I always thought that people who said even Diet soda could affect your diet were full of crap. That just didn't make sense to me, at all.
Now I'm working with an expert in this stuff for something I'm doing with my job and he says that acidic stuff (like diet soda) affects your body's pH level and ability to lose weight, and that you can even test your pH every day and try to bring it back into balance. Now I really want to find the pH tester thingies just because testing it daily would make me feel all official and sounds fun.
Here's the article about it, for anyone interested.