if you are truly doing a water fast you want to avoid EVERYTHING aside from water
you can do different types of fast where you add electrolytes, that would be the next step
but to truly get the health benefits you don't want to be taking stimulants doing your fast, one of the points of a fast is giving your body a break. that won't happen when you are using caffeine obviously right? but again there are different types of fasts.. some people with water fast and still drink coffee and tea with stevia.