There is however a link between drinking diet soda and obesity. By drinking or eating anything that is artificially sweetened you actually increase the craving for calories because the energy that the body is expecting (sweet taste) is not coming. I need to have a look but there have been some publications where it was found that drinking diet soda leads to bad diet habits. It is not the soda itself but the food you will eat extra. (Here is one I found really quickly - http://ajcn.nutrition.org/content/84/2/274.short - not sure it is the best)

Put it to the test and substituted diet coke with fruit juice. Lost about 5 kg in 2 weeks without doing anything else.