A Credit Suisse study on "the dietary impact of 'sugar and sweeteners' and their role in the ongoing health debate surrounding obesity and diabetes" shows a very interesting fact measuring Diet Coke consumption vs. education.

According to it, areas with more bachelor degrees consume higher levels of Diet Coke as a share of the general Coke intake.

Can we say for sure that smarter people drink Diet Coke? Probably not but the correlation is quite interesting.