Abstract
Background: The hygiene hypothesis suggests that early-life exposure to microbial organisms reduces the risk of developing allergies. Thumb-sucking and nail-biting are common childhood habits that may increase microbial exposures. We tested the hypothesis that children who suck their thumbs or bite their nails have a lower risk of developing atopy, asthma, and hay fever in a population-based birth-cohort followed to adulthood.
Methods: Parents reported children’s thumb-sucking and nail-biting habits when their children were ages 5, 7, 9, and 11 years. Atopic sensitisation was defined as a positive skin prick test (>2mm weal) to at least one common allergen at 13 and 32 years. Associations between thumb-sucking and nail-biting in childhood, and atopic sensitisation, asthma, and hay fever at these ages were assessed using logistic regression with adjustments for sex and other potential confounding factors: parental atopy, breastfeeding, pet ownership, household crowding, socio-economic status, and parental smoking.
Results 31% of children were frequent thumb-suckers or nail-biters at at least one age. These children had a lower risk of atopic sensitisation at age 13 years (OR=0.67 [95%CI: 0.48, 0.92], p=0.013) and age 32 years (OR=0.61 [95%CI: 0.46, 0.81], p=0.001). These associations persisted when adjusted for multiple confounding factors. Children who had both habits had a lower risk of atopic sensitisation than those who only had one. No associations were found for nail-biting, thumb-sucking and asthma or hay fever at either age.
Conclusion: Children who suck their thumbs or bite their nails are less likely to have atopic sensitisation in childhood and adulthood.