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This OnEarth column was written by Laura Fraser.

We all know that Americans — leading the way for the rest of the developed world — are getting fatter. We hear about the “obesity epidemic” on the TV news, with footage of people depicted from the waist down shuffling around in XXL sweatpants and carrying supersized sodas. The majority of us are overweight, complaining about how our jeans are getting tighter and wondering why, despite all our efforts to diet and go to the gym, the number on the scale keeps edging higher.

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: “We just aren’t evolving that fast,” one obesity expert noted.

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.

The idea of our surroundings contributing to weight gain is nothing new, of course. But past discussions about the role of the “environment” focused mostly on the fast-food culture that we live in, where highly processed, highly caloric foods are constantly available, eating times are chaotic, kids run around drinking sugar-saturated sodas all day, no one has time to cook, fruits and vegetables are scarce in low-income urban areas, a venti frappuccino has 760 calories, and muffins are the size of melons. Add to that our changing physical environment — the fact that everyone sits in front of computers every day, instead of working out or working on the farm — and the “calories in” excess of the weight equation seems obvious, and obesity over-determined.

But even allowing for such influences, something wasn’t adding up. There are plenty of people out there who eat well and exercise like Gwyneth Paltrow and still feel like their weight is out of control. Then there are those annoying people who eat everything they desire, never work out, and stay thin. There had to be more to it than calories. We know that hormones — the chemical messengers produced by our endocrine system to control things like blood pressure and insulin production — can fatten up animals for slaughter; that some drugs increase your weight; and that a change in hormones at midlife shifts where your fat is distributed. Researchers began to recognize that obesity is much more complicated than calories in and out, and that a lot of other mechanisms involving the hormonal regulatory system are involved in our bodies’ delicate weight balance.

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. “Overlooked in the obesity debate,” she wrote in 2002 in the Journal of Alternative and Complementary Medicine, “is that the earth’s environment has changed significantly during the last few decades because of the exponential production and usage of synthetic organic and inorganic chemicals.”

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.

Environmental researchers call these chemical calories “obesogens.” Bruce Blumberg, a University of California at Irvine professor of developmental and cell biology, studies the effects of endocrine disruptors on obesity in mice and sees clear differences between those who are exposed to them and those who aren’t. “Pretty much anyone who observes people knows that obesity is way more than eating and exercise,” says Blumberg. Instead, metabolism, appetite, and the number and size of fat cells you have come into play, all of which are affected by hormones, and therefore by hormone disruptors. Blumberg has shown that the organic pollutants tributyltin and triphenyltin derail the hormonal mechanisms that control the weight of mice. He’s found that when pregnant mice are fed a dose of organotins that is equivalent to normal human exposure to those chemicals, their offspring have 10 percent more fat cells than normal mice, the fat cells grow bigger than normal, and they end up, overall, 10 percent fatter than your average mouse.

Other compelling research that fat is not just about eating and exercise comes from studies that show that animals that live in human environments get fatter just by virtue of being around people. Researchers at the University of Alabama recently found that chimpanzees, macaques, mice, rats, dogs, cats, and other species that lived in proximity to humans got fatter than animals that didn’t live in an industrialized environment — even when their lab chow and exercise was highly controlled. The authors suggested that endocrine disruptors were one likely culprit in this cross-species obesity epidemic.

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. (The epigenome governs patterns of gene expression.) This reprogramming could explain how we are indeed evolving so fast.

Tang-Peronard says that it is impossible, now, to tease out how much of obesity is caused by chemicals, and how much by energy balance. They’re intertwined, anyway, with imbalances in appetite-regulating hormones like leptin and ghrelin causing us to want to eat more of the available food. “Endocrine disruptors may play

a significant role in obesity,” she says. But the research is in its infancy. She also points out that only a few of the tens of thousands of known environmental chemicals have been tested for their association with obesity. “We are only scratching the surface,” she says.

What to do about the problem of endocrine disruptors and obesity? It’s hard to say, given that virtually all humans have been exposed. Pediatrician Maida Galvez is involved in the Mt. Sinai “Growing Up Healthy” study of 330 children in East Harlem, monitoring their exposure to endocrine disruptors and their body weight. “Even if these chemicals play a small role in obesity, it’s a preventable exposure,” she says, explaining that if certain substances can be determined to have deleterious effects, we can avoid them at critical stages of development and ultimately replace them with safer alternatives.

For now, Galvez recommends that parents steer clear of Bisphenol-A — present in many plastic water and baby bottles, and in microwavable and dishwasher-safe food containers. (If you find a printed “7” on the bottom, get rid of it.) She also suggests avoiding shampoos, cosmetics, and soaps containing phthalates — up to 70 percent of “top-selling products,” according to a 2002 report by the Environmental Working Group. (Look for fragrance-free products, which are less likely to contain phthalates, or for anything from the Illumina Organics range or The Body Shop.). And, she says, eat fresh fruits and vegetables, instead of foods that are processed and/or packaged in plastic.

That’s one point on which traditional obesity researchers and environmental scientists agree: Eat plenty of fresh, organic vegetables. And while you’re at it, get out into the fresh air and get some exercise.