A chemical banned from baby's bottles has been linked to an increased risk of childhood obesity.
Children with high levels bisphenol A (BPA) in their urine are almost three times more likely to be obese than those with low levels, a study found.
The chemical, widely used in food and drink packaging, is an "endocrine disrupter" that can have hormonal effects.
It is present in the environment at low non-toxic levels. But many experts believe constant exposure to the substance may be harmful, pointing to associations with heart disease, breast and prostate cancer, neurological disorders, diabetes and infertility.
Last year the EU banned BPA from polycarbonate feeding bottles intended for infants up to 12 months of age. The ban applies to all EU member states including the UK.
The US Food and Drink Administration recently banned BPA from baby bottles and sippy cups.
In the latest study, scientists measured BPA levels in the urine of almost 3,000 children and teenagers aged six to 19.
Rates of obesity among those with the most BPA were 2.6 times higher than those with the least.
Among the participants with the highest levels, 22.3% were obese compared with 10.3% of children with the lowest levels.
"This is the first association of an environmental chemical in childhood obesity in a large nationally representative sample," said lead scientist Dr Leonardo Trasande, from New York University. "Our findings further demonstrate the need for a broader paradigm in the way we think about the obesity epidemic.
"Unhealthy diet and lack of physical activity certainly contribute to an increased fat mass, but the story clearly doesn't end there."
Previous studies have shown that BPA causes changes to human metabolism that may increase body mass, said the scientists.
The findings are published today in the Journal of the American Medical Association.
The researchers took account of a range of factors that might have influenced their results, including ethnicity, age, socio-economic background, calorie intake, and television watching.
Further analysis showed that the association was only statistically significant among white children and adolescents.
Obesity was also not linked to other similar chemicals found in products such as sunscreen and soap.
Most BPA exposure in the US is believed to be from aluminium drink cans, which are coated with the chemical on the inside, said Dr Trasande.
"This data adds to already existing concerns about BPA and further supports the call to limit exposure of BPA in this country, especially in children," he added. "Removing it from aluminium cans is probably one of the best ways we can limit exposure."
British expert Professor Richard Sharpe, from the Medical Research Council Centre for Reproductive Health at the University of Edinburgh, said: "This study does not prove that increased exposure to bisphenol A causes obesity, only that the two are associated. Such an association could arise because children who are obese choose to eat more of foods (eg canned drinks/foods) that contain more bisphenol A than do non-obese children, or that bisphenol A metabolism is altered by being obese, so that levels appear higher in fatter children.
"One unusual observation is that the association only applies to white children. As the prevalence of obesity is generally higher in black than in white children in the USA, this would certainly argue against bisphenol A as being an important cause of the childhood obesity crisis."
BPA was rapidly inactivated after being ingested, he pointed out. He added: "Nevertheless, the possibility that bisphenol A exposure could causally contribute to obesity cannot be dismissed, even if it seems unlikely."