Study: Thumb sucking, nail biting may keep allergies at bay

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Linus may have been onto something.
Children who sucked their thumb in elementary school  like the “Peanuts” comic strip character  were less likely to suffer from allergies as teenagers and adults.
The study, which analyzed a data set of just more than 1,000 subjects in Dunedin, New Zealand, also found a correlation between nail biting and fewer allergies in adolescence and adulthood.
The results of the study do add credence to the “hygiene hypothesis” the idea that the children who come into contact with more bacteria may develop more robust immune systems, which are able to ward off allergies.
“It adds more data to the thought that exposures to bacteria and other organisms may protect from allergic diseases,” said Alison Morris, director of the University of Pittsburgh Center for Medicine and the Microbiome. “I’m not going to go recommend that kids should suck their thumb, but there is this sense that we don’t need to be as sterile.”
Thumb sucking and nail biting, it should be noted, can cause dental problems, injuries to the gums and local hand infections.
The hygiene hypothesis has developed through previous studies that have found decreased incidences of allergies among children who have dogs, older siblings, or who live on a farm, said Allison Freeman, an allergist with Allegheny Health Network.
A recent study found that children whose parents clean their pacifiers by sucking on them are less likely to develop asthma or eczema.
The thumb-sucking study analyzed a cohort of data in which babies born in the early 1970s were followed for 38 years. When the children were 5, 7, 9 and 11 years old, their parents were asked whether they sucked their thumbs or bit their nails. At ages 13 and 32, most of the study participants were skin-pricked to test for allergens such as dust mites, grass, cats, dogs and wool. They were not tested for food allergies.
At age 13, 40 percent of children who either sucked their thumb or bit their nails showed evidence of at least one allergy, compared with 49 percent of children who did not have an oral habit. Children who did both  sucked their thumbs and bit their nails — had an even lower allergy prevalence, at 31 percent.
The lower rates of allergies were still present when the tests were done on the subjects at age 32, and were still statistically significant even when the authors controlled for other factors known to affect allergies, such as pet ownership, breastfeeding, parental allergies and parental smoking.
Thumb sucking and nail biting did not have a significant effect, however, on hay fever and asthma.
The hygiene hypothesis also has its detractors, the study notes, pointing out high allergy rates in children in “unhygienic” urban environments and the ineffectiveness of probiotics in preventing allergies.
Recommendations from doctors on introducing allergic foods has changed dramatically in recent years. While the American Academy of Pediatrics once urged parents to wait until their children were 2 years old to introduce peanuts, it now recommends giving them shortly after solid foods are introduced, before age 1  on the theory that multiple, early exposures build up the immune system.
Similarly, the idea of introducing bacteria early and often reflects a change in medical thinking.
“Maybe it’s not a sea change but it’s certainly a different approach,” said Dr. Freeman, who is also a clinical assistant professor of pediatrics at Temple University. “We went through this phase where we were building these bubbles of a sterile environment.”
Dr. Freeman recommends that parents de-emphasize antibiotics and anti-bacterial soap as much as possible.
“Kids love to run to the hand sanitizer and use it 40 times a day,” she said. “The baby does not need to sterilize their hands every time they sit on the floor or play in the grass outside.”

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