I forget where I read it, but the reasoning I heard for it is that your body still thinks the zero calorie sweeteners are sugar, so it will store more calories from the food you're eating because your body is compensating for the lack of calories in the "sugar". I've also heard that the causation is reversed though, and that obese people are more likely to drink diet soda because it's a low effort way to try and lose weight. I don't know which is true though, so I just try to stick to water or fruit/vegetable juice if I want some more flavor.