Diet coke contains citric acid which knocks you out of ketosis. Coke Zero contains Malic acid which shouldn't knock you out of ketosis, but can cause craving to fire up in some people. By CD guidelines it isn't allowed, but like sugar free chewing gum, it can be a real life saver when you need a change. Personally, I drink the odd can of Coke Zero and it causes me no problems. In moderation, I see no problem, just make sure you drink it on top of your 2.25 litre water allowance, and not count it toward.