Is your shampoo making you fat?

We all know that Americans -- leading the way for the rest of the developed world -- are getting fatter. For years, the explanation for weight gain was straightforward: it was all about energy balance, or calories-in versus calories-out. This Gluttony and Sloth theory held that obesity simply came from overeating and underexercising, and the only debate was about dieting -- whether it was better to join the low-fat or the low-carb camp. Some scientists explored genetic differences associated with fat, but others said genes couldn't possibly explain the rate at which Americans were gaining weight:

Environmental scientists have long suggested that there were likely external factors at work, but until recently, the traditional obesity-research community rejected such claims. Now it seems that the tide is turning: This month's issue of Obesity Reviews features an extensive look at the accumulating body of research linking the environment with obesity.

Paula Baillie-Hamilton, an expert on metabolism and environmental toxins at Stirling University in Scotland, was among the first to make the link between the obesity epidemic and the increase in the chemicals in our lives. Exposure to those chemicals, said Baillie-Hamilton, can damage the body's natural weight-control mechanisms. She calls toxic chemicals that act as endocrine disruptors -- mimicking hormones, and blocking or exaggerating our natural hormonal responses -- "chemical calories," and those in question include Bisphenol A, phthalates, PCBs, persistant organic pollutants such as DDE, a breakdown product of the insecticide DDT, and pesticides containing tin compounds called organotins. Many studies have shown that endocrine disruptors have been linked to early puberty, impaired immune function, different types of cancer, birth deformities, and other diseases. Now obesity and metabolism are on that list.

For her article in the new Obesity Reviews, Jeanett Tang-Peronard, of the Institute of Preventive Medicine in Copenhagen, looked at some 450 studies on endocrine disruptors and obesity and found that nearly all of them showed a correlation between exposure to those chemicals -- particularly in utero and in early childhood, when hormonal mechanisms are vulnerable -- and an increase in body size. She says that in early life, chemicals seem to alter the epigenetic regulation of certain genes, disrupting the programming of hormonal signaling pathways that affect fat storage, fat distribution, and appetite.

What to do about the problem of endocrine disruptors and obesity?