The news has recently carried this story about a study claiming that people's obesity risk goes up for every diet soda consumed.
The researchers call it the Mad Hatter theory, referencing the part of Alice's Adventures in Wonderland when Alice, vexed because she was offered tea but given none, eventually grabs not just tea but also some bread and butter.
So it is with our bodies, researchers theorize: Given a sweetish drink, but no calories, we go into search-obtain-and-eat mode. (Infant rats did this in an earlier study: Those given fake sweeteners craved calories more than those given real sugar).
Or perhaps the research has confused cause and effect: maybe people who are fat drink more diet soda because they drink more soda, and they're trying to control their weight.