To put it another way: I have NEVER had a BMI above the normal range. How much more awesome am I than you? 30%? After all, you have to work at it. My willpower is apparently 100% natural.
I fearlessly predict that more than one person will respond with some variation on "there were no fat people in concentration camps/but I told you, I totally lost 20 pounds last year by taking up marathon running!" Yes, we could solve America's obesity problem by putting everyone in the country on sawdust bread and cabbage soup. We could also just shoot anyone whose BMI is over 28. Are these good solutions? Because short of that, we don't have much.
4. Then we need to intervene with the kids, so they don't get fat in the first place
Schools have tried this; so far, it's no more successful than adult interventions. You can get a very small effect over the short term, but eventually, the kids start eating again. Yes, school meals are crap. I assure you, they were also crap thirty years ago, and sixty years ago. Yes, P/E has been cut in some schools, but there's little evidence that exercising makes you lose weight by itself. Unless you control their access to food completely--and you can't--those kinds of environmental interventions don't work.
5. So why is America getting fatter, Miss Smartypants?
Some combination of the following:
1. Hyperpalatability of food: the Seth Roberts/chain restaurant haters hypthesis. The processors have perfected combinations of fat, salt and sugar that addict us, causing us to eventually swell up like a balloon.
But French restaurants have been doing this for over a century, and for most of that century, thinness has been inversely correlated with poverty. Your body doesn't care whether it gets its fat and sugar from a Ho-Ho or a Chocolate Eclair.
2. Increasing prevalance of corn in the supply chain: the Michael Pollan/Cato hypothesis. Maybe: corn doesn't seem to be very good for you. But I'm skeptical of monocausal hypotheses.
3. Calories are getting cheaper. Self explanatory. In my view, the dominant reason. People eat more calories because they like it, and can afford to.
4. Animal fat. Eh, maybe. We sure eat a lot more of it than we used to. But we eat a lot more of everything. And without controlling for socioeconomic status, it's hard to tell whether vegetarians are thinner.
5. Larger portions. Special case of "calories are getting cheaper". I think it's less persuasive than many people think. It's true people will eat a great deal at a sitting if you give them a great deal. But if people were so easy to fool, long term, about their caloric intake, we'd all weigh eight zillion pounds.
As I pointed out elsewhere, a simple error of 50 calories a day--half a slice of Pepperidge Farm All-Natural Whole Wheat Bread--would make us gain five pounds a year apiece. Given inherent calculation error, no one is watching their calories this carefully. Our appetites are doing the work for us. Maybe you eat an extra 2,000 stealth calories at dinner, but you're not so hungry the next day. Conversely, try dining on Macaroni Grill's new 390 calorie scallop salad. Unless you're on a permanent diet, I bet you feel peckish before bed.