Does Cardio Cause Muscle Loss?

It is a physiological fact that muscle proteins are broken down and is used for energy during aerobic exercise. This process is known as "protein turnover." In a little more detail explanation, protein turnover is the balance between protein synthesis and protein degradation. More synthesis than breakdown indicates an anabolic state that builds lean tissues, more breakdown than synthesis suggests a catabolic state that burns lean tissues.

The human physiology fact of muscle metabolic is complicated to understand, and the average person often gets confused.  As such, this prevents people from not doing aerobic exercise for fear of losing muscle. It is highly unlikely to lose muscle by doing too much cardio workout even you do muscle lose some because over-training leans towards the catabolic side. You can do cardio training for 45 -60 minutes say 3 to 4 days a week without muscle loss as long as you incorporate proper nutritional support. Aerobics exercises is not a bad thing as you can enhance recovery from weight training and increase muscle growth by developing the circulatory pathways that provide nourishment to the muscles if you do it. Cardiovascular exercise is also important for fat burning, for good health as well as for building muscles.

If you are losing muscles, it has more to do with inadequate diet and not with extreme aerobics. If you suspect you are losing muscles here are 4 likely causes:

1. You are not eating enough protein. Protein is the only nutrient that is used to build muscle. To stay on the anabolic side, you have to eat five to six meals that contain protein. Make sure each meal are spaced out three hours apart. The recommended daily allowance (RDA) adult recommendation for protein is .8 gm ∙ Kg-1 ∙ d-1 and is met by a diet of having 12% of its energy as protein. 

Here’s an example, the RDA for energy for a 19 to 24 year old male weighing 72 kg is about 2.900 kcal/day. If 12% of that is protein, then 384 are taken in as protein (12% of 2,400 kcal). At 4 kcal/g, this person would consume approximately 87 g of protein per day, representing 1.2. g ∙ Kg-1 ∙ d-1 (87g/70 kg or 50% more than the RDA standard. Sounds a bit confusing but try to calculate to determine your own protein intake for practice. Keep in mind that physically active individuals need more protein than non-active people.

2. Your carbohydrate intake is too low. Low carb diets are often for fat loss, but your carbs intake you should not be cut too drastically. Carbohydrates are what we called protein-sparing. Even if you are consuming large amounts of protein, you can still lose muscle, if your carbohydrates are too small.

3. You are not eating enough calories to help your muscles grow ----- this is the most common cause of muscle loss. When you eat fewer calories, your body goes into a "starvation mode," which in turn slows down your metabolism. This actually causes you to burn muscle tissue to save energy. On a human physiology note, our muscle is metabolically active tissue that requires a great deal of caloric energy for maintenance. As such, this is why our body sheds muscle if it thinks we are hungry.

4. You are not adding weight training to your fitness regiment. Weight or strength training and a good diet plan are what keeps you from losing muscle, so it’s a good idea to include it. Strength training can burn or just as much, if not more, fat than cardiovascular exercises.
Although cardio exercises burn calories as well as fat, strength training when done with high repetition causes a physiological phenomena known as Excess Post-Exercise Oxygen Consumption (EPOC). EPOC is a term for saying how long your metabolism is elevated after exercise.

Studies show that strength training you can elevate your EPOC or metabolism for up to more than 38 hours after working out. This means that you continue to burn calories long after strength training, whereas once you stop cardio, the calorie burning stops as well.

Whether it is your fitness goal is to build muscles, lose fat or both, add aerobic activities as part of your training programme. Unless you are doing some kind of extreme-endurance regimen, aerobic exercises do not result in muscle loss, but it supports the pathways that help you build muscles. 

References: 

Powers, S. K., & Howley, E. T. (2007). Exercise physiology: Theory and application to fitness and performance.

Schuenke, M. D., Mikat, R. P., & McBride, J. M. (2002). Effect of an acute period of resistance exercise on excess post-exercise oxygen consumption: implications for body mass management. European journal of applied physiology, 86(5), 411-417.

By Sherldine Tomlinson, MSc, R. Kin