Infants who suck their thumbs or bite their nails appear less prone to allergies as adults, New Zealand researchers have found.
Their findings support the theory that early-life exposure to microbial organisms reduces the risk of developing allergies, Otago University scientists concluded.
A study, published this week in the US journal Pediatrics, recorded the thumb-sucking and nail-biting habits of 1,037 children when they were aged five, seven, nine and 11 years old.
Researchers then followed up by giving them allergy skin prick tests when they were aged 13 and 32.
They found 49 percent of 13-year-olds who did not suck their thumb or bite their nails tested positive to at least one allergy, compared to 38 percent who practised one of the habits.
The allergy level fell to 31 percent for children who did both.
The findings remained the same when participants were 32-years-old, regardless of factors such as parental history of allergies, pet ownership or being breast fed.
“(It) suggests that being exposed to microbes as a child reduces your risk of developing allergies,” lead researcher Bob Hancox said.
However, despite the laboratory skin-test results, the researchers said there was no evidence the habits reduced the risk of “real world” diseases linked to allergies.
“Although thumb-suckers and nail-biters had fewer allergies on skin testing, we found no difference in their risk for developing allergic diseases such as asthma or hayfever,” they said.
They added: “(We) do not suggest that children should be encouraged to take up these habits, because it is unclear if there is a true health benefit.”