Genetic basis for obesity

Olivia Judson’s blog on the New York Times today covers a topic that I have been very interested in the last five years.  I really liked her post because she is one of the few people in the popular press that has questioned the thrifty gene hypothesis for the recent obesity epidemic.  This theory proposed nearly 50 years ago is that we evolved during a time when food was scarce and famines were prevalent  so that we are genetically optimized to pack on as much weight as we can when food is available.  Hence, we are ill adapted to the modern age of plenty and as a result we are getting fat (as we should be).

The most prominent critic of the thrifty gene hypothesis is John Speakman, who proposes the “drifty gene” hypothesis where we are actually optimized to stay lean and regulate our food intake and it is only because evolutionary pressures to remain fit and slim have been reduced in recent times that we have genetically drifted from that genotype.  He has a lot of compelling arguments including the fact that people really don’t die very much during famines and  that the thrifty gene hypothesis doesn’t explain why 30% of the population is immune to the obesity epidemic.  I would also add that in all the attempts at finding genes related to obesity, which is believed to be highly heritable, the only genes that have been found thus far only explain a minuscule fraction of the variance.  For a nice summary of the results see here.

Judson proposes a slightly different theory for the obesity epidemic.  She writes

The thrifty gene hypothesis is contentious, and clear evidence for it is lacking. It isn’t known, for instance, whether fat people survive famine better than thin people (though it seems likely that they would). However, the logic of the argument seems to me to be reasonable.

But the experiments on moths suggest that a tendency for obesity could also arise without famine. In those experiments, animals exposed to an energy-rich diet quickly evolved to become immune to the excess; conversely, animals that were more prone to storing fat were those with no history of abundance in their family tree.

To me, this suggests that a tendency for obesity needn’t require that someone’s ancestors regularly went hungry; it could just mean that they didn’t gorge on doughnuts and cakes. Under this scenario, the fattest among us may not be those whose ancestors had the most experience of famine, but those with the least experience of excess.

References:

Speakman, J. R. 2008. “Thrifty genes for obesity, an attractive but flawed idea, and an alternative perspective: the ‘drifty gene’ hypothesis.” International Journal of Obesity 32: 1611-1617.