© ISTOCK.COM/WRAGGWithin the past 40 years, obesity has become more common in virtually all age groups in the U.S., and more recently, throughout much of the world. It has become clear, through the calculation of concordance rates in twin-pair and adoption studies, that 25–50 percent of the risk of obesity is heritable, the extent varying within specific environments. And cogent arguments can be made regarding the evolutionary enrichment of our genomes with alleles favoring the conservation of calories.
Body fat, which provides a calorically dense, disposable energy source, is critical to the functional integrity of the reproductive system and to survival in circumstances of caloric deprivation. Studies of weight perturbation in animals and humans indicate that the loss of body fat is defended against more vigorously than its gain. This makes sense if one assumes that we are the products of evolutionary processes designed to optimize survival in circumstances of limited food and to key reproduction to environmental circumstance. And this model is certainly consistent with the observation that humans more easily gain weight than lose it and sustain that loss.
But human genetic makeup has not changed sufficiently over the past several decades, so to account for recent major phenotypic shifts something ...
