It's said that diet soda causes an insulin release. I have to wonder about that. It's reproducible in the lab, when certain cells are exposed to certain sweeteners. But that's a long way from reproducible in a human.
One thing seems clear, diet sodas are associated with weight gain.
Here is what I think: People that choose diet sodas already have weight gain issues and are simply avoiding real Coke.
I've switched to unsweet iced tea and notice no difference what so ever. Other than I miss the taste of diet coke.
As someone with Hashimoto's thyroid failure, I can say with authority that low thyroid issues will cause uncontrolled weight gain. Ever wonder why some people don't gain weight and some like me have a deep breath for breakfast and lunch and a Marlboro light for dinner and still don't lose weight.... (a little joke about the breath and cigarette diet, I've never smoked) It's this simple, for those with low metabolism problems that involve the thyroid, a concentration-camp diet is the only thing that works. Daily calorie intake needs to be below 800.