It may sound radical, but it works: Eating peanuts slashes the chance of a peanut allergy, at least in children at high risk of developing one, a much-anticipated study finds. The results are likely to catapult a long-standing theory—that ingesting potential food allergens is a way to prevent allergies—into mainstream medicine.

“This is the study,” says Rebecca Gruchalla, a specialist in allergy immunology at the University of Texas Southwestern Medical Center in Dallas, who wasn’t involved in it. The data, she says, are “just mind-blowing.”

The trial, the results of which are published today in The New England Journal of Medicine to coincide with their presentation at a major allergy meeting in Houston, Texas, is by far the largest and longest running of its kind, with 640 children followed over 4 years. It was launched back in 2006, when both the United States and United Kingdom recommended that parents keep peanut products away from high-risk youngsters until they turned 3, advice families in other Western countries often followed, too. But as these children eschewed peanut butter, doctors were increasingly uncertain whether their recommendations were sound. Peanut allergies, which can be fatal, were soaring in the same countries that urged avoidance: In the United States, the prevalence of peanut allergy more than tripled between 1997 and 2008, to 1.4%. In Israel, meanwhile, where the environment is not appreciably different from other industrialized countries, only about 0.17% of schoolchildren were allergic. What was different there? For one, many Israeli babies consume a wildly popular snack of peanut puffs called Bamba by the time they’re 6 months old.

The rationale for avoidance was simple: It’s not possible to become allergic to a food unless exposed to it, and doctors had believed that the guts and immune systems of older children may be better able to tolerate potential allergens, making the body less likely to react badly to new foods. But avoiding peanuts completely—or, more specifically, the peanut protein that can trigger an allergic reaction in some people—is extraordinarily difficult. In the United Kingdom, one study more than a decade ago suggested that many peanut-allergic children had been slathered with diaper creams containing peanut oil in infancy. Gideon Lack, a professor of pediatric allergy at King’s College London, reported in 2013 that peanut protein persisted on hands and in saliva for up to 3 hours after peanuts were eaten, suggesting that babies might be exposed through their skin to miniscule amounts from parents or older siblings. We “really didn’t appreciate until quite recently … how much peanut protein is in the environment,” says Hugh Sampson, a pediatric allergist at the Icahn School of Medicine at Mount Sinai in New York City.

Although evidence has continued to mount, even 8 or 10 years ago avoidance was already being called into question. So Lack and his colleagues set out to test whether feeding babies and young children peanut products might help them learn to tolerate the peanut protein, inhibiting an allergy. All the babies were between 4 and 11 months old when they were enrolled, and all had either an egg allergy, severe eczema, or both—putting them at high risk of a peanut allergy down the road. Indeed, 98 of them were already heading in that direction: They tested positive for mild peanut sensitivity in a skin-prick test. This meant that these babies were already churning out antibodies to the peanut protein. Eating peanuts in the future could set off an allergic reaction.

The team divided the babies into two groups. Half were to avoid eating peanut products until they were 5 years old. The other half received at least 6 grams of peanut protein a week, spread across at least three meals, until they were 5 years old. Bamba was the preferred offering, though picky eaters who rejected it got smooth peanut butter.

Around the 5th birthdays of the trial subjects came the big test. The children consumed a larger peanut portion than they were used to in one sitting, and the results were clear-cut. Among 530 children who had had a negative skin-prick test when they were babies, 14% who avoided peanuts were allergic to them, compared with 2% of those who’d been eating them. In the even higher risk group, the children who were sensitized, 35% of the peanut-avoiders were allergic versus just over 10% of the peanut eaters.

“This study really proves cause and effect,” Sampson says, adding that he “certainly hopes” it will change clinical practice. “When someone asks me in my practice, I will encourage them to get peanuts in the diet in the first year of life.”

As with any study, there are still a number of unanswered questions. One is whether the allergy prevention will persist in the children if they drop their regular nutty meals. Lack and his colleagues plan to follow them to answer this question. Another is whether the results apply to other food allergens, too. Gruchalla sees no reason why they wouldn’t, and early results are encouraging: An Australian study of egg exposure in 86 high-risk babies reported in 2013 that it saw hints that babies eating eggs were less likely to become allergic. The same group, led by allergy researcher Debra Palmer at the University of Western Australia, is now testing this in 820 infants and hopes to have results in about a year.

Then there’s the difficult question of treatment: Can people who already have peanut allergies be exposed, with care, to peanut protein as a way of minimizing their reaction? Given the risks involved, researchers are treading carefully, but Sampson led a study presented yesterday at the annual meeting of the American Academy of Allergy, Asthma & Immunology, reporting results of a study aiming to do just that. The 221 adult and pediatric participants were offered either a patch delivering peanut protein through healthy skin or a placebo patch. After 1 year, those who got the highest dose patch could tolerate 10 times as much peanut protein—about four peanuts total—as they’d been able to when the study began. With such tolerance, allergy sufferers might be less fearful of accidental exposure to small amounts of the nut.

Exposure treatment like this still needs more testing. But for prevention of a life-threatening allergy, the landscape has changed. “Now,” Gruchalla says, “we have something to do.”