Yes, it's true. It's a scientifically proven fact that muscle proteins are broken down and used for energy during aerobic exercise. But don't worry, you are constantly breaking down and re building muscle tissue anyway. This process is called "protein turnover." Your body is constantly alternating back and forth between anabolic (building) and catabolic (breaking down) cycles. That's just a normal part of life. Your goal is simply to tip the scales slightly in favor of increasing the anabolic side and reducing the catabolic side just enough so you stay on the anabolic side and you gain or at least maintain muscle.

This fact of human physiology has often been taken out of context and used to scare people into not doing cardiovascular exercise for fear of losing muscle. When you fast overnight as you sleep, you lose muscle too, but that doesn't mean you should stop sleeping!

Sure, it's possible for you to lose muscle from doing too much cardio, but it's highly unlikely. Shying away from cardio completely because you think you'll lose muscle is a huge mistake. Only excessive amounts of cardio would cause you to lose muscle because over-training tips the scale towards the catabolic side. It's difficult to generalize and pinpoint one specific amount as too much, but I think it's safe to assume that just about anyone could do up to 45 -60 minutes of cardio a day, 6 to 7 days a week without losing any muscle - as long as the proper nutritional support is provided.

Trainer John Parillo has always been an advocate of lots of aerobics, even for his bodybuilder clients who are trying to gain muscle mass.

"Aerobics can enhance your recovery from weight training by promoting blood flow and oxygen transport to your muscles," says Parillo. "Aerobics forces oxygen through your body, increasing the number and size of your blood vessels. Blood vessels are the 'supply routes' that transport oxygen and nutrients to body tissues, including muscles, and carry waste products away for muscular growth, repair and recovery. The expansion of this circulatory network is called 'cardiovascular density.'"

So, according to Parillo, aerobics can actually enhance recovery from weight training and increase muscular growth by developing the circulatory pathways that provide nourishment to the muscles. Cardiovascular training is important for fat burning, for good health and for muscle-building.

Losing muscle has more to do with inadequate diet than with excessive aerobics. If you suspect you are losing muscle there are four likely causes:

1. You are not eating enough protein. Protein is the only nutrient that is actually used to build muscle. To stay anabolic you must eat five to six protein containing meals. Each meal should be spaced out approximately three hours apart. Research has proven that if you are physically active, you need a minimum of .8 grams to 1 gram of protein per pound of bodyweight.

2. Your carbohydrates are too low. Low carb diets are often used for fat loss, but it is a mistake to cut your carbs too drastically. Carbohydrates are protein-sparing, so even if you are eating large amounts of protein, you can still lose muscle if you your carbs are too low.

3. You are not eating enough calories to support muscle growth. This is the most common cause of muscle loss. When your calories are too low, your body goes into "starvation mode." Your metabolism slows down and your body actually burns muscle tissue to conserve energy. Muscle is metabolically active tissue, requiring a great deal of caloric energy just to maintain it. That's why your body will shed muscle if it thinks you are starving.

4. You are not training with weights. It is a common misconception that if you want to lose weight, you should start with cardio only and add the weights later - another big mistake! It is the weight training that keeps you from losing muscle while you are dieting.

You are much more likely to lose muscle from not eating enough than you are from doing too much cardio. All too often, people are afraid to eat a lot and do a lot of cardio at the same time. It doesn't seem to make sense. Logically, it seems like the two would cancel each other out - but the opposite is true. Many people believe they must "starve" the fat by drastically lowering calories. Unfortunately, this approach can cause you to lose muscle along with the fat. The only way to maintain your lean mass while losing fat is to feed the muscles with plenty of nutritious calories and at the same time, burn the fat off with cardio.

Whether your goal is muscle development, fat loss or both, you should always include some form of cardiovascular activity as part of your training program. Unless you're doing some kind of ultra-endurance regimen, AEROBICS DOES NOT CAUSE MUSCLE LOSS, in fact it supports the pathways that help you build it!