Could kids' thumb-sucking, nail-biting offer benefits?

(CNN) Thumb-sucking and nail-biting can cause health problems for kids and potential financial problems for parents paying for braces. Thumb-sucking can interfere with the alignment of children's teeth and nail-biting can increase the risk of spreading harmful germs from their fingers to their mouths.

Those two childhood habits, however, come with a surprising upside, according to a study published in the journal Pediatrics on Monday.

Children who suck their thumbs or bite their nails may have a lower risk of developing allergies, said Bob Hancox, a co-author of the study and an associate professor at the University of Otago's Dunedin School of Medicine in New Zealand.

"The study was done to test the hygiene hypothesis: the idea that reduced exposure to microbial organisms, in other words increased hygiene, is responsible for the rise in allergic diseases seen over recent decades," Hancox said. The hypothesis also has been described as including the use of antibiotics.

"We didn't know what to expect," he said. "We had hypothesized that we would find a reduced risk of allergies in children with these habits, but really had no idea whether this would turn out to be true."

Data from the U.S. Centers for Disease Control and Prevention show that food and skin allergies have increased among children in the United States in recent decades. The prevalence of food allergies jumped from about 3.4% in 1997 to about 5.4% now. The occurrence of skin allergies rose from 7.4% in 1997 to about 11.6% now.

Allergies -- from asthma to hay fever -- are the sixth leading cause of chronic illness in the United States, and more than 50 million people nationwide suffer from allergies each year, according to the CDC.

"Understanding why so many people have allergic disease is important," Hancox said.

For the new study, the habits and health of about 1,000 children born in New Zealand between 1972 and 1973 were tracked as they aged from about 5 to 32 years old.

When the children were 5, 7, 9 and 11, their parents were asked to complete questionnaires about their children's thumb-sucking and nail-biting habits. About 31% of the children were frequent thumb-suckers or nail-biters.

Then, when the children were 13 years old, the researchers administered a skin-prick test to determine whether they had allergies. Another skin-prick test was repeated when the participants were 32.

It turned out that children who were thumb-suckers, nail-biters or both had a 30% to 40% risk reduction in their chances of having allergies in childhood, which persisted in adulthood. No associations were found for nail-biting or thumb-sucking and asthma or hay fever.

In theory, the findings suggest that thumb-sucking and nail-biting may lead to a more diverse variety of environmental bacteria and other microbes entering the body, possibly boosting defense against developing allergies. However, that was not really tested.

"How exactly exposure to microbes alters immune function in this way is not known," Hancox said. "But it is suggested that increasing microbial exposure influences the immune system to fighting infections rather than developing allergies."

What do other experts think of the research?

The data would be much stronger if they showed a consistent strong correlation between thumb-sucking or nail-biting and allergies as well as asthma, after controlling for other known factors which affect these diseases, noted Dr. Purna Kashyap, a gastroenterologist at the Mayo Clinic who was not involved in the study.

"Overall, it is interesting but needs better data to support the conclusion," he said.