ok, well i can't really give you much in the way of science to go with this one, but i can give you my experience of it. i used to smoke a lot of weed, and i was a lot slimmer then. i dont think thc directly causes loss of muscle mass, however, it does speed up your metabolism.now i know what you're thinking, thats a good thing, isnt it?
not really in this case, seeing as when you're stoned chances are you're not going to be arsed to eat enough and/or properly. i do miss having a nice smoke after work/training, but i feel a lot better in general for quitting weed. thing is, you get used to it so you wont know how much more energy you'll have until you kick it. that said, unless you're a contest bodybulider and your physique depends upon you putting very strict amounts of only certain chemicals into your body, there's nothing wrong with having a VERY occasional smoke. hope this helps, as a said, maybe someone will be able to come along and give us a bit of science regarding this matter.