The idea of a "diet" coke was to cater to the needs of weight conscious people. It has hardly any calories in it and weight gain is not an issue with diet coke. However, there is some ongoing debate as to the safety of artificial sweeteners like Aspartame which is an important ingredient in diet coke. I would prefer not to drink coke at all or drink normal coke only if given a choice between the two. If you are active and burn enough calories by regular exercises you needn't worry about the sugar content in normal coke. Good Luck!