According to a new study artificial sweeteners could increase hunger by making the brain believe we are starving:
Scientific American: How Artificial Sweeteners May Cause Us to Eat MoreA vast body of research suggests that sugar substitutes, despite having far fewer calories than sugar itself, can wreak various forms of metabolic havoc such as upping diabetes risk and - perhaps paradoxically - causing weight gain in the long term.
The actual studies referred to were done using flies and mice, so the relevance for humans is not certain. But it matches several studies on humans showing for example that women lose weight by giving up drinking diet sodas.
Does this make you motivated to throw out the diet soda (if you haven't already)?