There is some interesting research out there indicating that drinking artificially sweetened foods actually causes weight gain, at least in rats.
The rats that were given artificially sweetened food tended to eat more of other high calorie foods when they were available. The hypothesis explaining the data is that “artificial sweeteners condition the body to no longer associate sweetness with calories, thereby disrupting its ability to accurately assess caloric intake. This disruption… may lead to overeating.”
See this for the simple summary, or this for the full paper from a medical journal.
This could help explain the surge in obesity in this country. Drinking diet drinks to lose weight may be like using gasoline to put out a fire.