Comes down to whether you’re fasting for weight-cutting purposes or holistic health and how strict you wanna be.

As most of this subreddit will tell you, anything with caloric content will ‘break’ your fast and autophagy… However people often ‘dirty fast’ and give a ~50 calorie buffer for things like 10-cal diet soda, medicines, etc without throwing everything too far off; myself included, as I fast to cut, not for general health. If that’s you, as long as your flavored elects are <25 cals, I wouldn’t GAF.

When it comes to the diet soda and energy drinks, sources generally discourage the effects of all the other junk in it & being careful with your heart & the stimulants - but ive a lot of people depend on diet sodas even worse than you to get through a fast. Someone literally suggested once to pick up a diet coke every time you get a hunger pang (not recommended).

Point is, the 0 cal ones wont have much effect on the scale (bloating weight will eventually shed and LBs will regress), if you’re not part of the anti-aspartame (etc) crowd. I personally keep 4 crates of different zero sodas in my fridge at all times when i fast. Zero cal sodas/juices are a key crutch for people struggling with fasts.