Aerobic exercise can burn muscle in some circumstances, depending on the volume and intensity of your training and your diet. Extensive aerobic exercise, such as marathon or triathlon training, is an extreme circumstance, and if you are training for an event like that, performance is your number one concern. Consult your physician before beginning any diet or exercise program.
Effects of Aerobic Exercise
Aerobic exercise primarily burns glycogen, or sugar, for fuel. Fat is utilized for energy in the process called lipolysis. While there are any number of equations as to optimal training range for your "fat burning zone," none is wholly accurate, as they do not take into account your diet or energy reserves. The level of your training does not matter; if you are bloated with carbohydrates, you are going to burn more sugar than fat.
De Novo Gluconeogensis
The term de novo gluconeogenesis is used to describe the breakdown of amino acids for energy. Amino acids are the building blocks of protein. To a degree, this happens all the time, but your goal should be to limit it. If you do not, you will lose muscle tissue. As an active person, you are going to have a greater degree of protein turnover secondary to muscle breakdown and repair and need more protein in your diet.
Hormones
Like all other forms of exercise, aerobic activity stimulates a hormonal response. Lengthy aerobic exercise can increase the output of thyroid and growth hormones, both of which can raise your metabolic rate, but the greater the intensity, the greater the release of growth hormone. Both hormones cause you to burn more fat and calories, but if you do not have enough of both, the increase in thyroid hormone output can also scavenge muscle tissue.
Avoiding Muscle Loss
One method to avoid muscle loss is to eat more, but loading up on pizza and beer might not be the best way to improve your physique. Time your calories for maximal recovery. Post workout is the ideal time to consume quickly digesting carbohydrates and proteins such as dextrose and whey. This not only helps you recover from training, it can increase your ability to build muscle. Given that nearly 35 percent of your muscles consist of the three branched-chain amino acids, valine, isoleucine and leucine, a BCAA supplement before training may help you recover and avoid muscle loss.
References
- "Journal of the American College of Nutrition"; Beyond the Zone: Protein Needs of Active Individuals; Peter W.R. Lemon; 2000
- "European Journal of Endocrinology"; Free Leptin Index and Thyroid Function in Male Highly Trained Athletes; G. Perseghin, et al.; December 2009
- "Growth Hormone & IGF Research"; Effect of High- and Low-intensity Exercise and Metabolic Acidosis on Levels of GH, LGF-I, LGFBP-3 and Cortisol; P. Wahl, et al.; October 2010
- "Journal of Applied Physiology"; Early Postexercise Muscle Glycogen Recovery Is Enhanced With a Carbohydrate-Protein Supplement; John L. Ivy, et al.; October 2002
- "The Journal of Sports Medicine and Physical Fitness"; Branched-chain Amino Acids Supplementation Enhances Exercise Capacity and Lipid Oxidation During Endurance Exercise After Muscle Glycogen Depletion; A.B. Gualano, et al.; March 2011
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