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.

About 10.8 percent of study participants had convincing food allergies, with anaphylactic symptoms; an additional 8.2 percent had other reactions to foods, such as symptoms of lactose intolerance. Prior studies had estimated that around 9 percent of adults had food allergies.

Of those with convincing allergies, 38 percent had experienced a reaction severe enough to send them to an emergency department, and 48 percent said at least one of their allergies had developed after age 18. Only about half of the people reporting convincing food allergies had ever had their allergies diagnosed by a doctor.

People who suspect that they have an allergy should seek medical care, Nadeau said. "They may be allergic to other foods and not know it yet, and they may have co-morbid conditions such as asthma that should be taken care of by a doctor as well," she said.

1/10/2019: Results reported in this story have been adjusted to accord with the final version of the study manuscript.

Photo by Marco Verch