"So, kids, in 2020 it was discovered that X was making people fat. Before that, nobody knew that it had any adverse effects. In fact, they used it almost everywhere, from X, Y and Z to J."
Basically, they'll be talking about X the same way we talk about X-rays or DDT or whatnot.
Whatever is making everybody huge can be traced to a change in the 1980s. That is a useful data point in that it rules out a lot of things. It's not exercise. It's probably not starch.
Personally I suspect it's a mix of telling everybody to cut out saturated fat and the increased prevalence of fructose.
Here are some actual numbers: http://en.wikipedia.org/wiki/File:USObesityRate1960-2004.gif What we see is a linear increase starting around 1980, where before then rates appeared to be constant.
Before 1980, 1/3 of Americans were overweight. Every overweight person has the same metabolic problem as the obese, just to a lesser extent. The fact that 1/3 of Americans are now obese 20 years later is what I would expect due to a gradual decline of American health that hit a tipping point for obesity around 1980.
Fructose found in fruit is indeed no different. But it is in fruit, bound up with fibre, protein, and many enzymes. In HFCS it's just fructose.
The delivery mechanism is more important here than the lab breakdown.
Modern fruit (due to selective breeding) is ultra sweet with what I can assume were pretty bland flavored fruit back in the day. Natures Candy.
My experience is that supermarket varieties of fruits are bigger and look better than local, traditional produce, or are more practical in other ways (thinner skin, less or no seeds) but are often insipid in comparison.
On the other hand (and this is just speculation), in the northern hemisphere most fruit is naturally available in the fall season. Fattening prior to winter was probably a desirable effect.
That's roughly equivalent to 7 apples, every day of the year. That's way, way above our actual fruit consumption levels.