People who ate canned soup for five days straight saw their urinary levels of the chemical bisphenol A spike 1,200 percent compared to those who ate fresh soup, US researchers said on Tuesday.

The randomized study, described as “one of the first to quantifyBPA levels in humans after ingestion of canned foods,” was done byHarvard University researchers and appears in the Journal of the American Medical Association’s November 23 issue.

“We’ve known for a while that drinking beverages that have been stored in certain hard plastics can increase the amount of BPA in your body,” said lead author Jenny Carwile, a doctoral student in the Department of Epidemiology at Harvard School of Public Health.

“This study suggests that canned foods may be an even greater concern, especially given their wide use.”