By Sumathi Reddy, The Wall Street Journal, Updated Aug. 21, 2017 11:12 a.m. ET

The rate of reports of severe allergic reactions to foods like peanuts has increased by nearly five times over the past decade, according to a new analysis of private insurance claims.

The analysis looked at private insurance claims with a diagnosis of an anaphylactic food reaction from 2007 to 2016. Anaphylaxis is a systemic allergic reaction in which the immune system affects multiple parts of the body at the same time, often leading to trouble breathing. It can be fatal if not treated promptly and requires an injection of epinephrine and a trip to the emergency room.

The analysis was conducted by FAIR Health, a New York City-based, independent nonprofit that has a database of 24 billion medical and dental claims from 150 million privately insured people.

“This is an incredibly important study,” says James Baker, chief executive officer and chief medical officer for the Food Allergy Research & Education (FARE), a Virginia-based advocacy group. “Clearly our own information suggests that not just the frequency of people having food allergy but the severity of food allergy in individuals has increased dramatically.”

The proliferation of food allergies in the western world, particularly to peanuts, has baffled medical experts who struggle with how to advise parents and children for a condition with no permanent cure. Studies have found that as many as 8% of children have a food allergy, with nearly 40% having a history of severe reactions.

The increase could be related to the increasing use of antibiotics, rising rates of C-sections that affect the microbiomes of babies, and an increasingly sterile environment, says Hugh Sampson, director of the Jaffe Food Allergy Institute at the Icahn School of Medicine at Mount Sinai in New York City. All have altered the good bacteria in our intestinal tracts, which alters the programming of our immune systems.

Many years of medical advice telling parents to avoid highly allergenic foods such as peanuts when children are babies may have contributed to increasing allergy rates, he says. Earlier this year medical organizations changed their advice recommending that babies at high and moderate risk of developing a peanut allergy, such as those with eczema, be introduced to the foods by six months.

The rate of food allergies to peanuts and tree nuts have doubled or even tripled over the past two decades in the U.S., Dr. Sampson says.

The FAIR Health analysis found that peanuts were the most common cause of anaphylaxis, making up 26% of claims. Tree nuts, such as walnuts and pistachios, and seeds accounted for 18% of claims, followed by eggs, crustaceans, and dairy. About 33% of claims were due to unspecified or unknown foods.

The analysis also showed a greater increase in claims in rural areas than in cities. “The one thing that surprised me was the bigger change occurring in rural areas as opposed to urban areas,” Dr. Sampson says.

The urban and rural area designations were based on the U.S. Census, says Robin Gelburd, president of FAIR Health.

The group will dig deeper into the data in a white paper to be released in October, she says, looking at geographical and gender variations and costs of services, among other factors.

About 34% of claims were in people over 18, she says.

“I think a lot of people assume children grow out of these allergies, and the fact that we’re seeing about a third of the claims attributable to those over 18-years-old is something that raises some interesting questions and invites some further study,” Ms. Gelburd says.

Kristin Osborne, a mother to three boys in Virginia Beach, Va., knows the fear of anaphylaxis. Her sons—ages 15, 11 and 5—all have multiple food allergies.

Her middle son, David Osborne, is allergic to peanuts, tree nuts, wheat, dairy and eggs. He has had two anaphylactic reactions in the past 12 months. The most recent was at the grocery store, when she suspects he was exposed to an allergen on the conveyor belt at the checkout line.

“He started wheezing and coughing and couldn’t breathe, so I administered epinephrine and rode to the emergency room,” recalled the 36-year-old, who works as a disability advocate.

“It’s extremely scary,” she says of the reactions. “It’s kind of like a roller coaster. We really prepare and try to be as proactive as possible, but when it happens, you never really know what the end result is going to be. We just hope to get to the ER in time.”

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