The surprising upside to childhood thumb-sucking and nail-biting

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Grimy fingers in the mouth leads to decreased risk of developing allergies, new study suggests.

The war on germs comes from an understandable place – much of history has been riddled with disease and squalor; combatting the microbes responsible for illness and death is a good thing. But as western culture manages to do so well, we’ve gone way overboard. To the point that, according to the hygiene hypothesis, we invite an increase of allergies by limiting exposure to dirt and germs. Our immune systems need things to fight in order to properly function; in the absence of dirt and germs it turns to allergens, which has likely led to the increase in allergic diseases.

Which is why it makes sense that a new study has found that kids who put their grubby little fingers in their mouths in order to suck their thumbs or bite their nails are less likely to be allergic to things like dust mites, grass, cats, dogs, horses or airborne fungi.

And, if they happen to suck their thumbs and bite their nails, they are even less likely to have allergic diseases.

"Our findings are consistent with the hygiene theory that early exposure to dirt or germs reduces the risk of developing allergies," said Malcolm Sears, from McMaster University's Michael G. DeGroote School of Medicine. "While we don't recommend that these habits should be encouraged, there does appear to be a positive side to these habits."

The study, which was conducted by Sears and researchers from New Zealand's Dunedin School of Medicine, specifically aimed to test the idea that 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.”

The researchers measured the incidence of the two habits in a longitudinal birth cohort of more than 1,000 New Zealand children at ages 5, 7, 9 and 11; and atopic sensitization was measured by skin-prick testing at 13 and 32 years old.

At the 13-year testing, 45 percent of all the participants showed atopic sensitization, but among those with one oral habit, only 40 percent had allergies. And for those with both habits, only 31 percent had allergies. This trend lasted into adulthood.

The new findings are "intriguing," Dr. Alison Morris, a professor of medicine in the division of pulmonary, allergy and critical care at the University of Pittsburgh and director of the university's Center for Medicine and the Microbiome, tells Today. "If parents can't get their kids to stop sucking their thumbs, this may make them feel better about that," Morris says. "But I don't think the study offers anything actionable at this point other than to be more relaxed about children's exposures to germs."

Which seems to be the takeaway here. For instance, handwashing is important, but antibacterial products can do much more harm than good. We need to be smart about what are kids are exposed to – like, don't lick the raw chicken, honey – but it's time to bid adieu to the obsession with eliminating germs entirely. We are so privileged to have good sanitation – we should respect that, for sure, but we should also remember that a grimy finger in the mouth isn't always such a bad thing. And in fact, may be beneficial.

The surprising upside to childhood thumb-sucking and nail-biting
Grimy fingers in the mouth leads to decreased risk of developing allergies, new study suggests.

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