So it seems that James Fallows and Marc Ambinder and I all agree that the increase in obesity in the American population is environmental, though they seem to think I disagree, despite my having made this point several times, and have thus spent a fair amount of time disproving a point no one has made. The very point of the height example offered in my first post was to note how environment interacts with genes.
It still remains to figure what the environmental change in America is that has caused this: whether the government is largely responsible, and regardless of that, whether the government can stop it.
As I've said elsewhere, I don't think the government is all that plausible as the primary source of the problem. Obesity is rising everywhere, even in poor countries. It seems to be rising fastest in the anglosphere, but then, most countries outside the anglosphere rely on self-reporting data, which produces lower estimates. Eyeballing it, people in other countries are a lot thinner. But there are also a lot more fat people in Europe than there used to be.
But leaving culpability aside, what can the government reasonably do to make us healthier? We could change our road building and build denser. But of course, as I pointed out elsewhere, while being rural is correlated with being fatter, it's also correlated with being healthier (though that advantage may be eroding). It's impossible to tease out the countervailing effects, so which should we do? Build up dense areas in which people will be thinner, but maybe sicker from the stress hormones of living in a noisier, more crowded area? This might be liking taking up smoking to lose weight.