In the McArdle vs. Frum diavlog I alluded to earlier there was a dispute centered around two seemingly contradictory results. First, the fact that heritability of obesity is rather high, in twins-separated-at-birth studies the correlation between monozygotic twins raised apart is on the order of ~0.75. And yet people tend to track the weight of their peer group. The causality here has to be teased apart of course, but consider this study, The presence of friends increases food intake in youth:

Design: Twenty-three overweight and 42 nonoverweight youths had the opportunity to play and eat with a friend (n = 26) or with an unfamiliar peer (n = 39). The dependent variables of interest were the amount of nutrient-dense and energy-dense foods children consumed and their total energy intake.

Results: Participants eating with a friend ate substantially more than did participants eating with an unfamiliar peer. Furthermore, overweight youth, but not nonoverweight youth, who ate with an overweight partner (friend or unfamiliar peer) consumed more food than did overweight participants who ate with a nonoverweight eating partner. Matching of intake was greater between friends than between unfamiliar peers.

Naturally twins raised apart were not placed into a "design" whereby they were forced to eat with strangers. Rather, they selected their peer groups. Heritability of many traits increases with age because individuals seek out particular environments which eventually dampen the "noise" which reduces the correlation between those with similar genetic propensities. Assortative friendship by weight then might result in amplifiers of mean deviation from phenotypic norms; that is, thin peer groups might model specific behaviors and apply certain pressures which differ greatly from overweight peer groups. Gene-environment correlation. So naturally in the interests of public health we need integration across weight classes....

Also see ScienceDaily.

Labels: Behavior Genetics, Origin of obesity

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?

Labels: Genetics, Origin of obesity

Obesity May Have Offered Edge Over TB:

Over the course of human evolution, people with excess stores of fat have been more likely to survive famines, many scientists believe, living on to pass their genes to the next generation.

But these days, obesity is thought to be harmful, leading to chronic inflammation and metabolic disorders that set the stage for heart disease. So what went awry? When did excess fat stop being a protective mechanism that assured survival and instead become a liability?

A provocative new hypothesis suggests that in some people, fat not only stores energy but also revs up the body's immune system. This subgroup may have enjoyed a survival advantage in the 1800s, when people were plagued by a disease that decimated Europe: tuberculosis.

The original paper is here. I'm skeptical, but I'd like people who know more about the history and distribution of tuberculosis to weigh in. My working assumption is that excess fat was helpful in most pre-modern contexts (i.e., female fertility) and obesity wasn't common and simply a modern overshoot.

Labels: Origin of obesity