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By Meghan Holohan

For some adults, certain foods they once enjoyed now seem intolerable. While some of that might have to do with aging tummies, a recent study indicates there might be another culprit: food allergies that developed in adulthood. And many adults don't understand what's making them sick.

The study from JAMA Network Open found that 10.8 percent of adults, more than 26 million adults, experience food allergies. Half of those adults report developing the allergies after age 18. By contrast, about 6 million, or 8 percent, of children, have food allergies. While some children grow out of their allergies many continue having them throughout their lives.

The study reveals that experts need to conduct more studies to better understand adult onset allergies for detection and treatment.

“We’re not sure why people develop allergies as an adult,” Dr. Scott Sicherer, a professor of pediatrics, allergy and immunology at Mount Sinai Hospital who did not participate in the study, told TODAY.

Because there are few studies on adult onset food allergies, experts are unsure whether rates are increasing or by how much.

Anecdotal evidence suggests they are on the rise, though. And they can come out of nowhere, said TODAY medical contributor Dr. Natalie Azar.

"Perhaps it's too much use of antibiotics which change our gut flora and that's not good for the immune system," Azar said. "Another popular theory is the hygiene hypothesis, which is that our environments are just too sterile and we're not exposing our immune systems enough to develop and mature and be able to distinguish the good from the bad."

What is known is that people with other ailments are more likely to develop adult food allergies.

"So if you have asthma, eczema, hay fever, you are more likely to develop food allergies as an adult,” Sicherer said.

The foods that trigger a reaction in adult onset allergies are slightly different from the most common food allergies, which include milk, eggs, fish, peanuts, shellfish, soy, tree nuts and wheat.

Most common adult food allergies include:

  • Shellfish
  • Fish
  • Almonds
  • Brazil nuts
  • Cashews
  • Hazelnuts
  • Hickory nuts
  • Macadamia nuts
  • Pecans

Common food allergy symptoms, include:

  • Hives
  • Rashes
  • Swollen lips
  • Tightening throat
  • Struggling to breathe
  • Chest tightness
  • Faintness
  • Repeated vomiting

For people who develop hives or a rash, an antihistamine taken at home should treat it without any more complications. But if a person starts wheezing or struggling to breathe they should go to the emergency room immediately.

But experts say an allergic reaction might not always be obvious.

“Your reaction might not be the same every time and it is a myth that it gets worse each time,” Sicherer.

He recommends that anyone who suspect they might have food allergies speak to their doctor and make sure they receive the proper treatment.

“A large percentage of people have severe reactions and a lot of people don’t have the right medication,” he said.