In July, a study was published in Pediatrics, the official journal of the American Academy of Pediatrics, which showed that children who suck their thumbs or bite their nails are less likely to have atopic sensation in childhood and adulthood. The Dunedin Multidisciplinary Health and Development study followed 1,307 participants from birth to age 38, completing follow-up assessments at ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32 and 38. Skin-prick testing occurred at ages 13 and 32.1
Results of the study showed that 31% of participants sucked their thumbs, bit their nails or both as children. Of the 724 participants who had skin-prick tests at age 13, the prevalence of sensitization showed significant changes depending on the oral habit(s) possessed by each participant: 49% with no oral habit showed atopic sensitization, as opposed to 38% with one oral habit and 31% with both habits.1
The Hygiene Hypothesis
While the Dunedin study may change the way that healthcare professionals think about allergies, it’s not the first study of its kind. In fact, the researchers were motivated by the “hygiene hypothesis,” which David P. Strachan suggested in 1989 would explain why children from large families or those with older siblings were less likely to develop hay fever. Strachan theorized that this phenomenon could be explained if “allergic diseases were prevented by infection in early childhood transmitted by unhygienic contact with older siblings, or acquired prenatally from a mother infected by contact with her older children.”2
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Similarly, in 2013, another study found that infants whose mothers “cleaned” their pacifiers by sucking them clean were less likely to develop asthma and eczema. The researchers of this study concluded that the exchange of maternal oral bacteria might protect against atopy, another finding that inspired the Dunedin study.3
Research strongly defends the hygiene hypothesis, and for good reason. According to Suman Golla, MD, associate professor of otolaryngology at the University of Pittsburgh School of Medicine, explained that exposure to allergens at an early age may lead to a shift in the balance of type 1 helper T cells (Th1), which are geared towards fighting off infection, and type 2 helper T cells (Th2), which increase the likelihood that the body will develop Immunoglobin E (IgE). “Anything that can alter that shift would be desirable,” she explained.
Altering Beliefs About Allergies
Research-based defense of the hygiene hypothesis directly contradicts what many healthcare professionals and parents alike previously believed regarding prevention of allergies. “We always thought that if you’re around very clean environments, you will have less disease and therefore, in the future, fewer health problems,” Golla said. “But now, with the hygiene hypothesis, the shift is towards increasing children’s exposure to antigens and infections and even allergenic foods at an earlier age.”
However, not all allergic predispositions arise due to being raised in sterile environments. If one parent is genetically predisposed to atopic sensitization, there is approximately a 30% chance that his child will also be sensitized. If both parents have allergies, that percentage jumps to 60-70%.
Helping children build a defense against allergies – even those to which they are genetically predisposed – goes beyond allowing thumb-sucking and nail-biting. “Introducing highly allergenic foods gradually can be helpful, including peanuts and even shellfish,” Golla recommended. “I wouldn’t have children eat shellfish first, but after they’ve had some of the more non-allergenic food, then introducing more dairy, eggs, peanuts and shellfish could be beneficial. The paradigm is changing to introducing allergens earlier so perhaps children will develop immunity to these agents in the future.”
An Unshakeable Theory?
While the hygiene hypothesis has been defended by several studies, Golla explained that it still has some major holes. In inner cities, where children often live in unclean environments among many other people, the rate of allergies is increasing. If the hygiene hypothesis were entirely true, the rate of allergies in inner cities would be extremely low.
With the holes in the hygiene hypothesis, further research must be done before professionals can have a clearer understanding of the true causes of allergies. “We need to study how children develop allergies and how their symptoms are manifested in years to come,” Golla said. “Long-term studies, like the recent study on thumb-sucking and nail-biting, are crucial, but there aren’t many about the hygiene hypothesis.”
Perhaps in the future, another long-term study will finally provide the explanation for the issues with the hygiene hypothesis. For now, though, it is just that: a hypothesis.
Sarah Sutherland is a staff writer at ADVANCE. Contact: [email protected].
References
1. Lynch, S., et al. Thumb-sucking, nail-biting, and atopic sensation, asthma, and hay fever. Pediatrics. 2016;138(2):1-8. doi: 10.1542/peds.2016-0443.
2. Strachan, D.P. Hay fever, hygiene, and household size. BMJ. 1989;299(6170):1259-1260.
3. Hesselmar, B., et al. Pacifier cleaning practices and risk of allergy development. Pediatrics. 2013;131(6):1-11. doi: 10.1542/peds.2012-3345.
