Kids and teens that had higher levels of the chemical bisphenol A in their urine were more likely to be overweight or obese, in a new nationally-representative U.S. study.

The findings can't prove BPA - which has been banned from baby bottles but is still found in aluminum cans and other types of packaging - causes kids to gain weight.

But previous studies have also suggested a link between the chemical, a type of synthetic estrogen, and body weight in adulthood. And researchers say earlier exposures could be more influential.

"Children are uniquely vulnerable to environmental chemicals," said Dr. Leonardo Trasande from the New York University School of Medicine, who worked on the study.

It's possible, he said, that ingesting extra BPA could throw off young people's hormonal balance and disrupt their metabolism. But for now, that's just a theory.

"Unhealthy diet and poor physical activity are still the biggest causes of childhood obesity," Trasande told Reuters Health.

He and his colleagues analyzed data from a nationwide health and nutrition survey conducted between 2003 and 2008. Close to 3,000 kids age six to 19 were weighed, measured and had their urine tested for BPA. They also answered a range of diet and lifestyle questions.

In total, about one-third of the kids were overweight and 18 percent were obese.

The average kid had close to three nanograms - three billionths of a gram - of BPA in every milliliter of urine.

The researchers found that just over 10 percent of kids with the lowest BPA levels were obese, compared to 22 percent of those with the highest BPA, according to results published Tuesday in the Journal of the American Medical Association. That was after taking into account how much kids ate overall, as well as their age, race and gender.

Trasande said he was struck by the strength of that link. But it still doesn't mean extra BPA in kids' diets was responsible for their extra pounds.

There are a couple of other theories, according to Trasande.

"Obese children could ingest food that has higher BPA content - it could be what we call reverse causation," he said. Or, "They could have higher BPA stores in their bodies and release more BPA. Those are both very plausible explanations."

Still, he said the findings point to hormone-like chemicals as one factor to consider in the rise of childhood obesity, after diet and exercise.

Karin Michels, an epidemiologist from the Harvard School of Public Health in Boston, agreed there is "accumulating evidence" that BPA may be linked to obesity and related diseases like diabetes - although the majority of that research still comes from animals.

Her own study found a link between urinary BPA and adult weight, based on the same health and nutrition survey used by Trasande's team.

"We still don't really know how safe bisphenol A is," Michels, who wasn't involved in the new report, told Reuters Health.

While more research is underway, Michels said it makes sense to avoid polycarbonate bottles, aluminum cans and other products containing BPA if there are other options.

"I think we should be on the safe side," she said. But, "I don't think we have to panic about it at this point."