super low body fat percentage were to perform excessive amounts of cardiovascular exercises without consuming a sufficient amount of calories, they will most likely start to burn muscle.

However, for most of us, this is nothing to worry about.

Your body burns muscle only as a last resort, specifically as a means of survival.

Consuming carbohydrates before a cardio workout

It is important to consume a sufficient amount of carbohydrates before and during exercise to give your body the fuel it needs so that it doesn’t enter a catabolic state.

There is a higher possibility of cardio burning muscle during crash diets.

Crash diets are extreme diets that involve consuming a dangerously low amount of calories for rapid, temporary weight loss.

During these diets, since calorie intake is very low, so is protein intake. Without protein in the diet, your body has no way of attaining essential amino acids.

And without essential amino acids, your body will have no choice but to break down your own muscle to get them.

Doing cardio along with a crash diet will further increase the loss of your muscle mass.

Is it the cardio that burns muscle?

No, it is not necessarily the cardio that is burning the muscle. Rather, it is the lack of calories, dangerously low body fat percentage, and low protein intake that is.

Proper caloric intake is key to ensure a lack of muscle loss.

After How Long Does Cardio Burn Muscle and Enter a Catabolic State?

In the fitness world, cardio performed at medium to high intensity for OVER an hour is seen as the marker for your body to enter the muscle-burning state.

Try to keep your cardio within the hour and it will be very safe that your body will not lose your lean muscle mass.

What Can I do to Prevent Muscle Loss if I Perform Cardio for More Than an Hour?

There are several things that you could do.

For one, I would make sure to consume 1-2 servings of complex carbohydrates 2 hours before you exercise to ensure that your carbohydrate energy stores are at full capacity when you exercise.

Examples of a good pre-exercise meal would consist of something along the lines of 2 pieces of whole-wheat toast and a banana.

Secondly, during the long exercise session, I would make sure to consume a simple carbohydrate such as a protein bar, sports drink, or energy gel.

Consuming one of these simple carbohydrates will provide an instant energy source for your body and will help prevent your body from “hitting the wall” and entering a catabolic state.

Does Cardio Burn Muscle if I just do Cardio-Based Lifts?

Absolutely not! Cardio, if done correctly and in moderation, may actually help gain muscle.

One way it does this is via DOMS (delayed onset muscle soreness) where your body will become sore after an intense workout as the body recovers and grows.

Another way cardio helps gain muscle is by getting you in better shape for your weight lifting workouts.

Cardio exercise actually helps to build muscle by enhancing recovery from weight training by promoting blood flow and oxygen transport to your muscles.

A lot of weight lifting workouts require you to be in good shape to maintain a high intensity throughout your workout.

If you are not in good enough shape, then you will simply be unable to complete the workout and you will not get the muscle gains you could have gotten had you been in better cardiovascular shape.

Studies To Back It Up

A study published in the  Journal of The American College Of Nutrition showed that cardio performed on a low-calorie diet may result in lean muscle loss.

The subjects only consumed 800 calories daily while intensely exercising for an hour a day 4 days a week. This is equivalent to performing cardio on a crash diet. As shown, it may result in lean muscle loss.

In another study conducted in 2009, the trans-Europe foot race was monitored closely to see the effect that endurance running has on the body.

The race spans over 64 days accumulating 2800 miles (43 miles per day!)!!!

That makes me tired just thinking about that, but the study showed some interesting results.

As expected, the runners lost a lot of fat, but their muscle mass showed some interesting insight.

The runners only lost muscle mass in their legs and not in their upper body. This means that they lost muscle only as a product of overtraining the leg muscles and not from the cardio itself.

In conclusion…

In summary, cardio will burn muscle if you are severely overtraining or you are not consuming enough calories.

So next time someone says they’re not doing cardio because they’ll lose all their muscle, you can tell them they may be wrong!

Daniel Maman

Daniel Maman's two biggest passions in life are health and fitness. He is a certified personal trainer, has a degree in exercise science, and spends his free time keeping up to date with the latest research. In his free time, you can find him playing basketball, bouldering, and spoiling his dog, Luna.

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