Nail Biting, Thumb Sucking Tied to Lower Allergy Risk

Beth Skwarecki

July 12, 2016

Children who sucked their thumbs or bit their nails while in grade school were less likely than their peers to be sensitized to common allergens as teenagers and adults. The habits did not appear to affect asthma and hay fever risks. The results come from a population-based birth cohort study out of New Zealand.

"Our findings lend support to the hygiene hypothesis that avoiding oral environmental microbial exposures increases the risk for allergic sensitization to inhaled allergens," Stephanie Lynch, from the Dunedin School of Medicine at the University of Otago in New Zealand, and colleagues write in the study, published online July 11 in Pediatrics.

The 1037 children in the study were born in 1972 and 1973 in or near Dunedin, a coastal city in New Zealand surrounded by rural areas. Most were ethnically New Zealand European, and the authors note that the cohort "represents the full range of socioeconomic status" in the area.

At follow-up visits from ages 5 to 11 years, parents answered questions about their child's thumb-sucking and nail-biting behavior. Allergy sensitivity testing was done at age 13 years in 70% of participants and at age 32 in 93%, using a skin-prick test for house dust mite, grass, cat, dog, horse, kapok, Aspergillus fumigatus, Alternaria, Penicillium, and Cladosporium. The test for 32-year-olds also included cockroach allergens.

The 31% of children who sucked their thumbs or bit their nails frequently, according to parents, were less likely to show sensitization to at least one allergen at age 13 years (odds ratio, 0.67; 95% confidence interval, 0.48 - 0.92; P = .013) and age 32 years (odds ratio, 0.61; 95% confidence interval, 0.46 - 0.81; P = .001) compared with their peers who did not bite nails or suck thumbs.

The investigators adjusted for factors known to be associated with atopy, including whether the children were breast-fed for at least 4 weeks, presence of a cat or dog in the home, and parental history of atopy. They also adjusted for household crowding, socioeconomic status, and parental smoking.

Self-reported hay fever and asthma were not associated with thumb sucking or nail biting (P = .8 for asthma, P = .9 for hay fever) at age 13 years. The authors note that the development of asthma is thought to be less closely related to immune function than atopy, and that only one third of childhood asthma appears to be related to atopy.

If future studies confirm the link, the authors write, thumb sucking and nail biting could be considered to have a long-term benefit in preventing allergies. At this time, however, the authors write that they "do not suggest that children should be encouraged to take up these oral habits," citing concerns about dental development and possible hand infections.

One of the authors holds the AstraZeneca Chair in Respiratory Epidemiology at McMaster University. The other authors have disclosed no relevant financial relationships.

Pediatrics. Published online July 22, 2016. Full text

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