Thumb-Sucking Could Prevent Allergies, Study Finds

Worried about your child's thumb-sucking habit? A new study suggests that the oft-discouraged behavior may have long-term health benefits: In particular, children who suck their thumbs or bite their nails appear to have lower rates of allergies later in life.

Why is this important? First, it supports the hygiene hypothesis, which holds that the increase in allergic conditions found in wealthy countries is due to low rates of exposure to microbial organisms early in life because of an excessively "clean" environment.

"It seems likely that thumb-sucking and nail-biting would introduce a wide variety of microbes into the body, thus increasing the diversity of the child’s microbiome," the authors write. "If the hygiene hypothesis is correct, it is plausible that this would influence the risk for allergies."

Second, it gives parents some reason to ease up on young kids who might self-soothe by sucking on their thumb or biting their nails.

The study was conducted by Stephanie Lynch of the Dunedin School of Medicine, Malcolm Sears of Firestone Institute for Respiratory Health and Robert Hancox of McMaster University, and it was published in the highly respected medical journal Pediatrics.

To carry out the research, the authors investigated the effects of thumb-sucking and nail-biting over the lives of more than 1,000 individuals involved in the Dunedin Multidisciplinary Health and Development Study, which has followed its subjects since 1972.

Parents of the subjects were asked about their child's thumb-sucking and nail-biting habits at ages 5, 7, 9 and 11. These same kids were then tested for allergic sensitivity at age 13 and again at age 32.

Researchers used the "skin-prick test," which exposes individuals to small levels of common allergens like dust mites, wool, dog and cat dander, and grass and measures any reaction. Individuals who sucked their thumb, bit their nails or displayed both behaviors at one or more of the ages when the researchers asked, were around 30 to 40 percent less likely to develop these kinds of allergies later in life. (However, the researchers also measured rates of asthma and hay fever, and they found no evidence that thumb-sucking or nail-biting protected against these conditions.)

According to the the hygiene hypothesis, the increased exposure to potential environmental allergens as a result to the oral behaviors reduced the children's immune sensitivity, leading to lower rates of allergies.

Dr. Claire McCarthy, writing for the Harvard Health Blog, thought this was a plausible interpretation of the study's findings.

"[The hygiene] hypothesis doesn’t explain everything we see about allergies and other examples of the body attacking itself, but it certainly may play a role," she writes. "[W]hen kids suck their thumbs or bite their nails, they do put all sorts of new germs into their mouths and therefore their bodies."

This, of course, may be a primary reason parents discourage the behavior — they want their kids to avoid germs. But a growing body of evidence suggests that there's such a thing as being "too clean," and we know that some bacteria are essential to health. For example, a 2013 study found that infants whose parents cleaned their children's pacifiers with their own mouths were also less likely to develop allergies.

Thumb-sucking can cause problems that parents should be aware of. The American Dental Association warns against kids sucking their thumbs past the age of 4, because the behavior can affect tooth development. And obsessive nail-biters can do damage to their fingers, which should certainly be prevented.

Generally, most kids will stop these behaviors without much intervention, and McCarthy only recommends positive reinforcement for avoiding the behavior, if guidance is required at all.

"The bottom line is that parents should take a deep breath and try not to worry," she says. "Not only are the habits likely to either go away by themselves or not cause any problems, they may even help your child’s health!"

The study used sophisticated statistical controls to rule out other potential causal factors that could confound the studies findings, such as correlations between the child's gender and economic circumstances, but the results proved robust. The authors also think reverse causation, that children with low rates of allergies would be more likely to engage in oral behaviors, is unlikely.

While the experiment was not a randomized controlled trial, usually considered the most reliable type of study, these considerations add to the weight of the study's findings. Nevertheless, the authors recommend further research efforts pursue these lines of questions.

"[T]he findings suggest that thumb-sucking and nail-biting reduce the risk for developing sensitization to common aeroallergens," the authors note.

But if now you're worried because your child doesn't suck his or her thumb, they also add, "[W]e do not suggest that children should be encouraged to take up these oral habits."

Photo Credit: Rikrok Yu Castro via Flickr (edited)