You can't win bro. I'm in the same situation now and it's always been that way with every girlfriend I eventually spilled the beans to. (Of course, it's hard to hide it when they see me at 235 for 3 months and then I bulk up to 260 in no time).
Really, for most girls, I don't think it is so much that they are really concerned about any harmful effects, because no matter how much proof you give them that juice is not that harmful if used properly (and the idiots are responsible for its bad rap), they never accept it. I think it's really about them feeling self-conscious (as fine as they may be, girls seem to always be comparing themselves to other girls) as we make drastic improvements. Especially if you not interested in competing or anything, they probably wonder if were looking to upgrade and dump 'em. Probably makes them feel insecure, I'd guess, whether they are conscious of it or not.
Basically, I just ignore them when they bitch about that shit. If she leaves becasue of that, then adios baby!