My CDC said no about Coke Zero. Although it doesn't contain citric acid like other diet drinks, it does contain forms of sugars / sweetners that may knock you out of it. That being said, I had a huge coke zero cup last time I was on Cambridge at the cinema once, and it didn't affect me, but I was like 10 weeks into 100% SS and therefore I think it would have taken a lot to knock me out of ketosis.