Eh it depends on the study. One study has shown that, while others have shown no change at all. Studies involved nutrition are extremely difficult because it's hard to control what people do with their diet and outside of their diet and their genetics. A diet coke or two a day won't kill you. Some people can eat perfectly and still get a stroke and die young while others can eat whatever they want, not exercise and smoke and live til 100 with no health problems. It's hard to control for genetics. Obviously water is better, but the caffeine in diet coke is probably worse for you than the aspartame