A new study finally looks at the frequency of adult-onset allergies.
peanut allergies developed even in adulthood
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If you're like most adults, you think that if you didn't suffer an unfortunate trail-mix incident when you were five, you are peanut-allergy-free—and you would be wrong.

A new, yet-to-be-published national study shows that some 53 percent of American adults with reported food allergies developed that condition after the age of 18, according to the New York Times.

Dr. Ruchi Gupta, food allergy researcher at both Ann and Robert H. Lurie Children's Hospital of Chicago and Northwestern University Feinberg School of Medicine, led the study after she heard that more and more adults were reporting food allergies. Yet there was no research to prove adult-onset food allergies were such a frequent occurrence, Gupta told the newspaper. So Gupta set out to get the data for herself.

Gupta and colleagues surveyed more than 40,000 American adults and found that shellfish allergies are the most common food allergies to develop after age 18, with 3.9 percent of the population affected. Next, 2.4 percent of adults said they'd developed a peanut allergy. And 1.9 percent reported adult-onset tree nut allergies.

So why do more than half of American adults with food allergies report developing those allergies after they are fully grown? Many adults may suffer from oral allergy syndrome, a condition that often affects adults who also have seasonal allergies. The syndrome "involves your body getting tricked," Dr. Sharon Chinthrajah, who participated in the research, explained to the New York Times. Tree and flower pollens can be similar to those in vegetables and fruits, and so, "when your body eats the raw form of those foods, it thinks you're eating tree pollen," Chinthrajah said. Pregnancy can also cause new allergies to crop up, Gupta said, as can suffering from and treating a viral infection.

Of course, if you suspect you've developed a new food allergy, it's best to head to an allergist to get tested.