EDIT: Here is what I believe to be a source of many such claims, a 2014 study suggesting that artificial sweeteners can alter gut flora in mice to lead them to being glucose intolerant: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4615743/
I'd be interested if there's anything more recent or conclusive about this - plenty of sensationalistic articles seem to be out there either extrapolating from or challenging this study, but I'm not able to deduce anything further.