Related To:  The Unfinished Mystery of the Shangri-La Diet and Missed Distinctions 

Megan McArdles blogs an interview with Paul Campos, author of The Obesity Myth.  I'll let anyone who is interest read the whole thing, but here's some interesting excerpts:

I mean, there's no better established empirical proposition in medical science that we don't know how to make people thinner. But apparently this proposition is too disturbing to consider, even though it's about as well established as that cigarettes cause lung cancer. So all these proposals about improving public health by making people thinner are completely crazy. They are as non-sensical as anything being proposed by public officials in our culture right now, which is saying something. 

It's conceivable that through some massive policy interventions you might be able to reduce the population's average BMI from 27 to 25 or something like that. But what would be the point? There aren't any health differences to speak of for people between BMIs of about 20 and 35, so undertaking the public health equivalent of the Apollo program to reduce the populace's average BMI by a unit or two (and again I will emphasize that we don't actually know if we could do even that) is an incredible waste of public health resources

and

Megan: An economist recently pointed out that we don't encourage people to move to the country, even though rural people live more than three years longer than urban people, and the diffefence in their healthy life expectancy is even more outsized. Nor do we encourage people to find Jesus or get married. We target "unhealthy" behaviors that are already stigmatized.

Paul: Right, as Mary Douglas the anthropologist has pointed out, we focus on risks not on the basis of "rational" cost-benefit analysis, but because of the symbolic work focusing on those risks does -- most particularly signalling disapproval of certain groups and behaviors. In this culture fatness is a metaphor for poverty, lack of self-control, and other stuff that freaks out the new Puritans all across the ideological spectrum, which is why the war on fat is so ferocious -- it appeals very strongly to both the right and the left, for related if different reasons.