Parents feeding their kids fast food all the time, and not encouraging exercise, is undoubtedly the primary cause, but poor nutrition in school lunches almost certainly has some additional effect.
I ate so much sugar and fat back when I was in school, in retrospect I'm disgusted. It probably wasn't as much as kids today eat, but still way too much. I did exercise a lot though, and I've always been fairly thin. Still, I'm sure I was not very healthy back then if I'd had detailed tests done. I was probably pushing my luck with my glucose levels. Exercise can only do so much to negate the effects of bad diet.
Is there any compelling reason to compromise adult "health food" standards when constructing food plans for kids? No cholesterol, very little saturated/trans fat, and if they want sweet stuff, there's this stuff people used to eat called fruit! And there's yogurt! It's not the end of the world to eat doughnuts or a greaseburger once in a while, but if you get into a habit of eating healthy food, why would you want a burger or a doughnut... ever?
Maybe if the government wasn't subsidizing corn and soy feed for animals so that cheap low-quality meat was readily available for fast food, school meals, and microwave dinners, we would all be healthier. If the government wants to subsidize something, it should subsidize fresh produce. The status quo is nothing more than corporate welfare for the meat farming industry (and the corn industry which produces insane amounts of corn-derived sugar additives and corn-derived, hydrogenated oils).
