The Athletic Foodie has a very good article about diet sodas and artificial sweeteners I think you should take a peek at. The most interesting part (to me anyway) was this:
Diet sodas (and any food containing artificial sweeteners) trick the body into thinking that it is consuming a sugary food. This causes a spike in insulin, which decreases the body’s ability to burn fat effectively. The increase in insulin causes an increase in storage of fat. After the body takes glucose that is in the bloodstream, the person feels lethargic and tends to overeat. This is where the weight gain is seen. People who consume artificial sweeteners also develop a preference for sweet foods, overall leading to an increase in caloric consumption from foods like cookies, cakes, and sugary cereals.
Makes sense, doesn’t it?