Could thumb sucking and nail biting prevent allergies in later life?

This blog post was written by Dr Harish Nair.

Parents have long fretted about their children sucking thumbs and biting nails, worried that this would cause infections in young children. The “hygiene hypothesis” proposed in 1989 has been used often to explain (unconvincingly) the role of infections in early childhood for preventing allergies including asthma and hayfever. Therefore, the story in the Independent, and the Telegraph that children who suck their thumbs and bite their nails suffer fewer allergies is not really novel. This story is based on a paper published in the journal Paediatrics earlier this month. The study was conducted in Dunedin (New Zealand) based on 1037 participants from a birth cohort recruited in 1972 – 1973, and followed up at various stages until age 38 years.

The Research

The authors used parental recall at age 5, 7, 9 and 11 years to enquire about their child’s thumb sucking, nail biting and other oral habits. They conducted a skin prick test in 70% of study participants at age 13 (724 of 1031), and conducted questionnaire-based respiratory assessments at regular intervals from age 9, using self-reporting for asthma or hay fever to diagnose the outcome. The authors report that children who reported thumb sucking or nail-biting at age 13 and again at age 32 were less likely to react positively to a skin prick test for allergic sensitisation. However, this did not translate to these oral habits having a protective effect against asthma or hay fever (that cause a substantial burden on healthcare systems). The parental recall and self-reporting for disease are the major limitations in the study by Stephanie Lynch and colleagues.

How did media handle coverage of the study and its findings?

Both the Independent and the Telegraph as well as BBC reported the findings as reported in the paper. However, the Daily Mail headlines screamed that “Children who bite their nails or suck their thumb cut their risk of developing an allergy as an adult by a fifth”. In the text, this was then reported as the risk being reduced by a third. The research paper reported a 49% reduction in the risk for allergic reaction in children who reported either of these habits (compared to not having these habits at all). Someone should check the facts…

Bottom line

The hygiene hypothesis has evoked a lot of interest, sparking research on the role of microbiota (microorganisms in the gut) and potent physiological and immune “crosstalk” between distant organs in contact with the microbiota. Recent studies looking at the gut-lung axis have shown that bacteria in the gut affect the lungs’ immunological response to environmental challenges, such as allergens and infectious agents.

Diversity of gut bacteria have been shown to be related to asthma (in older children) and bronchiolitis (in young children). However, these studies are based on small sample sizes. What is needed is a prospectively designed birth cohort study that can take faecal samples at regular intervals, and other information including oral habits, details of respiratory illnesses in infancy and early childhood, and administer objective tests for allergies and asthma to provide more convincing evidence on this perplexing question. Until then, do not worry…be happy. Let your child be content with their oral habits (thumb sucking and nail-biting), and let their gut bacteria have some fun too.

Lynch, S. J. et al. (2016). Thumb-Sucking, Nail-Biting, and Atopic Sensitization, Asthma, and Hay Fever. Pediatrics. DOI: 10.1542/peds.2016-0443