People who eat canned soup should be aware that a chemical used to line the tin can leach into the food and end up in the body, say scientists.
Tests on 75 volunteers revealed the compound bisphenol A (BPA) was readily ingested and detected in large amounts in the urine, the Journal of the American Medical Association reports.
Past studies have linked high BPA levels with adverse health effects.
In the EU the chemical is already banned from baby bottles.
But it is still used in cans as a coating to prevent rusting and keep the food fresh. Some soft drink cans and bottles also contain BPA.
Dr Jenny Carwile, lead author of the latest study at the Harvard School of Public Health, said: "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. This study suggests that canned foods may be an even greater concern, especially given their wide use."
Her team asked the volunteers to eat either a freshly made 12oz serving of vegetarian soup or one out of a can once a day for five days.
After a weekend of rest, the groups switched over so that the fresh soup group now ate the canned variety and vice-versa.
Tests on their urine revealed detectable BPA in 77% of samples after fresh soup consumption and 100% of samples after canned soup consumption.
A serving of tinned soup a day appeared to increase BPA 20-fold.
The average concentration of BPA was 1.1 μg/L after fresh soup consumption compared to and 20.8 μg/L after eating soup from a tin.
The researchers say levels like these are "among the most extreme reported in a non-occupational setting".
The study did not look at what the health impact of this might be, but they say this warrants further investigation, even if rises might be temporary.
Fellow researcher Karin Michels said: "The magnitude of the rise in urinary BPA we observed after just one serving of soup was unexpected and may be of concern among individuals who regularly consume foods from cans or drink several canned beverages daily.
"It may be advisable for manufacturers to consider eliminating BPA from can linings."
The UK's Food Standards Agency said: "Our current advice is that BPA from food contact materials does not represent a risk to consumers but the agency will be looking at this study, as it would at any new piece of work, to see if it has any implications for our advice to consumers."
The North American Metal Packaging Alliance said that BPA is "highly unlikely" to affect health.