It's literally impossible to gain weight from drinking diet soda. It doesn't have food energy. It can't break thermodynamics. It can only be explained by other factors like people over compensating with other calorie consumption.

Could things such as chemical imbalance or just the chemicals themselves affect metabolism / digestion, causing weight gain?

That requires some good research to prove.

What is known is that suddenly cutting caffeine after large regular intake will cause weight gain.

On the other hand, I stopped drinking all diet sodas and just drank water / carbonated water the last year. Absolutely no difference. Didn't even notice the lack of caffeine going from 6 diet dr peppers per day down to 0.

Which means of you drank unsweetened caffeine you might lose weight.

You think? I'll be on the lookout for carbonated caffeine water.

If you switch to diet soda from regular soda because of weight issues or diabetes risk, that seems like a pretty serious self-selection bias for later weight issues or diabetes risk.

https://img.buzzfeed.com/buzzfeed-static/static/enhanced/web...

Number 1: humans are not mice, the research cited does not even suggest that mice gain weight or how large dose is required. (BTW circumventing paywalls sucks)

Number 2: major confounder is that people who eat unhealthy or are already obese drink those diet drinks. People who aren't drink normal ones. Needs interventional study not dumb epidemiology.

Number 3: see number 2.

These experiments are mainly just correlational, they do not control the usage of diet sodas. So, of course, people who drink diet sodas are going to be heavier: they're the ones trying to loose weight and hence the ones who will gain the most weight! There's no reason to drink diet soda if your body doesn't have any problems with weight.