This is an interesting article on the link between diet drinks and depression: http://www.foxnews.com/health/2013/01/08/drinking-soda-linked-to-depression/.  The study did not prove any cause and effect, just a correlation.  But I'd be curious to know which came first.  The depression or the diet soda.  Are people who are depressed more likely to gravitate towards diet soda?  Or is it something else that leads people to drink a lot of diet soda and the diet soda results in depression?  This particular study doesn't say.  But as a major diet-coke drinker myself, and a professional working with people struggling with food and body image issues, I'd like to know the answer to this question.  If there is one...  Many people who struggle with their weight also drink diet sodas, and have depression.  But many others drink regular sodas or no sodas at all and have depression.  This subject definitely deserves more research.