Excerpted from The End of Food Allergy by arrangement with Avery Books, a member of Penguin Group (USA) LLC, A Penguin Random House Company. Copyright © 2020, Kari Nadeau, MD, PhD, and Sloan Barnett

Less justified, though, is the guilt that many parents suffer when a child tests positive. Did they inherit it from me? What should I have done differently? How could I have stopped this? Why did I have to crave eggs so much?

For all that we don’t know about food allergy, we have enough evidence to assuage the blame. And we have enough information to help parents make sensible changes where and when it’s possible and to exercise appropriate caution as their infants move from breast milk or formula to solid foods.

Do Genes Matter?

The answer to this first and most obvious question is yes and no. Studies over the past two decades have illuminated the hidden ways in which our DNA may make us more prone to food allergy. But the findings aren’t conclusive. There’s no such thing as a “food allergy gene,” and as to those genes that do seem to influence the risk, we aren’t sure how or when they come into play. Still, knowing whether a food-allergic parent is more or less likely to have a food-allergic child—and knowing what genetic entanglements are completely random—may help families prepare and cope with a diagnosis.

Many studies have tackled the question of inheritance. In an early attempt to figure out whether peanut allergy was inherited, research led by Scott Sicherer at the Icahn School of Medicine at Mount Sinai in New York recruited 58 pairs of twins among whom at least one member had a confirmed peanut allergy. Because some of the pairs shared all their genes (identical twins), some shared just half (fraternal twins), and the environment was the same for each pair, the researchers could calculate the hand that DNA had in the allergy. To do this, they used a statistic known as the concordance rate, which calculates the likelihood of both members of a twin pair having the same trait. By comparing the concordance rate of the identical twins to that of the fraternal twins, the researchers could figure out the likelihood that peanut allergy was inherited. The study estimated that peanut allergy was about 82 percent inheritable. The genetic influence, the researchers concluded, was “significant.” More recently, a group of Swedish researchers looked at data on asthma, eczema, and food allergy from more than 25,000 Swedish twins at age 9 or 12 years. “Asthma and allergic diseases of childhood are highly heritable,” the 2016 paper stated.

Another way to find out whether food allergy is inherited is to look at entire families. Does having a parent or sibling with food allergy make it likelier that you will, too? A HealthNuts study looked at just that. Among more than 5,200 infants, those with one immediate family member who had food allergy were just a bit more likely to have the same condition than infants without that tie. But if two or more family members had food allergy, the infant’s risk spiked. The researchers also uncovered some unexpected connections. Mothers who had battled asthma or eczema in their lives and siblings who’d had hay fever were each linked to egg allergy in the infant. Mothers and fathers with a history of asthma and hay fever were more likely to have infants with peanut allergy.

The heritability of food allergy is still being studied. We know that peanut allergy can be passed down from grandparent or parent to child, but we also know that this doesn’t always happen. We know that food allergy may cluster within families, but we also know that just because one sibling has this disease, that doesn’t mean the others will. However, the presence of food allergy within a nuclear family gives parents a reasonable starting point for caution.

The First Environment

The mixing and mingling of parental DNA may explain some newborn food allergy. But there may be other routes—namely, via the umbilical cord. Many new mothers begin worrying about the health of their babies from the moment the pregnancy test comes out positive. Out goes the caffeine, in come heaps of leafy greens. And as food allergy has become more common, so has the tendency to eliminate certain foods from pregnancy diets. Paranoia and too much Googling could lead many mothers to wonder if their nine-month cheese habit led to their baby’s milk allergy.

The recent history surrounding the link between pregnancy diet and infant food allergies is entangled with the history of food allergy research as a whole. As our understanding of this disease has evolved, so has our thinking about what mothers should or shouldn’t consume. For parents suffering from guilt, or pregnant women panicking in the supermarket, the facts offer a sigh of relief.

In 2000, the American Academy of Pediatrics recommended that pregnant women avoid peanuts as a way to reduce the risk of peanut allergy in the growing fetus. That recommendation came in the form of a single sentence at the end of a publication focused on infant formula and breastfeeding. “No maternal dietary restrictions during pregnancy are necessary,” the Academy stated, “with the possible exception of excluding peanuts.” But that was enough to send many expectant women fleeing from the peanut butter aisle. The notion that infants could develop food allergy based on what mothers ate while pregnant seeped into our collective consciousness, giving women one more worry, one more cause for guilt, one more reason to think they weren’t doing it right.

Hints that maternal diet may not be the allergy driver many women came to fear already existed. In the 1980s, two Swedish researchers looked for atopy among infants whose mothers had avoided milk and eggs during pregnancy. Starting at halfway through their pregnancies, 212 women were randomized to either eat or abstain from these foods. After their babies were born, many of the newly breastfeeding mothers assigned to the allergen- free diet decided to keep their intake of milk and eggs low. But it turned out that allergies—eczema, asthma, hay fever, and food reactions— were equally common in both groups. Maternal diet, it seems, had little impact on the risk of food allergies.

A shift began at the start of the twenty-first century when a group of British researchers turned to a massive bank of data for some better intelligence. Their 2003 study, published in The New England Journal of Medicine, found no link to prenatal diet among children with peanut allergy. In 2008, the American Academy of Pediatrics reversed course. Studies, the Academy pronounced, “have not supported a protective effect of a maternal exclusion diet.” The publication offered a clear summary: there’s no persuasive evidence in favor of eliminating potential allergens from the prenatal diet.

All this research on the link between pregnancy diets and food allergy shows one fact very clearly: science isn’t always clear-cut. More often than not, the results are subtle or contradictory. It can be difficult for even the most seasoned expert to tease out the truth. We offer all this research not to confuse you but rather to equip you with the information.


Book cover for The End of Food Allergy by Kari Nadeau and Slaon Barnett

Excerpted from The End of Food Allergy by arrangement with Avery Books, a member of Penguin Group (USA) LLC, A Penguin Random House Company. Copyright © 2020, Kari Nadeau, MD, PhD, and Sloan Barnett

Liquid Lunch

This brings us to one of the first worries parents have once babies leave the womb: breastfeeding. We know that the nutrients from our diets seep into breast milk and thereby into our infants. So it’s logical for a new mother to wonder whether the newly developed food allergy in her toddler derived from the peanuts she fueled herself with on the go, the eggs she inhaled for breakfast, or some other component of her diet.

Studies have found that nursing reduces the risk for food allergy compared to formula. According to the American Academy of Pediatrics, breastfeeding for at least four months reduces the risk of eczema and milk allergy for the first two years of life. But ultimately the decision of whether to nurse or use formula is personal.

With regard to whether the diet a nursing mother consumes has any bearing on her child’s likelihood of food allergy, the trajectory of the research has been much the same as for the prenatal months. After earlier suggestions to avoid risky foods while lactating, medical authorities changed their views. Avoiding some food allergens may help prevent eczema in children, and some data point toward avoiding peanuts when babies are at high risk because the allergy is in the nuclear family. But taken as a whole, the data don’t show that the mother’s diet matters much in the quest to prevent food allergy.

What About Formula?

Parents who choose to feed their newborns formula may feel stigmatized in light of the current recommendations to breastfeed exclusively for at least four months. However, when it comes to food allergy, the evidence that formula feeding increases the risk is unpersuasive. And there is no proof that so-called hypoallergenic formulas prevent food allergy. Parents giving their infants formula should consult their pediatricians or allergists if there is a family history of food allergy and if they are concerned about what product to use, or any other issue surrounding feeding during the first few months of life.

The Effect of Cesarean Section on Food Allergy

Many parents of food-allergic children wonder whether C-sections increase the risk of food allergies. The answer is far from clear. Putting reasons for choosing birth by cesarean aside, parents hoping to avoid food allergy in their children may want to consider the evidence surrounding the link between the two.

There is no cause for blame or shame here. Cesarean sections save lives. Mothers of children who were born by this method and who have food allergy do not need to feel guilty. As we will see, the evidence is not conclusive. And in the studies that have shown this link, the data are not that stark. Still, a body of research has emerged over the past decade indicating that the risk of food allergy could be increased by cesarean section birth. And the research also touches some of the central questions surrounding food allergy— namely, how microbes, including beneficial bacteria, may protect against the condition.

Understanding why cesarean delivery could increase the risk of food allergy means returning to the gut microbiome. The correlation between allergy and the human microbiome may begin at birth. Several studies have found that allergic and nonallergic infants have different gut flora. In one Swedish study, children with food allergy had higher amounts of a bacterial species known as Staphylococcus aureus and lower amounts of Bacteroides and bifidobacteria, two other types. In a study from 2001, children with food allergy again had lower amounts of Bacteroides and bifidobacterial throughout their first year of life. That same year, a Finnish study probed even deeper. The researchers wanted to know if differences in the composition of gut flora in infants preceded the onset of allergies to egg and milk. “Neonatal intestinal microflora changes precede the development of atopic sensitization during infancy,” the authors wrote. In other words, the differences in gut flora arose before the allergy. That meant that variations in the gut microbiome may do more than correlate with food allergy— they could actually cause it.

What does all this have to do with birth by C-section? In the many studies unpacking the links between gut flora and allergy, two types— Bacteroides and bifidobacteria— pop up again and again. The more of these bacteria colonizing the gut, the less likely a child is to have an allergic disease. C. difficile (bacteria often associated with hospital-borne illnesses) also makes frequent appearances in the scientific literature, for the opposite reason— more C. diff equals more allergy. And it turns out that infants born by cesarean are more likely to have gut microbiomes with this exact profile: fewer Bacteroides, fewer bifidobacteria, and more C. difficile. By contrast, infants born vaginally are colonized exactly in reverse. We have enough data to know that gut flora differs between infants born by C-section and those born vaginally, at least for a period of time. It also appears that these differences fade after not too long, especially after nursing is finished.

Let’s be clear: Mothers who had C-sections should not blame themselves for their child’s food allergy. There is no evidence that this delivery route causes the condition on its own. C-sections are lifesaving procedures for many women and their newborns, and there should not be any guilt attached to having one. Parents who are concerned that their future child may be at high risk for food allergy due to family history should speak with their obstetricians and allergists about how a C-section delivery may play into that risk and what to do about it.

For families coping with a new diagnosis, it may also help to know that children may outgrow their food allergy. A study by David Fleischer at the University of Colorado found that people with peanut allergy had a 50 percent chance that the condition would disappear. Other studies have reported rates closer to 20 percent. Predicting which children may outgrow their allergies is currently not possible. However, we do know that many children who are allergic to eggs early in life are later able to tolerate cooked eggs. Children who eat baked goods containing eggs and dairy may be more likely to outgrow these allergies without any intervention, though it’s not common and there’s no guarantee. No one diagnosed with food allergy should ever just try a bit of the allergen to see if it’s no longer a threat. For people who develop food allergy as adults, the immune system is unlikely to overcome the condition without medical intervention. Food allergy may also recur after it appears to have faded away.

References and Resources

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Ullemar V, Magnusson PK, Lundholm C, et al. Heritability and confirmation of genetic association studies for childhood asthma in twins. Allergy. 2016;71(2):230–38.

Koplin JJ, Allen KJ, Gurrin LC, et al. The impact of family history of allergy on risk of food allergy: a population-based study of infantsInt J Environ Res Public Health. 2013;10(11):5364 –77.

Hourihane JO, Dean TP, Warner JO. Peanut allergy in relation to heredity, maternal diet, and other atopic diseases: results of a questionnaire survey, skin prick testing, and food challengesBMJ. 1996;313(7056):518–21. 

Greer FR, Sicherer SH, Burks AW; American Academy of Pediatrics Committee on Nutrition; American Academy of Pediatrics Section on Allergy and Immunology. Effects of early nutritional interventions on the development of atopic disease in infants and children: The role of maternal dietary restriction, breastfeeding, timing of introduction of complementary foods, and hydrolyzed formulasPediatrics. 2008;121(1):183–91. 

Falth-Magnusson K, Kjellman NI. Development of atopic disease in babies whose mothers were receiving exclusion diet during pregnancy – a randomized study. J Allergy Clin Immunol 1987;80(6):868–75.

Lack G, Fox D, Northstone K, Golding J. Factors associated with the development of peanut allergy in childhoodN Engl J Med. 2003;348(11):977–85. 

Greer FR, Sicherer SH, Burks AW. The effects of early nutritional interventions on the development of atopic disease in infants and children: The role of maternal dietary restriction, breastfeeding, hydrolyzed formulas, and timing of introduction of allergenic complementary foods. Pediatrics. 2019;143(4).

Muraro A, Dreborg S, Halken S, et al. Dietary prevention of allergic diseases in infants and small children. Part III: Critical review of published peer-reviewed observational and interventional studies and final recommendations. Pediatr Allergy Immunol. 2004;15(4):291–307. 

Bjorksten B, Sepp E, Julge K, et al. Allergy development and the intestinal microflora during the first year of lifeJ Allergy Clin Immunol. 2001;108(4):516–20.

Kalliomaki M, Kirjavainen P, Eerola E, et al. Distinct patterns of neonatal gut microflora in infants in whom atopy was and was not developingJ Allergy Clin Immunol. 2001;107(1):129 –134. 

Stinson LF, Payne MS, Keelan JA. A critical review of the bacterial baptism hypothesis and the impact of cesarean delivery on the infant microbiomeFront Med (Lausanne). 2018;5:135. 

Fleischer DM, Conover-Walker MK, Christie L, et al. The natural progression of peanut allergy: Resolution and the possibility of recurrenceJ Allergy Clin Immunol. 2003;112(1):183–89. 

Dhar M. Can you outgrow your allergies? 

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Dr. Kari Nadeau DrGreene.com contributor

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