I think the only sweet tastes allowed on LT are Vanilla Shake, Strawberry Shake & tablet form sweetner's, they don't allow granulated sweetner's and I think it's to do with the type of sugar.
I found the following on the internet:
"Coke Zero is sweetened with aspartame and acesulfame potassium (ace-k) and has zero calories. The only chemical difference between Coke Zero and Diet Coke is that Coke Zero has about half the aspartame but has more ace-k".
I've heard aspartame is a carcinogen and I also heard that the over sweet taste (compared to the Vanilla and Strawberry shakes) of these diet drinks stimulate the stomach acid as a result of this you feel more hungry and therefore harder to control your cravings.
This is what happens to me anyway when I've deviated off the LT program in anyway in the past. (Same goes for salt, which I was totally addicted to hence high blood pressure).
Suse
It's the only thing I don't like about the diet, as I avoid Aspartame usually, but prepared to take the risk to get the weight off as think the weight is more of a risk to me. 
