Being overweight may not be just the result of personal choices about what you eat, combined with inactivity. It could even be exposure to an array of chemicals while in the womb.

Pollutants acting on genes in the developing fetus and newborn are believed to turn more precursor cells into fat cells, which stay with you for life. And they may alter metabolic rate, so that the body hoards calories rather than burning them, writes Sharon Begley in a recent issue of the Newsweek.

In 2002, an unknown Scottish academic published a paper about the link between obesity and synthetic chemicals.

In Japan, scientists were finding that bisphenol A (a chemical compound used to make plastic drinking bottles and baby bottles, among other things) pushed certain cells to become fat cells in experiments performed in the lab and also acelerated the growth of existing fat cells. If their results held true outside the lab in people, it would mean that BPA, and potentially other synthetic chemicals, were in fact contributing to obesity.

In 2006, Bruce Bloomberg at the University of California, Irvine exposed pregnant mice to a chemical called tributyltin, which is found in marine paints and plastics and often ends up in people through drinking water.

The tributyltin activated a receptor called PPAR gamma, which acts like a switch for cells' fate: in one position it allows cells to remain fibroblasts, in another it guides them to become fat cells. (It is because the diabetes drugs Actos and Avandia activate PPAR gamma that one of their major side effects is obesity.) The effect was so strong and so reliable that Blumberg thought compounds that reprogram cells' fate like this deserved a name of their own: obesogens.