We may not know all the ways in which the chemical bisphenol-A (BPA) affects our health, but we can be sure that we’re exposed to it frequently — BPA is used in plastic products and lines nearly all food and beverage cans.
Exposure to BPA, an endocrine-disrupting compound that mimics the body’s hormones, has been linked to heart disease, diabetes and obesity, and to potential problems during development in fetuses and young children. In Canada and Europe, the chemical has been banned outright from baby bottles, and while many manufacturers have removed BPA from baby products in the U.S., it hasn’t been regulated yet by the government.
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To understand how much of the BPA in food packaging shows up in our bodies, researchers led by Jenny Carwile, a doctoral student in epidemiology at the Harvard School of Public Health, focused on the BPA in canned foods. Carwile and her colleagues recruited 75 fellow students and staff members; half of the participants were asked eat one 12-oz. can of vegetarian soup each day for five days, while the other group ate soup made from fresh ingredients. Then, after a two-day interim of no canned soup, the two groups switched roles for another five days. This way, says Carwile, she could be sure that whatever else the participants were eating during the two-week study wouldn’t affect their BPA levels, since the only thing that changed was the source of their soup.
To measure levels of BPA, the researchers then asked all the participants to give urine samples after each soup-eating period. In the fresh-soup group, average levels of urinary BPA were about 1.1 micrograms per liter, roughly equivalent to what’s seen in the average American adult. After five days of eating canned soup, however, those levels rose to 20.8 mcg per liter, a more than 1,000% increase.
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The study — the first to measure how much BPA is absorbed by eating canned food — found some of the highest recorded levels of BPA in urine outside of manufacturing facilities where BPA is used. “We were surprised,” says Carwile. “Other studies have quantified the amount of BPA in canned food itself, so we were expecting a modest association. But this is really big.”
Although the study, published in the Journal of the American Medical Association, looked only at canned soup, the results likely apply to other canned foods and beverages as well.
The researchers did not explore the health effects of the spike in BPA levels or how quickly those levels may return to normal. Other studies have noted that BPA levels do fluctuate depending on people’s exposure to the chemical, but it’s not clear yet whether repeated spikes of BPA concentrations are particularly harmful or not. “We see an increased amount of BPA in urine. We don’t know how long that lasts, and we don’t know the effect of a fluctuating BPA level on health outcomes. But the results definitely deserve further study,” says Carwile.
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According to the Centers for Disease Control and Prevention, 93% of Americans have a detectable amount of BPA in their bodies. BPA comes not only from food and drink in cans, but also those packaged in polycarbonate plastic. The chemical is also found on thermal register receipts, which people receive at checkout at nearly every retailer.
The Food and Drug Administration says the small amount of BPA exposure we typically get doesn’t appear to be toxic, but notes that recent studies have led to “some concern about the potential effects of BPA on the brain, behavior, and prostate gland in fetuses, infants and young children.” A growing number of studies also links BPA exposure to changes in liver and heart function, and to detrimental effects on insulin levels. In a study involving a U.S. government health database, for example, British researchers reported that people with the highest levels of BPA in their urine were twice as likely to have heart disease or diabetes as those with lower levels.
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The FDA is currently conducting further studies on the effects of BPA exposure and supporting other research seeking alternative ways to manufacture food and beverage cans without BPA in the lining. In the meantime, the agency is continually urging manufacturers to stop using BPA in baby bottles and feeding cups.
Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.