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Turning what was once conventional wisdom on its head, a new study suggests that many, if not most peanut allergies can be prevented by feeding young children food containing peanuts beginning in infancy, rather than avoiding such foods.

About 2 percent of American children are allergic to peanuts, a figure that has more than quadrupled since 1997 for reasons that are not entirely clear. There have also been big increases in other Western countries. For some people, even traces of peanuts can be life-threatening.

An editorial published Monday in The New England Journal of Medicine, along with the study, called the results “so compelling” and the rise of peanut allergies “so alarming” that guidelines for how to feed infants at risk of peanut allergies should be revised soon.

The study “clearly indicates that the early introduction of peanut dramatically decreases the risk of development of peanut allergy,” said the editorial, by Dr. Rebecca S. Gruchalla of the University of Texas Southwestern Medical Center and Dr. Hugh A. Sampson of the Icahn School of Medicine at Mount Sinai in New York City. It also “makes it clear that we can do something now to reverse the increasing prevalence of peanut allergy.”

In the study, conducted in London, infants 4 to 11 months old who were deemed at high risk of developing a peanut allergy were randomly assigned either to be regularly fed food that contained peanuts or to be denied such food. These feeding patterns continued until the children were 5 years old. Those who consumed the foods that had peanuts in them were far less likely to be allergic to peanuts when they turned 5.

Dr. Gideon Lack, a professor of pediatric allergy at King’s College London and the leader of the study, said the common practice of withholding peanuts from babies “could have been in part responsible for the rise in peanut allergies we have seen.”

Whether infants should be fed peanuts and other foods associated with allergies is one of the most common questions parents ask about introducing solid foods to their children, said Dr. Ruchi Gupta, associate professor of pediatrics at Northwestern University, who was not involved in the study. “And until now most of what we can say is there’s not very conclusive data.”

The American Academy of Pediatrics, in guidelines released in 2000, recommended that peanuts be withheld from children at risk of developing allergies until they were 3 years old.

In 2008, the academy revised its stance, saying there was no conclusive evidence that avoidance of certain foods beyond 4 to 6 months of age helped stave off allergies, but stopped short of recommending that parents give their young children such foods.

“There was no study showing that that was the right thing to do,” said Dr. Wesley Burks, chairman of pediatrics at the University of North Carolina, who was not involved in the new research. Now, with the new study, he said, there is such evidence.

The results of the study were presented on Monday at the annual meeting of the American Academy of Allergy, Asthma and Immunology in Houston.

Pediatricians caution that parents should not feed whole peanuts to infants because of the choking risk, but rather peanut butter or other foods.

Dr. Lack said he first began to question the conventional wisdom about 15 years ago when he gave a talk in Tel Aviv and asked doctors in the audience how many had patients with peanut allergy. Only three hands went up. “In the U.K., if you had asked that question, every single member of the audience would have put up their hand,” he said.

So Dr. Lack and colleagues conducted a survey, published in 2008, that found the rate of peanut allergy in Israeli children was only about one-tenth that of Jewish children in Britain. The best explanation, they concluded, was that Israeli infants consumed high amount of peanut protein in the first year of life while parents in Britain avoided giving such foods.

Some other studies have also found that earlier feeding of allergy-inducing foods was associated with lower allergy rates. But such observational studies are not definitive because there could be unexplored factors that account for the differences.

The new study, by contrast, was a randomized trial in which the only difference between two groups of children was whether they were fed peanut protein. The study, sponsored by the National Institutes of Health and other organizations in the United States and Britain, involved infants 4 to 11 months old deemed to have a high risk of peanut allergy because they already had severe eczema or were allergic to eggs.

The infants were given skin-prick tests for peanut allergy. Those already allergic to peanuts were excluded from the study.

Some 530 children were did not have peanut allergy on that initial test. Parents of half of them were told to avoid peanuts. The other half were told to feed their children at least six grams of peanut protein per week, the equivalent of about 24 peanuts, spread over three or more meals. The preferred food was Bamba, an Israeli snack made of puffed corn and peanut butter.

The children were given another allergy test when they turned 5. Only 1.9 percent of those who were fed peanuts were allergic to them, compared with 13.7 percent of the children in the group that avoided peanuts.

An additional 98 infants had a weakly positive test when the study began, suggesting they were on their way to developing a full-fledged allergy. Among those children, only 10.6 percent of those fed food containing peanuts developed that allergy by age 5, far less than the 35.3 percent rate for children whose parents avoided feeding them peanuts.

“You got a definitive outcome, which I think is a very important thing,” said Dr. James R. Baker Jr., chief executive of FARE, or Food Allergy Research and Education, an advocacy group that helped pay for the study.

There are some caveats. Parents knew which group they were in. And the study was done at a single site in London with 75 percent white children. However, the results were the same for the small number of children of other ethnic groups in the study, the researchers said.

Moreover, it is still unknown whether allergies might yet develop if the regular feeding of peanuts stopped. To test this, the children in the study were taken off peanuts after they turned 5 and are being followed for a year.

It is also unknown if the same strategy would work with other foods or for children not considered at a high risk of getting a peanut allergy. Some pediatricians said that if feeding peanut foods to children at risk was beneficial, there would be no reason to withhold such food from children not prone to allergies.

Changing practice might not be easy. “I do think parents have the highest fear of introducing peanuts,” said Dr. Gupta of Northwestern. Eight years ago, she withheld peanuts from her own baby daughter, who had eczema and was allergic to eggs. The girl developed a peanut allergy.

In light of the new study, Dr. Gupta said, “I’m kind of slapping myself on the wrist.”