It's not so much that each CDC says something different as our guidelines are pretty clear... zero calorie drinks are no longer recommended. However, EACH PERSON is different and some people are not as ketosis sensitive as others and can, therefore, add little extras here and there with little or no effect whilst others are affected by the tiniest thing.
Trouble is how do you know which one you are until you try it and is it really worth it for a fizzy drink that's no good for you anyway when you can have sparkling water with a 1/2 tsp of water flavouring????