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.
Its Phosphoric Acid in Coke Zero and Dr Pepper zero (in the UK). Maybe its Mal...something in another county. I have some everyday (500mls), but I have my own theories on drinking or not drinking it, have only been on it a week will see if my weighloss slows, but I keep checking and im in Ketosis. Each to their own I suppose.
Im really tempted to ring head office and ask if there is any research backing this advice. x