I'm not a nutritionist. My understanding, however, is that sugars are the culprit. When you look at what has changed over the years, that's the big one.

Sugar: The Bitter Truth http://www.youtube.com/watch?v=dBnniua6-oM

There are lots of other articles concerning this, but I felt this was the best one for pure information. He's not alone in discussing this, so I don't have to just take his word for it. But like I said, I'm not a professional in this area.