Enzo,
Ok, it's my turn to be confused. You want to lose fat and gain muscle? This is an accumulation of reading and trying to help with a wide number of things. Here's the deal if I understand correctly. This couldn't be better for you. When people who are skinny start working out they usually drop weight for some time as they strenghten muscles with no visable gain in bulk. But, if you have some body fat, no matter where, you can make the conversion of fat to bulk muscle. Oh, yah, this works when you are 18. Not so much when you are 30 an onward.
Join a gym or take up long distance swimming and find all the "gain weight you 98lb weakling" protein powders, brewers yeast and such you can. Start with a 110lb weight set and couple of barbells and pump iron dude. All any weight reducing potion or pills you might take is going to stress your body like you have never felt before. You are already running on adenaline with CF, you are eighteen and have so many issues that have to put you in more stress. A wad of stimulants, and God forbid, the only way to burn fat artificially is to ignite all of your cellular metabolism with stimulant or the effect on your body of a stimulant, it nets out the same. Every brain cell is going to amped up into a metaboism and it thinks it is fat anyway so...... Please walk away from this idea. Maybe consider another low level test with creatine, having your doctor monitor it closely. It legitimately ignites your celluar furnaces and should then be discontinued once you are pumping and getting some sweat for it. I also respect your discontinuing it as you have so just consider it.
LL
