Few things are more subject to change and passing fancies than dietary advice. And that can be true even when the advice comes from trusted health authorities. A dozen years ago the standard recommendation to new parents worried about their child developing an allergy to peanuts, eggs or other common dietary allergens was to avoid those items like the plague until the child was two or three years old. But in 2008 the American Academy of Pediatrics (AAP) dropped that guidance, after studies showed it did not help. And in its latest report, issued in April, the AAP completed the reversal—at least where peanuts are concerned. It recommended that high-risk children (those with severe eczema or an allergy to eggs) be systematically fed “infant-safe” peanut products as early as four to six months of age to prevent this common and sometimes life-threatening allergy. Children with mild or moderate eczema should receive them at around six months.

These are not whimsical changes. They match advice from a federal panel of experts and reflect the results of large randomized studies—with the inevitable cute acronyms. One called LEAP (Learning Early About Peanut Allergy), published in 2015, found that feeding peanut products to high-risk infants between four and 11 months old led to an 81 percent lower rate of peanut allergy at age five, compared with similar babies who were not given that early exposure. Another trial, known as EAT (Enquiring About Tolerance), published in 2016, found that after parents carefully followed a protocol to begin feeding peanut protein, eggs and four other allergenic foods to healthy, breastfed infants between three and six months of age, the babies had a 67 percent lower prevalence of food allergies at age three than did a control group. The results were strongest for peanuts, where the allergy rate fell to zero, compared with 2.5 percent in the control group. Egg allergies also fell, but the AAP is waiting for more data on eggs, says Scott Sicherer, a professor of pediatrics, allergy and immunology at the Icahn School of Medicine at Mount Sinai and an author of the April report. “We don't want to tell people to do something where there isn't really good evidence.”

How food allergies develop and why they have become so commonplace remain dynamic areas of research. Both the allergies and eczema (a major risk factor) have been on the rise. A 2010 study by Sicherer and his colleagues found that the prevalence of childhood allergies more than tripled between 1997 and 2008, jumping from 0.6 to 2.1 percent.

A leading theory about how these allergies develop and the role of eczema has been proposed by Gideon Lack, a professor of pediatric allergy at King's College London and senior author of both LEAP and EAT. The “dual allergen exposure hypothesis” holds that we become tolerant to foods by introducing them orally to the gut immune system. In contrast, if a child's first exposure is through food molecules that enter through eczema-damaged skin, those molecules can instigate an allergic response. Research with mice strongly supports this idea, whereas in humans the evidence is more circumstantial. Lack points out that peanut allergy is more prevalent in countries where peanuts or peanut butter is popular and widespread in the environment, mustard seed allergy is common in mustard-loving France and buckwheat allergy occurs in soba-loving Japan. “Parents are eating these foods, then touching or kissing their babies,” Lack suggests, “and the molecules penetrate through the skin.”

A modern emphasis on hygiene may also contribute, Lack notes: “We bathe infants and shower young children all the time, very often once a day or more, which you could argue breaks down the skin barrier.” Researchers are examining whether applying barrier creams such as CeraVe can help stave off food allergies.

Eight foods account for 90 percent of food allergies: cow's milk, eggs, fish, shellfish, tree nuts, peanuts, wheat and soybeans. Some scientists believe this is so because these foods contain proteins that are unusually stable to digestion, heating and changes in pH and are therefore more likely to cause an immune response.

Early dietary exposure is now the confirmed preventive strategy for peanuts and, pending more research, perhaps the other foods, although this is more easily said than done. In EAT, parents had to get their babies to swallow at least four grams per week of each of the allergenic edibles, and many found it to be challenging. As Lack observes, “It's just not part of our culture to feed solids to very young babies.”