Children who suck their thumb or bite their nails are less likely to suffer from allergic conditions such as asthma or hay fever, according to research.
While both habits may cause parents anxiety, the study from New Zealand suggests both may enhance health, especially if a child does both things.
The apparent link is down to young thumbsuckers and nailbiters inadvertently increasing their exposure to microbes at an early age, which in turn boosts their immune system.
The findings, reported in the journal Pediatrics, conclude: “Children who suck their thumbs or bite their nails are less likely to have atopic sensitisation [sensitivity to allergens] in childhood and adulthood.”
The research seems to lend further weight to the hygiene hypothesis. This holds that people exposed to potentially harmful bacteria as young children are more likely to grow up able to resist them, whereas those reared in cleaner environments are more likely to suffer from allergic ailments such as eczema, asthma and hay fever.
“Early exposure in many areas is looking as if it’s more protective than hazardous, and I think we’ve just added one more interesting piece to that information,” Malcolm Sears, a co-author of the paper, told the New York Times.
The findings have emerged from the ongoing Dunedin multidisciplinary health and development study. It has been tracking the health of 1,037 people born in Dunedin, New Zealand, in 1972 and 1973 as they have grown up.
Sears and co-authors Stephanie Lynch and Robert Hancox asked the children’s parents about their offsprings’ thumb sucking and nail biting when they were five, seven, nine and 11 years old. The subjects were then given skin-prick tests when they reached 13 and 32 to see if they were allergic to common allergens such as house dust mites, grass and household pets, which can trigger allergic reactions.
Almost a third (31%) of the children analysed in the study were “frequent” participants in one or both habits at either age.
“These children had a lower risk of atopic sensitisation at age 13 and age 32 years. These findings persisted when adjusted for multiple confounding factors,” the study says. “Children who had both habits had a lower risk of atopic sensitisation than those who had only one. No associations were found for nail biting, thumb sucking and asthma or hay fever at either age.”
However, the authors stressed that their findings did not explain exactly how thumb sucking and nail biting had the potentially protective effect.
“Even if we assume that the protective effect is due to exposure to microbial organisms, we don’t know which organisms are beneficial or how they actually influence immune function in this way,” said Hancox.
