I dunno, 1.23 increase in risk is tiny; seems likely that it's just lifestyle stuff (which you can try to control for a bit, but you can't know every single choice a diet soda drinker is making -- quite likely an overall less healthy lifestyle).
Compare this to e.g. 20x lung cancer incidence for smokers, where even w/ observational data, you can be pretty sure there's a causal relationship.
Maybe do some RCTs giving diet soda to animals or something and see what happens. And look into the chemistry to see what the mechanism might be...
(That said, probably prudent not to go too crazy with diet soda, or any other novel ingredients.)
Compare this to e.g. 20x lung cancer incidence for smokers, where even w/ observational data, you can be pretty sure there's a causal relationship.
Maybe do some RCTs giving diet soda to animals or something and see what happens. And look into the chemistry to see what the mechanism might be...
(That said, probably prudent not to go too crazy with diet soda, or any other novel ingredients.)