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Though there's no immediate need to turn your PB&J's into jelly-only sandwiches, there is a chance that peanut butter may contain the carcinogen aflatoxin. The soft and porous shell that encases peanuts can allow fungus with aflatoxin to penetrate into the nut, explains Organic Authority.
Although aflatoxin has yet to be proven to cause cancer in the United States, it has been documented as causing liver cancer in developing countries where corn, peanuts and grains are grown without strict soil quality regulation. Currently, all commercially-produced peanut butters must be tested for aflatoxin, but grind-your-own peanut butter may actually be at a higher risk because the peanuts sit around the longest without refrigeration, allowing more mold to develop, according to Organic Authority. Planet Green also adds that natural peanut butters may be more susceptible since "they are less processed and have a shorter shelf life, therefore the mold is more likely to thrive."
This alarm bell has been sounded before. In an informative write-up, Celeb doc Andrew Weil explains that the Consumers Union found the exact same results about a decade ago, when it was discovered that supermarket brands like Peter Pan, Jif and Skippy contained the less afloxtin than natural brands. Consumer Reports has researched aflatoxins since 1972, but still says peanuts are worth eating for their health benefits.
There's a simple precaution if you're concerned. Refrigerate your peanut butter -- it'll keep out the mold.