A new study suggests the long-held industry assumption that bisphenol-A breaks down safely in the human body is incorrect. Instead, researchers say, the body transforms the ubiquitous chemical additive into a compound that might spur obesity.

The study is the first to find that people’s bodies metabolize bisphenol-A (BPA) — a chemical found in most people and used in polycarbonate plastic, food cans and paper receipts — into something that impacts our cells and may make us fat.

The research, from Health Canada, challenges an untested assumption that our liver metabolizes BPA into a form that doesn’t impact our health.

“This shows we can’t just say things like ‘because it’s a metabolite, it means it’s not active’,” said Laura Vandenberg, an assistant professor of environmental health at the University of Massachusetts Amherst who was not involved in the study. “You have to do a study.”

People are exposed to BPA throughout the day, mostly through diet, as it can leach from canned goods and plastic storage containers into food, but also through dust and water.

Within about 6 hours of exposure, our liver metabolizes about half the concentration. Most of that — about 80 to 90 percent — is converted into a metabolite called BPA-Glucuronide, which is eventually excreted.

The Health Canada researchers treated both mouse and human cells with BPA-Glucuronide. The treated cells had a “significant increase in lipid accumulation,” according to the study results. BPA-Glucuronide is “not an inactive metabolite as previously believed but is in fact biologically active,” the Health Canada authors wrote in the study published this week in Environmental Health Perspectives.

Not all cells will accumulate lipids, said Thomas Zoeller, a University of Massachusetts Amherst professor who was not involved in the study. Testing whether or not cells accumulate lipids is “a very simple way of demonstrating that cells are becoming fat cells,” he said.

“Hopefully this [study] stops us from making assumptions about endocrine disrupting chemicals in general,” he said.

The liver is our body’s filter, but it doesn’t always neutralize harmful compounds. “Metabolism’s purpose isn’t necessarily a cleaning process. The liver just takes nasty things and turns them into a form we can get out of our body,” Vandenberg said.

BPA already has been linked to obesity in both human and animal studies. The associations are especially prevalent for children exposed while they’re developing.

Researchers believe BPA does so by mimicking estrogen hormones, but its metabolite doesn’t appear to do so. In figuring out why metabolized BPA appears to spur fat cells, Zoeller said, it’s possible that BPA-Glucuronide is “hitting certain receptors in cells”.

Health Canada researchers were only looking at this one possible health outcome. “There could be other [health] impacts,” Zoeller said.

In recent studies BPA-Glucuronide has been found in human blood and urine at higher concentration than just plain BPA.

Industry representatives, however, argue the doses used were much higher than what would be found in people.

Steve Hentges, a spokesperson for the American Chemistry Council, which represents chemical manufacturers, said the concentrations used in which the researchers saw increased fat cells were “thousands of times higher than the concentrations of BPA-Glucuronide that could be present in human blood from consumer exposure to BPA.

“There were no statistically significant observations at lower BPA-G concentrations, all of which are higher than human blood concentrations,” he said in the emailed response.

Zoeller agreed the dose was high but said “the concentration is much less important than the fact that here is a group testing an assumption that’s uniformly been made.” Vandenberg said the range is not that far off from what has been found in some people’s blood.

The U.S. Food and Drug Administration is reviewing the Health Canada study but couldn’t comment before Environmental Health News’ deadline, said spokesperson Marianna Naum in an email.

The agency continues to study BPA and states on its website that federal research models “showed that BPA is rapidly metabolized and eliminated through feces and urine.”

Health Canada, which was not able to provide interviews for this article, has maintained a similar stance to the U.S. FDA, stating on its website that it “has concluded that the current dietary exposure to BPA through food packaging uses is not expected to pose a health risk to the general population, including newborns and infants.”

However, the fact that Health Canada even conducted such a study is a big deal, Vandenberg said.

“Health Canada is a regulatory body and this is pretty forward thinking science,” she said. “Hopefully this is a bell that can ring for scientists working for other regulatory agencies.”