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Are environmental chemicals making us fat?

We are getting fatter. The World Health Organization estimates that a seventh of the world’s population is overweight and about 300 million people can now be classified as obese. What’s going on? The answer would appear to be pretty simple. We are eating more and exercising less.

We are getting fatter.  The World Health Organization estimates that a seventh of the world’s population is overweight and about 300 million people can now be classified as obese.  What’s going on?  The answer would appear to be pretty simple.  We are eating more and exercising less.  But that answer may be a tad too simple.  Some researchers maintain that our increased calorie intake and decreased calorie expenditure cannot account for the current “epidemic of obesity.”  We better have a look, they say, not only at the gluttonous amounts of processed foods we consume, but also at the packaging it comes in.

Until recently, such a notion would have seemed absurd.  But now some intriguing research suggests that exposure in the womb to environmental chemicals, such as some of the fluorinated compounds used in greaseproof packaging, can be a predisposing factor for obesity.  Of course, developing babies don’t order pizzas, but their carriers have been known to make the odd call.  And what mommy eats, embryo eats.  And if mommy consumes hormone-like chemicals, baby may pay the price.

The usual suspects here include chemicals in detergents (nonylphenol ethoxylates), pesticides (atrazine, DDT, lindane), flame retardants (polybrominated diphenyl ethers) antifouling paints (tributyltin) and compounds leaching from plastics (bisphenol A, phthalates).  Connecting these to weight gain may seem to be off the wall, but there may be something to the argument.  After all, hormones are commonly used to make cattle gain weight more quickly.  While evidence linking human weight gain to endocrine disruptors is very sketchy, some animal studies do point to that possibility.

Retha Newbold’s work at the U.S. National Institutes of Health is a case in point.  Her interest was in diethylstilbesterol (DES), the classic estrogen-like compound that was once used to prevent miscarriage in women, unfortunately with tragic consequences.  Their “DES daughters,” as they came to be called, had an increased risk of clear cell adenocarcinoma, a rare cancer.  Newbold was investigating how DES might interfere with hormone systems when she made a surprising discovery.  Injecting pregnant mice with tiny amounts of DES resulted in their offspring exhibiting unusual weight gain.  Although food consumption and activity levels in the exposed mice were no different than in controls, by sixteen weeks of age, they had 25% more body fat!

Diethylstilbesterol is not the only compound that has shown such an effect.  Other researchers have connected in utero exposure to bisphenol A, phthalates and perfluorooctanoic acid (PFOA) with weight gain in rodents.  Bruce Blumberg of the University of California actually coined a new term for such substances, calling them “obesogens.”  His original interest was in the hormone-like effects of tributyltin, a fungicide used in paints, especially those used to protect ships’ hulls from barnacles.  Blumberg was taken aback when female mollusks exposed to the chemical grew male sex organs.  Better see what is does to mice, he thought.  Well, when pregnant mice were treated with tributyltin, their offspring showed unusual weight gain.

But, as the common saying goes, humans are not giant rodents.  And obviously we can’t expose pregnant women to suspected endocrine disruptors.  So human evidence for the obesogen theory is hard to come by.  Surprisingly, some support comes from of all people, smokers.  Smoking generally is associated with weight loss, but when women puff away during pregnancy, (as unbelievable as that is) their offspring are twice as likely to be obese by the time they reach school age.  In animal models, prenatal exposure to nicotine produces a similar effect.

What about post-natal exposure?  Richard Stahlhut and colleagues at the University of Rochester have linked higher levels of phthalates found in men’s urine with more belly fat.  Previously, low levels of testosterone have been associated with abdominal obesity, and phthalates, at least in animal studies, depress testosterone.  So a connection between phthalates and jiggling bellies is plausible.  And the connection may not be limited to men.  Recently researchers at Mount Sinai School of Medicine in New York investigated exposure to phthalates by looking for the metabolites of these compounds in the urine of some four hundred girls in East Harlem.  The heaviest girls had the highest levels of phthalate metabolites.  Of course, such an association does not prove that phthalates are responsible for weight gain.  Maybe the heavier girls ate more high calorie processed foods, which would expose them to more phthalates from packaging.

Obesity is a complex phenomenon, with multiple confounding factors.  For example, sleep deprivation can lead to weight gain, and data show that during the past few decades the average daily sleep has decreased from nine to seven hours.  Smoking reduces weight and there are fewer people smoking.  Some drugs, especially antidepressants and antipsychotics produce weight gain, as do anti-diabetics and beta-blockers.  The age at which women have babies is also a factor.  Having an older mother is a risk factor for obesity, and since 1970 the average age of first pregnancy has increased by about two years.

Believe it or not, even air conditioning and heating may be connected to obesity.  Maintenance of body temperature requires energy expenditure, meaning that more calories are “burned” when we have to cope with high or low temperatures.  Interestingly, in the Southern U.S., which has an extremely high obesity rate, the percentage of homes with central air conditioning has increased from about 30 to 75% since 1980.  When people are comfortable, they eat more.

Taking all this into account, jumping on the endocrine-disruptor/weight gain bandwagon is premature.  When it comes to packaged food, it is more important to worry about how much of the food in the package we put into our mouth than about the trace amounts of chemicals the package puts into the food.  And until the laws of thermodynamics are repealed, the best way to lose weight is still the old fashioned way.  Eat less and exercise more.

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