Deepening the mystery surrounding the health effects of bisphenol A, a large new study has linked high levels of childhood and adolescent exposure to the industrial chemical to higher rates of obesity — in white children only.
The latest research, published in the Journal of the American Medical Assn., measured bisphenol A, or BPA, levels in the urine of a diverse group of 2,838 Americans ages 6 to 19. Researchers from New York University also reviewed data on the participants' weight, dietary intake, physical activity and socioeconomic backgrounds.
At first blush, the link between BPA and obesity appeared to be powerful: Compared with children and teens with the lowest apparent exposure to the ubiquitous chemical, those with the highest exposure were roughly 2.5 times more likely to be obese.
But upon further analysis, the researchers found that for African Americans and Latinos, the link was so small it could have been a statistical fluke. And for young Caucasians, the association strengthened: Compared with white children with the least BPA in their urine, those with the most were six times more likely to be obese, the researchers found.
Bisphenol A is a chemical agent widely used to protect aluminum food cans from corrosion and to strengthen plastic bottles, toys and containers. In recent years, alarm has grown among consumer groups and public health officials amid mounting evidence that BPA readily accumulates in the body's fat stores and can disrupt hormones that play crucial roles in sexual development, energy use and fat deposition.
Indications of BPA's "endocrine disrupting" effects in humans has been weak and inconsistent, and as recently as July, the U.S. Food and Drug Administration reiterated its belief that BPA is safe for use in food containers.
At the same time, the agency said it would no longer support the use of BPA in baby bottles and sippy cups after the American Chemistry Council declared that its members had discontinued use of the chemical in such products.
Although the latest study is likely to strengthen concerns about BPA's effects on human hormones, its authors caution that they are far from finding a cause-and-effect link between BPA and obesity. Their findings, they wrote, are "at best hypothesis generating."
The absence of an association between BPA exposure and obesity in African Americans was particularly perplexing because researchers found some of the highest levels of BPA in the urine of black participants. (By contrast, some of the lowest BPA-exposure readings researchers found in the study were in young Latinos, the researchers noted.)
Lead author Dr. Leonardo Trasande called the ethnic differences "striking" and acknowledged that he and his colleagues were mystified by them.
"We know of no behavior in obese white children" that would make them more prone to weight gain when exposed to BPA than children of other ethnicities, Trasande said in a news conference.
What was clear is that, where BPA exposure and obesity were linked, it did not take much to make a difference.
The researchers used data collected in 2003 and 2004 as part of the National Health and Nutrition Examination Survey, which allows the Centers for Disease Control and Prevention to track national trends in health behaviors and obesity. They divided the participants into four groups on the basis of the BPA content in their urine — a crude measure of ongoing exposure to the chemical.
White children whose BPA readings fell in the lowest 25% were clearly less likely to have a weight problem: Only 4.7% of the group weighed in as obese.
Above that low level of exposure, however, obesity rates rose rapidly and did not correspond closely with stronger doses of BPA: Among the group whose BPA exposure lay in the second-lowest quartile, 17.5% were obese, along with 17.1% of those in the next-highest category of BPA exposure. Among the 25% of white childrenwith the highest level of BPA in their urine, 22.8% were obese.
When the dose of a potentially harmful agent does not track well with an observed effect such as obesity, "it's tricky to know where to go with it," said David B. Allison, a University of Alabama at Birmingham epidemiologist who was not involved in the study. It's possible that BPA exposure could be causing the obesity, but unseen factors could be intervening to create the effect, he said.
The NYU researchers offered several potential explanations for the apparent link. Perhaps, they suggested, the acidity of a canned food affects the degree to which BPA is taken up by the body. Perhaps differing dietary choices may account for differences in ethnic groups' response to BPA. Perhaps exposure to BPA in a child's earliest years have a more potent effect than exposure later in childhood.
It would be difficult to conduct the clinical trials that could establish a firmer causal link between BPA and obesity because neither researchers nor parents would want to expose children to unnecessarily high levels of BPA, said Allison, who directs the Nutrition Obesity Research Center at his university.
Trasande said such trials would be done "in the ideal world." In the meantime, he added, "it would be too early to make particular recommendations" to consumers about reducing BPA exposure in a bid to prevent obesity.