Nail-Biting and Thumb-Sucking in Childhood Might Prevent Allergies – July 15, 2016

Allergies & Asthma


According to a new study published in the Pediatrics journal, children who bite their nails or suck their thumbs are less likely to develop allergic sensitivities later in life. Those who have both habits have even a lower risk of developing allergies.

Nail-biting and thumb-sucking are common habits among children, often seen as undesirable and are discouraged by many parents. These habits can potentially increase exposure to microorganisms in the environment and have been linked to the spread of parasitic and bacterial infections such as intestinal parasites and Escherichia coli infections. It seems plausible that a wide variety of microorganisms would be introduced into the body through nail-biting and thumb-sucking, thus increasing the diversity of the collection of microorganisms that inhabit the child’s body.

The hygiene hypothesis proposes that a lack of exposure to microbial organisms in early childhood increases susceptibility to developing allergies by suppressing the natural development of the immune system. If the hygiene hypothesis holds true, it is possible that nail-biting and thumb-sucking habits would influence the risk for allergies. Currently, there is no evidence to suggest whether nail-biting and thumb sucking affect function of the immune system or allergy risk. As a result, a group of researchers examined the effect of nail-biting and thumb sucking during childhood on the development of asthma, hay fever, and atopic sensitization (allergic reactions) in children. They hypothesized that introducing microbes into the body via nail-biting and thumb sucking would decrease the development of atopic sensitizations and have an influence on the child’s immune system.

The researchers used data from the Dunedin Multidisciplinary Health and Development Study, a prospective longitudinal population-based birth cohort study of 1037 participants (52% males) born in Dunedin in 1972-1973, mainly of New Zealand European ethnicity, and followed to 38 years of age. Children’s nail-biting and thumb-sucking habits were reported by their parents when the children were 5, 7, 9, and 11 years old. Skin prick testing was conducted at 13 and 32 years of age; a positive skin prick test indicated atopic sensitization to one or more allergens. Associations between nail-biting and thumb-sucking in childhood, and atopic sensitization, hay fever, and asthma at these ages were evaluated considering potential confounding factors known to be related to atopic sensitization, including sex, pet ownership, breastfeeding, household crowding, parental smoking, socioeconomic status, and parental history of atopy or asthma.

The study, published in the Pediatrics journal, showed that 31% of children frequently bit their nails or sucked their thumbs. Moreover, the findings support the hypothesis that children who often bite their nails or suck their thumbs have a reduced risk of developing atopic sensitization. Compared to children who did not engage in any of these habits, those who did were less likely to have positive skin prick tests at 13 years of age and this protective effect remained until 32 years of age. The results of the first skin-prick tests revealed that 38 per cent of children who were either thumb-suckers or nail-biters had allergic sensitizations, compared with 49 per cent of children who had neither habit. These associations were not accounted for by sex, a parental history of atopy, or a range of environmental factors known to be related to atopic sensitization.

Additionally, at age 13 years, there also seemed to be a dose response relationship for atopic sensitization; there seemed to be an even lower incidence of atopic sensitization in children who engaged in both nail biting and thumb-sucking compared to those who had only one of the habits (38% vs. 31%). At age 32 years, however, this dose-response relationship was no longer evident. In contrast, the researchers did not find any link between nail-biting and thumb-sucking and asthma or hay fever at either age 13 or 32 years (or at any age). It is unclear what the reasons for this inconsistency are.

In conclusion, teenagers and adults who had the habits of nail-biting and thumb-sucking during childhood showed a lower prevalence of atopic sensitization. The findings suggest that the immune response and risk of allergies may have been influenced by exposure to bacteria or other microorganisms through engaging in these childhood habits. Moreover, they promote the hygiene hypothesis, in that keeping away from exposures to microbes increases the risk for allergic sensitization to inhaled allergens. While the researchers do not recommend that children should be encouraged to take up these habits, it is suggested by the study that these habits may reduce the risk for developing sensitization to common aeroallergens. Even though these habits were not found to influence asthma or hay fever, the reduction in atopic sensitization could possibly have long-term benefits to health. Further studies are needed on the long-term effects of these childhood habits.

Written By: Nigar Celep, BASc



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