I asked Dr., Professor and Chair of the Department of Health and Exercise Science at the College of New Jersey, about the theory that sports supplements are a gateway to anabolic steroid use. He replied in an email:
I do not believe that this is the case. There really isn’t any documented evidence to support such a claim. Although all anabolic steroid users I would venture to guess use sport supplements – I do not necessarily support the hypothesis that increasing supplement use would increase anabolic steroid use.