The real issue here is over-training and/or dieting. Over-training occurs when one performs physical activity at frequencies, intensities, and/or durations that overly taxes the body's ability to recover. Dieting in this context means caloric reduction. Over-training and/or dieting are not conducive for muscle anabolism (growth). Rather, muscle breaks down (catabolism) as the body resorts to its protein stores for much-needed energy.
So the key here is to perform cardio exercise in moderation and eat plenty of carbs and protein to spare muscle tissue. Moderation means three to five days per week at 65-85% MHR from 20 to 45 minutes. Recommendations for how much in the way of carbs and protein one should eat to preserve muscle mass can be found in the nutrition section of this blog.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."