Thursday, August 20, 2009

Nudge the fat; satiety & the implicit mind   posted by Razib @ 8/20/2009 09:58:00 PM

Megan McArdle has has been talking about the high heritability of BMI again. I have expressed concern about her putting the high heritability numbers out there when it comes to its relevance for public policy, though I do tend to agree with her general stance that glib assertions about the importance of will-power are probably non-starters. And, rather than point to arguments such as "I have a slow metabolism," it is probably more critical emphasize the complexity of the chain of events and framing of how we make decisions, much of which occurs "under the hood" and outside the purview of conscious explicit control. Interestingly, the reality that choice is highly conditioned by details of our environment combined with innate predispositions, and proximately is driven by many implicit factors, has pushed me in a less libertarian direction.

In any case, the whole discussion got me interested in the topic of obesity & heritability, and I found this review, Human Obesity: A Heritable Neurobehavioral Disorder That Is Highly Sensitive to Environmental Conditions. You can read the full text, it's Open Access now, but this part caught my attention:
...He hypothesizes that random natural variation in "hypothalamic energy balance set points" has occurred over millions of years of primate evolution. Whereas variants that would tend to produce a state of low energy stores would have been systematically selected against, at least in part because of their adverse impact of reproductive success, upward drifts in such set points would have been allowed to persist (rather than being positively selected for, as the “thrifty gene” hypothesis would have it). This upward drift would be particularly prominent because the formation of organized social groups and the discovery of fire, both of which occurred around 2,000,000 years ago, made our ancestors less susceptible to predation. Not particularly emphasized by Speakman, but likely to be important, is the probability that such natural tendencies toward an upward drift in adipose stores may rarely have actually manifested themselves as obesity because of the high energy cost of obtaining food during most of human evolution. It is only in the past 50 years or so, when for the first time in human history the majority of people in the developed and developing world can readily access sufficient daily calories to exceed the calories expended in acquiring them, that those with intrinsically higher set points have manifested their "obesity potential" on a grand scale. Unlike the “thrifty gene” hypothesis, this scenario provides a credible explanation for the fact that even in places where obesity is very common, a substantial proportion of the population remains lean.

This is an old hobby horse of mine: if you see a quantitative trait which can be conceived of as normally distributed with a high degree of heritability, such as body mass index, then its fitness implication can't have been too stark. In other words, if a very heritable trait still has a great deal of extant genetic variation, then it is either in transient, or, more likely the fitness implication of any particular trait value was low or there is balancing dynamics preserving the variance. Like IQ, body weight has been increasing over the past century. Many people think that they know the reason why this is occurring. If the reasons are ever established to a high degree of certitude, is it possible to reverse the slouch toward obesity without coercion?

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