Allergist and immunologist Dr. Gideon Lack’s first inkling that some peanut allergies might be preventable came more than 20 years ago while he was giving a talk in Tel Aviv.

Lack, a professor of pediatric allergies at King’s College London, asked an audience of roughly 200 Israeli allergists how many children with peanut allergies they had treated in the last year. When he asked that question during similar talks in the U.S. and U.K., nearly every hand in the room shot up. To his surprise, only two or three Israeli doctors raised their hands.

He did some research and zeroed in on a key difference: Parents in the U.S. and U.K. were told not to give their infants any peanut products until the age of 3 as a precaution against future peanut allergies. In contrast, puffy peanut snacks were a favorite staple of many Israeli babies’ diets.

Lack and colleagues decided to test the theory that early oral exposure could actually prevent children from developing peanut allergies. After tracking hundreds of children from infancy to early adolescence, they recently concluded that babies who eat the stuff early and often in their first five years of life are 71% less likely to be allergic to peanuts at age 12.

The Learning Early About Peanut Allergy (LEAP) clinical trial ultimately overturned the official guidance given to new parents and has potentially prevented countless new cases of a serious and potentially deadly allergy.

“It was revolutionary,” said Dr. Rita Kachru, a UCLA allergist and immunologist. “It really completely shifted the paradigm and the understanding of food allergy.”

The team recently published the third and final report of their longitudinal study.

In the first phase, whose results were published in 2015, the team recruited 640 babies between the ages of 4 and 11 months deemed at high risk for developing allergies, either because they were already allergic to eggs or had severe eczema.

Half the babies were prohibited from consuming any peanut product in their first five years. The other half had to eat at least 6 grams of peanut protein per week.

At the five-year mark, 13.7% of peanut-avoiding kids who had no sensitivity to peanuts at the start of the trial had peanut allergies by the end.

But only 1.9% of the peanut-eaters in this group did — an 86% relative reduction in peanut allergy risk. For kids who showed some initial sensitivity to peanuts at the start of the test, eating peanuts was associated with a 70% relative reduction in developing a full-blown allergy.

“The results have the potential to transform how we approach food allergy prevention,” Dr. Anthony Fauci said at the time. Fauci was then director of the National Institute of Allergy and Infectious Diseases, which helped fund the study.

In the second phase, the researchers asked 556 participants from the original study to avoid peanuts entirely for a year, to see if continuous peanut exposure was necessary to prevent allergies from forming. Only a few kids who had previously eaten peanuts without issue developed an allergy after going without them for 12 months.

In the third phase, published last month in the New England Journal of Medicine, the researchers tested 508 children who had participated in the first two studies.

Participants had been free to eat or avoid peanuts as they wished in the six years since they were last studied. The team found that 15.4% of participants from the group that avoided peanuts in early childhood had peanut allergies at age 12, while only 4.4% of those who ate peanuts early on did.

“It was doubly gratifying because our hypothesis was correct, but more importantly, we now have a strategy to prevent — and I would argue, nearly eradicate — the development of peanut allergy in the population,” Lack said over Zoom from London.

Incidence of food allergies began rising sharply in the 1980s, particularly in industrialized Western nations. In 1997, 0.4% of people in the U.S. had diagnosed peanut allergies. Today, about 1.8% do.

Amid the search for explanations, one 1989 study found that infants whose exposure to common allergenic foods was severely restricted in their first two years of life ended up with fewer allergies than those in a control group.

Largely based on that research, in 1998 the U.K. instructed women to not eat peanuts during pregnancy or while breastfeeding if they or their partner had a family history of allergies, and to prevent their child from eating peanuts until the age of 3. The American Academy of Pediatrics adopted similar guidelines in 2000.

After the first two LEAP reports came out, both the American Academy of Pediatrics and British Society for Allergy and Clinical Immunology issued new guidelines in 2017 incorporating the results. They now advise children at greater risk of developing a food allergy — those with eczema, egg allergies or both — to start eating peanut products between 4 and 6 months. For children without risk factors, the AAP says, peanuts can be introduced whenever the baby starts eating solid foods.

“Previous guidance and recommendations prior to the LEAP study, where we were just avoiding peanuts because we were afraid of peanut allergy, was completely thrown out the window,” said Dr. Jenny Lee, a UC Irvine allergist and immunologist. “It changed the way that we practice.”

Nine years after the initial findings were published, there are signs that the approach is preventing new allergy diagnoses. In Australia, where guidelines also now encourage early peanut consumption, a large study published in 2022 found that 2.6% of 1-year-olds were allergic to peanuts in 2018-2019, compared with 3.1% in 2007-2011.

Despite the strong evidence, the updated AAP guidelines haven’t translated into clear communications to all parents that early peanut introduction prevents allergies, said Dr. Katie Marks-Cogan, an allergist and immunologist who practices in Culver City.

Marks-Cogan says she asks parents of children with newly diagnosed food allergies if their pediatrician talked to them about early introduction of allergenic foods. Most of the time, they say no.

“They will still say … ‘Aren’t you supposed to wait until a year for milk, and three years for tree nuts and peanuts?’ So a lot of parents still think that, and it’s because it’s slow to change things in medicine,” Marks-Cogan said. “Introducing early is actually safer and it’s better.”

Times staff writer Karen Kaplan contributed to this report.