The issue of obesity is addressed in Tuesday's online edition of the Journal of the American Medical Association.
In one U.S. study, researchers wanted to test the idea that hormone-like chemicals like bisphenol A, also called BPA, could be contributing to childhood obesity by disrupting kids' metabolism.
BPA is used to make hard plastics for food and beverage containers. It also found in the lining of many metal cans.
Dr. Leonardo Trasande of the New York University School of Medicine and his co-authors looked at BPA concentrations in the urine of 2,838 Americans aged six to 19 as well as body mass index scores.
"Urinary BPA concentrations was significantly associated with obesity in this cross-sectional study of children and adolescents," the study's authors concluded.
The researchers weren't able to tell which came first, the obesity or BPA concentrations.
"Explanations of the association cannot rule out the possibility that obese children ingest food with BPA content or have greater [fat] stores of BPA," the researchers said.
The obese children may have simply eaten or drunk more from cans and bottles containing BPA.
The researchers called it the first report of an association of an environmental chemical exposure with childhood obesity in a nationally representative sample.
Trasande noted that unhealthy diet and lack of physical activity remain the leading factors contributing to obesity in the U.S.
"This cross-sectional study, when considered in isolation, is at best hypothesis generating," the study's authors said.
In the study, the association between BPA and excess weight was higher than chance only in white children, not black or Hispanic children. The researchers weren't able to say why.
Experimental studies in animals have suggested that BPA may disrupt metabolism and lead to larger fat cells.
Studies of Americans and Canadians have found high levels of BPA in urine of people of all ages. But the body excretes it in hours, so the measurements may just reflect recent consumption.
It's also unknown how much BPA the kids and teens were exposed to in infancy. Regulators like Health Canada banned BPA in baby bottles because of concerns about exposure in the very young.
In a statement responding to the study, the American Chemical Council said "because of the way BPA is processed in the body, it is very unlikely that BPA could cause health effects at any realistic exposure level."
"Furthermore, regulators from Europe to Japan to the United States have recently reviewed hundreds of studies on BPA and repeatedly supported the continued safe use of BPA."
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