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Pollen, Pets

Thumb-sucking and Nail-Biting May Prevent Some Allergies

Malcolm SearsDr. Malcolm Sears

We discourage our kids from “bad habits,” but a new study finds that children who suck their thumbs or bite their nails are less likely to develop to certain environmental allergies.

The study, published on July 11 in the medical journal Pediatrics, also finds that those who indulge in both “bad habits,” are even less likely to react to allergens including dust mites, cats, dogs, horses, grass or mold.

In long-running allergy and asthma research involving a New Zealand population group, Dunedin School of Medicine researchers have followed 1,000 people from childhood, with follow-ups at ages 13 and and 32. In the Pediatrics paper, they found that by the age of 13 and continuing into adulthood, about half of the people (49%) who didn’t suck their thumb or bite nails as children tested positive to environmental allergens in skin-prick tests.

However, for those who had had one of the two oral habits, the level of skin-test sensitization (which does not always develop into a full-blown allergy) dropped to 40%. And the researchers found the lowest level of positive tests – 31% – among those who had both bitten their nails and sucked their thumbs.

“Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies,” says Dr. Malcolm Sears, an epidemiologist, respiratory specialist and professor of medicine at McMaster University in Hamilton, Canada, who has been involved in the study since his days in New Zealand in the 1970s.

“While we don’t recommend that these habits should be encouraged, there does appear to be a positive side to these habits,” he says.

The research is also in keeping with a growing body of research showing that greater diversity of microbial exposure can lessen allergies. One similar 2013 study found that Swedish mothers who tended to suck their child’s pacifier as a “cleaning technique” reduced their baby’s odds of asthma and eczema. Other studies have shown that dogs introduce greater microbial diversity into the home and can have a role in allergy prevention.

The study authors note that even when they analyzed factors as diverse as gender, parental allergies, breastfeeding, pet ownership, parental smoking, household crowding and economic status, the associations between oral habits and atopic sensitization remained significant.

However, the study did not find a direct link between the oral habits and onset of asthma or hay fever.

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