Originally Posted by teamawe

This isnt entirely true. If you are say a marathon runner, and you dont lift weights, then yes, you are going to develop your slow twitch fibers and your body will let the fast twitch atrophy. Over time your muscle will 'lean out'. However, a blanket statement that cardio is catabolic isnt valid. Cardio taken into anerobic levels is catabolic, but if kept within the aerobic zone your body will continue to burn glycogen just fine, sparing that muscle mass.