Large trial finds maternal egg and peanut intake doesn't prevent infant allergies

In the early months of pregnancy, when the fetal immune system is still forming its first responses to the world, researchers in Australia wondered whether what a mother ate might shape her child's tolerance to common allergens. The question was reasonable: egg-specific immune responses can appear in some infants as early as four months old, before they've ever tasted an egg. If tolerance could be seeded earlier—during pregnancy, through amniotic fluid, or later through breast milk—perhaps allergies could be prevented before they began. To test this, researchers enrolled 2,137 pregnant women in four Australian cities and asked half of them to eat substantially more eggs and peanuts than usual, starting before week 23 of pregnancy and continuing through at least four months of breastfeeding. The other half ate a standard amount. The results, published in the New England Journal of Medicine, offered no support for the theory.

The trial, called PrEggNut, was carefully designed. Researchers recruited only women carrying a first or second child who had a strong family history of allergic disease—asthma, eczema, allergic rhinitis, or food allergy in at least two biological relatives. These were the infants at highest risk. The high-intake group was asked to consume at least six eggs and 60 peanuts per week in any form, while the standard-diet group was limited to three eggs and 30 peanuts per week, amounts the researchers considered typical for Australia and consistent with current guidelines that do not recommend avoiding allergens during pregnancy or breastfeeding. Monthly surveys tracked adherence. Among those in the high-intake group, about 64 percent met the egg target and 67 percent met the peanut target across at least three-quarters of the surveys. The standard-diet group actually showed higher adherence to its own targets—73 percent for eggs and 80 percent for peanuts—suggesting the intervention created a real difference in consumption between the two groups.

When the infants reached approximately one year of age, researchers tested them for IgE-mediated allergy to egg and peanut using skin-prick testing and, when necessary, oral food challenges conducted in a hospital setting. The results were stark in their simplicity: 7.8 percent of infants in the high egg-peanut group developed allergy, compared to 8.4 percent in the standard-diet group. This difference was not statistically significant. Even when the analysis was restricted to women who breastfed for the full four months and adhered well to their assigned diet, the allergy rates were 5.6 percent and 6.8 percent respectively—still no meaningful difference. The pattern held across subgroups: birth order, household intake of these foods, and socioeconomic status made no difference to the outcome. Individual allergies to egg alone or peanut alone showed the same null result. Serious adverse events were rare and similar in both groups.

The study's null finding was not entirely surprising to the researchers, who noted a significant limitation: the actual prevalence of egg or peanut allergy in their population turned out to be only about 8 percent, roughly half the 16 percent they had anticipated when designing the trial. This lower-than-expected rate likely reflects a shift in infant feeding practices following updated Australian guidelines in 2016 that recommend introducing eggs and peanuts directly into infants' diets around six months of age. The confidence interval the researchers calculated does not completely rule out a clinically important 30 percent reduction in risk, but the smaller number of allergies overall meant the study had less statistical power to detect such a benefit if one existed.

The implications point in a specific direction. The authors concluded that modifying maternal diet during pregnancy and breastfeeding neither reduced nor increased the risk of food allergy in high-risk infants. Instead, the evidence continues to support what current guidelines already recommend: introducing eggs and peanuts directly into the infant's diet around six months of age, during the window when the infant's own immune system appears most receptive to establishing tolerance. The shift in thinking over the past decade has been profound. Decades ago, the conventional wisdom held that delaying exposure to allergens might prevent allergies. That approach failed. The newer approach—early introduction—appears to work better, at least for these common allergens. What a pregnant woman eats, it seems, matters less than what her child eats once solid foods begin.