Somone from Curves for Women tried to tell me that 40% of what I've lost over the past year has been muscle until I got on the weight machines and proved her wrong. The trainer pronounced my upper body strength better than most women, and my lower body strength above average for someone my age. Not bad for someone who's lost (supposedly) 25 lbs of muscle in the last year. It's metabolically inefficient for your body to consume it's own lean muscle mass in the presence of adequate dietary protein.