A few days ago I stumbled upon a graph of the obesity rate in the U.S..
It was steadily slightly increasing since the 1960s and then around 1980 (the data points are too sparse to exactly locate the point), there was an inflection point where it started to climb more rapidly.
Now, look at this graph of U.S. consumption of sugars. HCFS gets introduced in the 1970s, it's cheap, and so we started to eat more sugar -- increasing our per-capita consumption about 20 pounds a year.
Notice the similarity in the shapes of the graphs. Hmm.
That 20 pounds works out to extra 25 grams of sugar a day, an extra 100 calories from sugar alone. So sugar itself is only a fraction of how many calories we over-eat -- our caloric intake increased about 25% between 1970 and 2000. But there's a pretty clear link -- put sugar in any food, and we'll eat more of it!
Correlation is not causation, as they say; but it does jump up and down and point and say, "Look here! Look here!"
I don't know how much of a role subtle biochemical effects involving fructose might or might not play a role on top of increased caloric consumption; the research seems mixed, though I've admittedly only scratched the surface. But the simple fact that food got sweeter, we started eating more of it, and got fat, explains a lot.