anthrax,
i am working on a thread about cardio and muscle loss. so far it seems there is some evidence that cardio specifically running, can be beneficial to muscle growth and fat loss. i'm researching on PubMed mostly.
most people think that you burn muscle when you perform cardio. the simple fact is you use what is stored in the muscle cell to create immediate energy. you can replace what is striped from the muscle cells after performing cardio (aerobic) exercise with little to no effect on the amount of muscle mass.
The fact that most people lose "size" while doing cardio is because they are burning more calories resulting in weight loss. the reason we lose strength and size (muscle) is not because of the exercise. its because of the calorie deficit which in-it-self is catabolic.
aerobic exercise can do the following:
increase fat oxidation
improve insulin sensitivity
increase levels of gh, tsh, prostagladins, and numerous others
raise resting metabolism
improve HDL/LDL lipids
improved blood flow
and tons of others