Is plastic making us fat? (Boston Globe)
Wednesday January 23rd, 2008
Being fat has long been seen as a personal problem, fixed only by struggling against the proliferation of fast food restaurants, unlucky genes, and a sedentary life.
But could something in the environment also be making Americans fat in epidemic numbers?
Animal studies in recent years raise the possibility that prenatal exposure to minuscule amounts of common chemicals - found in everything from baby bottles to toys - could predispose a body to a life of weight gain. The chemicals, known as endocrine disrupters, mimic natural hormones that help regulate, for example, how many fat cells a body makes and how much fat to store in them.
These findings have led some scientists to put forth a provocative argument: They say diet and too little exercise clearly are key reasons for the worldwide rise in obesity in the past 20 years, but they may not be the only ones. Food intake and exercise just haven't changed that much in that period, they argue. And while genetics obviously play a role - just think of someone you know who can eat three Big Macs a day and never gain an ounce - these researchers say it would be impossible to see such widespread genetic change in just two decades, giving them more reason to suspect the environment.
"This is a really new area . . . but from multiple labs on multiple levels we are getting preliminary data that all say the same thing: Chemicals can play a role," said Jerry Heindel, a program administrator for the National Institute of Environmental Health Sciences. "We know that nutrition and exercise are very, very important, but underlying that could be environmental exposures during development that alter your physiology, including how you respond to food and exercise."
Thousands of chemicals have come on the market in the past 30 years, and some of them are showing up in people's bodies in low levels. Scientists studying obesity are focusing on endocrine disrupters - which have already been linked to reproductive problems in animals and humans - because they have become so common in the environment and are known to affect fat cells.
One key researcher in the field, Bruce Blumberg of the University of California, Irvine, has even coined a new word for chemicals that can make you fat: Obesogens.
A recent US Centers for Disease Control and Prevention study found that about 93 percent of the US population had bisphenol A, a chemical that can be found in canned goods and in hard, clear plastic items such as baby bottles and hiking containers, in their body. A study at the University of Missouri-Columbia showed that mice fed bisphenol A during early development - at lower amounts than what would have resulted in the levels found in most people in the CDC study - become markedly more obese as adults than those that weren't fed the chemical. Tufts University scientists observed similar phenomenon in rats.
The chemical industry, however, disputes those studies and says dozens of others that examined bisphenol A showed no weight gain.
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"The scientific evidence shows that bisphenol A . . . does not have any effect on body weight," said Steven Hentges, executive director of the polycarbonate/BPA global group of the American Chemistry Council, which represents chemical manufacturers.
Bisphenol A is only one of the chemicals scientists are studying. Blumberg's lab has also studied tributyltin, an endocrine disrupter that is used as an antifungal agent in agriculture and in marine paints to keep ship hulls free of barnacles. Female mollusks exposed to the chemical were seen to grow male sex organs. Lab mice exposed to tiny levels of tributyltin during prenatal development became fatter adults than those not given the chemical.
"It predisposed them for life," said Blumberg.
These scientists are focusing on early development because it is a critical time for determining a baby's long-term health and weight. For example, studies show that babies born underweight are likely to be fatter later in life, possibly because undernourished fetuses learn to use fat cells more efficiently - and it gets embedded in their physiology. Researchers suspect the same thing may be taking place with chemical exposures.
Exposure "can be critical on the front end of one's life where the rest of your life's physiology is being programmed," said Frederick vom Saal, a biological scientist at the University of Missouri-Columbia who studies bisphenol A.
His lab is studying genes in the fat cells of mice to better understand why the animals became fatter when exposed to the chemical.
Growing up with more fat cells isn't necessarily a problem if you are running around a lot, says Pete Myers, chief scientist for Environmental Health Sciences, which publishes the online journal Environmental Health News. But in a world where exercise is down and poor diets abound, it could exacerbate a weight problem.
Vom Saal says as people become adults, they may be able to shake off the weight with extreme diet and exercise, but it won't be easy. "It is a very intractable thing to change," he said.
Scientists who study obesity's link to chemicals say the research is still in its infancy. Among the many unanswered questions that remain: How do the changes happen? What about the combined impact of exposure to many chemicals? Are humans affected by the chemicals the same way as animals?
For those who don't want to wait until all the evidence is in, there is another question: How to avoid these chemicals now?
"It can be difficult," said Felix Grun, assistant researcher in the department of developmental and cell biology at the University of California who works with Blumberg. To minimize exposure to bisphenol A, Grun said people can avoid buying plastics with the recycling number 7 marked on the bottom, but similar types of chemicals abound in other products, too. "These compounds are everywhere, the carpet fibers, the PVC piping, etc," he said.
Scientists say years of research into a once-popular synthetic hormone - diethylstilbestrol (DES) - also bolsters their belief that chemical exposure during early development can affect weight later in life. DES was once given to women to prevent miscarriages until it was linked to cancer in female offspring. Now, research by Retha R. Newbold at the National Institute of Environmental Health Sciences shows that mice exposed to DES in utero are fatter than those not given the chemical.
Ana Soto, a Tufts University professor who studies endocrine disrupters and development, says scientists already know that the most serious health problems of DES impact mice and humans similarly. Now that mice exposed to low levels of bisphenol A are behaving much the same way they do when exposed to DES, it makes sense to conclude that humans may be at risk too. She wants the chemicals like bisphenol A to be regulated by the federal government.
"What else are we waiting for?" Soto asked. "There is evidence these chemicals have a multitude of deleterious effects in animals. . . We should be worried."