wouldnt it be slander/defamation?
no, only if you said brand names, that might be a problem. Print out some scientific studies to back up your claims. You can find reports to back up almost anything.

If you do this, pick a GNC that is not in a mall or has a big parking lot or some shit. Find one next to a side walk and have a friend film the whole thing.In case you get your ass kicked.

Makes brochures that have quotes for the studies. Post vid on getbig

IMO