Answer: Two main factors will contribute to you burning muscle. First, if your calorie intake is too low, then your body has no choice but to break down muscle for energy. A typical weight loss plan with no exercise will allow about 26% of the weight lost to come from muscle. Adding exercise and not cutting calories too low can help prevent this loss of muscle. Second, you will use more muscle the longer your exercise session lasts. In continuous activity lasting over an hour, muscle can supply up to 10% of the energy used.
So to limit muscle loss, follow a sensible, reduced-calorie diet and maximize your cardio workouts by increasing the intensity, not just the time. Also, supplementing with a multi-vitamin and mineral and/or branched chain amino acids (BCAA), as well as proper pre and post workout eating can reduce muscle breakdown--and even increase muscle building--while trying to lose fat.