Adding to the mixed bag of research on bisphenol A and diabetes, a new study suggests that people with higher urinary levels of the controversial chemical do have a higher risk of diabetes.
Bisphenol A—better known as BPA—is a so-called endocrine disruptor, which means it may affect normal hormone activity in the body.
It's also all around us. BPA has been used for decades to make hard plastic containers, as well as linings for metal food and drink cans. Research suggests that most people have some amount of BPA in their blood, including about 95 percent of Americans.
Recent animal studies have hinted that the chemical could play a role in certain cancers, heart disease and abnormal brain development in children. But BPA's true effects in humans remain unknown.
Two large studies have found a link between higher BPA levels and higher heart disease risk. And a 2008 study found that of Americans in a government health survey, those with higher BPA levels showed a higher diabetes risk.
None of that, however, proves cause-and-effect. And a recent study of Chinese adults found no link between BPA levels and diabetes risk.
This latest study is based on data from a federal health study done between 2003 and 2008. Researchers found that of nearly 4,000 U.S. adults involved, those with the highest urinary BPA levels were more likely to have diabetes.
Just under 12 percent of all study participants had diabetes, based on blood sugar tests. And the odds of having the disease rose as urinary BPA increased.
Of people with the highest levels (more than 4.2 nanograms per milliliter, ng/mL), almost 13 percent had diabetes, versus 8 percent of adults with the lowest BPA levels (less than 1.1 ng/mL).
For comparison, the typical urinary BPA level among Americans has stood at about 2 ng/mL in recent years.
The findings, reported in the Journal of Clinical Endocrinology & Metabolism, do not prove that BPA is responsible for the higher diabetes risk.
"Since BPA measurements as well as diabetes diagnosis were conducted at the same time, we cannot say for sure that BPA exposure preceded diabetes development," lead researcher Dr. Anoop Shankar, of the West Virginia University School of Medicine, said in an email.
The researchers did account for a number of other factors in diabetes risk—like body weight, age and race. And the BPA-diabetes link still held; people with the highest levels had a 68 percent greater risk of diabetes than those with the lowest levels.
But what's needed, according to Shankar, are long-term studies that start with diabetes-free adults, measure their BPA levels, then see who develops diabetes over time.
Shankar said he and his colleagues are planning such a study.
Exactly how BPA might promote diabetes is unclear. Lab research suggests that BPA can act like a hormone in the body and, in animals at least, promote weight gain.
In this study, Shankar's team found that BPA levels were related to diabetes risk in both heavy and normal-weight people. But there may be pathways other than weight gain, according to Shankar.
BPA may, for instance, promote body-wide inflammation, which is linked to diabetes and a range of other chronic diseases. Again, though, that's based on animal research.
In general, experts say that people who are concerned about BPA can try cutting down on canned foods and avoiding food containers made of polycarbonate plastics—especially for reheating food, since high heat may transfer small amounts of BPA into food.
Polycarbonate plastics are usually marked with the recycling code "7."
Due to the controversy over BPA, the major manufacturers of infant bottles and feeding cups in the U.S. have stopped using the chemical.