Newly published research found that children who are thumb-suckers or nail-biters are less likely to develop atopic sensitization or allergic sensitivities (as measured by positive skin-prick tests to common allergens). And, if they have both 'habits', they are even less likely to be allergic to such things as house dust mites, grass, cats, dogs, horses, wool, or airborne fungi. The finding emerges from a longitudinal study which followed the progress of 1,037 persons born in Dunedin, New Zealand in 1972-1973 from childhood into adulthood. However, the researchers found no relationship to these 2 habits to allergic asthma or "hay fever" - a contradictory finding that the researchers don't have an answer for.
"Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies," said Professor Sears (one of the researchers). The researchers were testing the idea that the common childhood habits of thumb-sucking and nail-biting would increase microbial exposures, affecting the immune system and reducing the development of allergic reactions also known as atopic sensitization. 31% of the children were frequent thumb suckers or nail biters.
Among all children at 13 years old, 45% showed atopic sensitization, but among those with no habits 49% had allergic sensitization; and those with one oral habit - 40% had allergic sensitization. Among those with both habits, only 31% had allergic sensitization. This trend continued into adulthood, and showed no difference depending on smoking in the household, ownership of cats or dogs; or exposure to house dust mites.
Excerpts of the study from Pediatrics: Thumb-Sucking, Nail-Biting, and Atopic Sensitization, Asthma, and Hay Fever
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. Parents reported children’s thumb-sucking and nail-biting habits when their children were ages 5, 7, 9, and 11 years. Atopic sensitization was defined as a positive skin-prick test (≥2-mm weal) to ≥1 common allergen at 13 and 32 years.
Thirty-one percent of children were frequent thumb-suckers or nail-biters at ≥1 of the ages. These children had a lower risk of atopic sensitization at age 13 years and age 32 years. These associations persisted when adjusted for multiple confounding factors. Children who had both habits had a lower risk of atopic sensitization than those who had only 1. No associations were found for nail-biting, thumb-sucking, and asthma or hay fever at either age.
What This Study Adds: Children who sucked their thumbs or bit their nails between ages 5 and 11 years were less likely to have atopic sensitization at age 13. This reduced risk persisted until adulthood. There was no association with asthma or hay fever.
The “hygiene hypothesis” was suggested by Strachan1 to explain why children from larger families and those with older siblings are less likely to develop hay fever. Strahan hypothesized that this 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.” The hypothesis is supported by evidence showing that children who grow up in large families are at greater risk of coming into contact with more infections....The hygiene hypothesis remains controversial, however, as it is unable to fully explain many associations, including the rise of allergies in “unhygienic” inner-city environments, and why probiotics are ineffective at preventing allergic diseases.3
Thumb-sucking and nail-biting are common oral habits among children, although the reported prevalence varies widely, from <1% to 25%.4–7 These habits have the potential to increase the exposure to environmental microorganisms, and have been associated with the oral carriage of Enterobacteriaceae, such as Escherichia coli and intestinal parasite infections.8–12 It seems likely that thumb-sucking and nail-biting would introduce a wide variety of microbes into the body, thus increasing the diversity of the child’s microbiome. If the hygiene hypothesis is correct, it is plausible that this would influence the risk for allergies....
Of 1013 children providing data, 317 (31%) had ≥1 oral habit: there was no significant sex difference in prevalence of these habits. Of the 724 children who had skin-prick tests at age 13 years, 328 (45%) showed atopic sensitization. The prevalence of sensitization was lower among children who had an oral habit (38%) compared with those who did not (49%) (P = .009). The lower risk of atopic sensitization was similar for thumb-sucking and nail-biting. Children with only 1 habit were less likely to be atopic (40%) than children with no habit at all (49%), but those with both habits had the lowest prevalence of sensitization (31%) .