About 26 million U.S. adults have food allergies, nearly half of which develop after age 18, according to a study published in JAMA Network Open.

The research, a collaboration between Stanford food allergy expert Kari Nadeau, MD, PhD, and scientists at Northwestern University, is the first comprehensive examination of the prevalence of food allergies among the country’s adults. More than 40,000 people were surveyed.

Many more adults have food allergies than previously suspected. The findings also contradict a long-held assumption that these allergies usually show up in childhood.

The emergence of new allergies in adulthood is especially alarming given that anaphylaxis — a severe allergic reaction — requires quick treatment. People who knew they had an allergy could carry injectable epinephrine for accidental exposures to foods that triggered their allergies. But others could be caught unprepared, the study found.

“They were eating shrimp salad for the 30th time, for example, and that’s when they had their first anaphylactic event,” Nadeau said. Experts need to determine how to predict adult-onset food allergies, she said. Nadeau leads a wide variety of studies on food allergies and related conditions as the director of the Sean N. Parker Center for Allergy and Asthma Research at Stanford University.

Because the term “food allergy” often gets colloquially used to include many types of reactions to foods, Nadeau and her collaborators included questions in their survey to zero in on anaphylactic symptoms, such as itching; hives; swelling of the lips, tongue and throat; and difficulty breathing.

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