The bloggers are working on designing such a study. The good news is that weight loss studies almost all show small effect sizes so if remove a chemical exposure factor is the solution you would expect the results to be pretty obvious. I mean, if getting rid of PFAS "prevents obesity" but a McDonalds ad undoes the effect, then it was probably the McDonalds ad causing the obesity, not the PFAS. The bad news is that preventing chemical exposure is really hard to do.