Found this while doing some reading this morning:

Triathletes and marathoners must train their bodies to use fat instead of carbohydrates as fuel in long bouts of exercise, says Brendan Brazier, professional Ironman triathlete and author of "Thrive: The Vegan Nutrition Guide." When you do low-intensity exercise that lasts more than three hours, your body burns 20 percent carbohydrate, 70 percent fat and 10 percent protein. This compares to 90 percent carbohydrate, 3 percent protein and 7 percent fat with high intensity exercise lasting one hour or less. If you do moderate intensity exercise that lasts one to three hours, your body will burn 60 percent carbs, 35 percent fat and 5 percent protein, according to Brazier.

I am assuming ( I know, I know) this is how fuel conversion breaks down for non-fasted cardio. Does anyone know if this holds true with fasted cardio? I am trying to get a grip on how much lean mass I am burning because I have lost ~2 lbs of lbm over the last 3 weeks after making good gains the prior 3 months or so.