Microevolutionary Hypothesis of the Obesity Epidemic

The obesity epidemic represents potentially the largest phenotypic change in Homo sapiens since the origin of the species. Despite obesity’s high heritability, a change in the gene pool has not generally been presumed as a potential cause of the obesity epidemic. Here we advance the hypothesis that a rapid change in the obesogenic gene pool has occurred second to the introduction of modern obstetrics dramatically altering evolutionary pressures on obesity - the microevolutionary hypothesis of the obesity epidemic. Obesity is known to increase childbirth related mortality several fold. Prior to modern obstetrics, childbirth related mortality occurred in over 10% of women. After modern obstetrics, this mortality reduced to a fraction of a percent, thereby lifting a strong negative selection pressure. Regression analysis of data for ∼ 190 countries was carried out to examine associations between 1990 maternal death rates (MDR) and current obesity rates. Multivariate regression showed MDR correlated more strongly with national obesity rates than GDP, calorie intake and physical inactivity. Analyses controlling for confounders via partial correlation show that MDR explains approximately 11% of the variability of obesity rate between nations. For nations with MDR above the median (>0.45%), MDR explains over 20% of obesity variance, while calorie intake, and physical inactivity show no association with obesity in these nations. The microevolutionary hypothesis offers a parsimonious explanation of the global nature of the obesity epidemic. Significance Statement Humans underwent a rapid increase in obesity in the 20 th century, and existing explanations for this trend are unsatisfactory. Here we present evidence that increases in obesity may be in large part attributable to microevolutionary changes brought about by dramatic reduction of childbirth mortality with the introduction of modern obstetrics. Given the higher relative risk of childbirth in women with obesity, obstetrics removed a strong negative selection pressure against obesity. This alteration would result in a rapid population-wide rise in obesity-promoting alleles. A cross-country analysis of earlier maternal death rates and obesity rate today found strong evidence supporting this hypothesis. These findings suggest recent medical intervention influenced the course of human evolution more profoundly than previously realized.