There's fresh evidence that the chemical bisphenol A, or BPA, may play a part in childhood obesity.
BPA is a chemical that is widely used in food packaging. Government studies have shown that 92 percent of Americans have detectable levels of BPA in their bodies.
There's intense scientific interest in BPA because it is chemically similar to the hormone estrogen, and there's some concern that it may mimic estrogen's effects in the body, causing harm to the brain and reproductive organs, particularly in children.
Last year, the US Food and Drug Administration formally banned BPA from baby bottles and sippy cups, though manufacturers had already stopped using it. The agency declined to ban it from other food containers, pending further research.
How the study was done
In a new study published online in the journal PLoS One, researchers measured BPA levels in the urine of more than 1 300 children in China and compared those levels to their body weights.
The study authors also asked the kids about other things that may influence body weight, such as how often they ate junk food, fruits and vegetables, how much exercise they got, whether their parents were overweight and how long they played video games, on average, each day.
After taking all those factors into account, the investigators found that girls aged nine to 12 who had higher-than-average levels of BPA in their urine were about twice as likely to be obese as those with lower-than-average levels. The researchers didn't see the same association for boys or for older girls.
One explanation for the results may be that girls who are entering puberty are uniquely vulnerable to the effects of hormone-disrupting chemicals, said study author Dr. De-Kun Li, an epidemiologist at Kaiser Foundation Research Institute and the Stanford School of Medicine, in California.
"Human studies are starting to confirm animal studies that show BPA can disrupt energy storage and energy metabolism," said Li.
One of the most recent questions raised about BPA is whether or not it may be an obesogen, or a chemical that contributes to the development of obesity.
In laboratory studies, BPA produces many of the molecular hallmarks of obesity. It makes fat cells bigger, it blocks the function of a protein called adiponectin, which protects against heart disease, and it disrupts the balance of testosterone and estrogen -- hormones that are important for maintaining a healthy body mass.
One expert found the study results troubling.
Environmental chemicals and lifestyle
"Clearly, unhealthy diet and physical activity are still the leading causes of the childhood obesity epidemic worldwide, but this study adds further concern to the notion that environmental chemicals may be independent contributors," said Dr. Leonardo Trasande, an associate professor of pediatrics, environmental medicine and health policy at NYU Langone Medical Center, in New York City.
In a study of more than 2,800 U.S. children published last year in the Journal of the American Medical Association, Trasande reported that boys and girls who were exposed to higher levels of BPA were more likely to be obese than those exposed to lower levels of the chemical. That was true even after they took into account how many calories kids ate, how much TV they watched and household income.
Still, he said, neither of these studies can prove that BPA causes children to become obese. One explanation could be that obese kids eat more packaged and processed foods, which in addition to having more fat and calories could also contain more BPA. Another explanation is that obese kids may have higher levels of BPA because the chemical is stored in body fat, Trasande said.
Other studies that follow children as they grow are needed to clarify the nature of the association.
The American Chemistry Council, a trade group that represents the interests of the chemicals industry, said in a statement that the new study did little to shed light on the true causes of childhood obesity.
"Attempts to link our national obesity problem to minute exposures to chemicals found in common, everyday products are a distraction from the real efforts underway to address this important national health issue," the statement from the council said. "Due to inherent, fundamental limitations in this study, it is incapable of establishing any meaningful connection between BPA and obesity. In particular, the study measures BPA exposure only after obesity has developed, which provides no information on what caused obesity to develop, a limitation noted by the study's authors."
For more on BPA, head to the U.S. National Institute of Environmental Health Sciences.
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