BPA makes plastic hard and shatterproof. (CBC)

People exposed to years of low levels of bisphenol A, a chemical commonly found in plastic food containers, may be at higher risk of developing heart disease and diabetes — the first such finding in humans — U.S. researchers reported Tuesday.

BPA makes plastic hard and shatterproof. The chemical is also used to line cans and is found in water bottles and baby bottles, as well as consumer products like CDs.

In Wednesday's issue of the Journal of the American Medical Association, David Melzer, a professor of epidemiology and public health at Peninsula Medical School in Exeter, England, and his colleagues said those in the top one-fifth for BPA concentrations in their urine had nearly three times the odds of cardiovascular disease, compared with those showing the lowest levels.

"It is the first evidence we’ve ever had on ordinary U.S. adults living ordinary lives," Melzer said in a release.

Those with the highest levels also had 2.4 times the odds of Type 2 diabetes, compared with those in the lowest one-fifth, after adjusting for age and sex.

Also, higher BPA concentrations were linked with "clinically abnormal" concentrations of liver enzymes that are a marker for liver damage.

The findings were based on an analysis of data from the 2003-04 National Health and Nutrition Examination Survey, which looked at 1,455 adults who gave urine samples as part of U.S. government study that monitors exposure to environmental pollutants.

"Higher BPA exposure, reflected in higher urinary concentrations of BPA, may be associated with avoidable morbidity in the community-dwelling adult population," the study's authors concluded.

Limit exposure

"Measuring who has disease and high BPA levels at a single point in time cannot tell you which comes first," noted Dr. Lisa Schwartz of the Dartmouth Institute for Health Policy and Clinical Practice.

In an editorial accompanying the study, Frederick vom Saal of the division of biological sciences at the University of Missouri at Columbia, agreed longer-term follow-up studies are needed.

"While the evidence is preliminary in humans, the findings should spur U.S. regulatory to follow the recent action taken by Canadian regulatory agencies, which have declared BPA a 'toxic chemical' requiring aggressive action to limit human and environmental exposures," vom Saal and chief scientist John Peterson Myers of the Environmental Health Sciences, a non-profit group in Charlottesville, Va., wrote.

Both are longtime critics of the chemical.

It's not clear why BPA may increase the risk of disease in humans. Earlier studies involving laboratory animals may provide an explanation, said study co-author Tamara Galloway of the University of Exeter.

"They have suggested that bisphenol A acts like a hormone where it disrupts the actions of hormones, particularly estrogen, and that might be having some effect on insulin resistance and on the way that fat is distributed in the body," Galloway said.

Risk greatest in youngest

A causal role for BPA is also plausible, vom Saal said, given that BPA changes the programming of genes during critical periods during development in fetuses and newborns. Vom Saal has served as an expert witness and consultant on BPA litigation.

Reports by Health Canada, the U.S. National Institutes of Health expert panel, and U.S. toxicology program also concluded that exposure to BPA poses the greatest risk of adverse effects in these young age groups.

Since production of BPA worldwide has now reached about seven billion pounds per year, eliminating exposure from food and beverage containers would be easier than finding safe ways of disposing it that don't pollute landfills or waterways, vom Saal said.

"Thus, even while awaiting confirmation of the findings of Lang et al, decreasing exposure to BPA and developing alternatives to its use are the logical next steps to minimize risk to public health," the editorial said.

The report was released early to coincide with a hearing on BPA held Tuesday by the U.S. Food and Drug Administration, which defended its assessment that the chemical is safe.

"Right now, our tentative conclusion is that it's safe, so we're not recommending any change in habits," said Laura Tarantino, head of the FDA's office of food additive safety.

To lower their exposure, consumers can avoid plastic containers displaying the recycling number 7, many of which contain BPA, or the letters "PC" for polycarbonate, the type of plastic.

The American Chemical Council, a trade group representing chemical makers, has long maintained that BPA does not pose a risk to consumers at the levels to which they are exposed. In a statement, the group called the study flawed, saying it has major limitations.

"As the authors themselves note, they do not conclude that the presence of BPA is causing adverse health effects — they merely noted a statistical association," the council said.

But the editorial in the medical journal said the FDA is relying on studies that use outdated testing methods, and fails to take into account that since BPA is hormonally active, it does not follow the classic concept that "the dose makes the poison."

With files from the Associated Press