Thumb sucking and nail-biting early in life may reduce allergies later in life.

Linus’ thumb-sucking habit just might reduce his risk of common allergies. An article in Pediatrics found that kids who had their fingers in their mouths at ages 5, 7, 9, and 11 years old were less likely to have a reaction to a skin prick test for allergies later in life (ages 13 and 32 years) [1]. Study participants were pricked with common allergens including: house dust mites, grass, cat, dog, horse, wool, and several fungi at ages 13 and 32. Redness and swelling of the skin indicated a sensitivity to the allergen.  Kids who were both thumb-suckers and nail-biters were least likely to react to the skin test, though thumb-sucking only seemed to have a stronger effect than nail-biting alone.  Sucking or biting on their fingers seemed to lower kid’s allergy levels, even after considering whether or not the parents had allergies or smoked, the number of people and pets in the house, breastfeeding, and socioeconomic status. Thumb-sucking and nail biting had no effect on asthma or hay fever, perhaps because these were self-reported whereas the skin prick test is more definitive.


Charles Shultz PEANUTS comic strip featuring Linus letting Snoopy try out his thumb.
Allergy-wise, Linus may have the right idea about sucking his thumb. Sharing it with Snoopy might not be such a bad idea either.

Are fingers early “allergy injections”?

Why might spending your childhood with your fingers in your mouth reduce the chance of allergies? Maybe exposure early in life makes a person less sensitive. A type of environmental microbe “vaccination” or “allergy shot”. Early, low dose exposures to fungi, bacteria, dust mites, pollen, and animal dander may train a child’s immune system that these items are not harmful. This “fingers in mouth” study joins several others suggesting that exposure to “dirt” might reduce the chance of allergies in later life. Children who grow up on farms or with pets [2] and children whose parents sucked their pacifiers “clean” [3] have reduced instances of allergies. If that is the case, I certainly wish that I’d spent more of my childhood with my fingers in my mouth instead of doing a series of allergy shots after moving to different parts of the U.S. as an adult.

Close-up of a girl who is biting her nails.
Could nail biting actually help “inoculate” Jac’s immune system against common allergies?

What about breastfeeding and allergies?

This ~40 year study followed over 1, 000 people in New Zealand from birth is yet another example of the powerful findings of such “cohort population studies”. Collecting a wide variety of detailed data on such a large group of individuals in one country allows for differences in people’s lifestyles, environment, and sometimes genetic diversity to be removed from the study. Kids who sucked their thumbs, but had parents who had allergies or smoked were still less likely to be allergic than kids who didn’t suck their thumbs. I was surprised that breastfeeding didn’t seem to have an effect on allergies since other research has seen an association [4, 5], though the findings on breastfeeding and asthma is mixed. Breastmilk’s unique sugar composition influences an infant’s gut microbiome [6-14]. The different bacteria in the gut microbiome are hypothesized to influence immune development and reduce allergies [7, 15-18]. Switching to the sugar composition of formula or even between formula and breastmilk change the gut microbiome [19-21] as does introduction to solid food and cessation of breastfeeding after solid food introduction [22]. However, in the New Zealand cohort “breastfed” was defined as 4 weeks or longer. Is there a difference between exclusive breastfeeding and mixed feeding? Does length of time of exclusive breast feeding make a difference? Hopefully more data and future studies that take these considerations into account will give us further insight.

When I told my family about this study around the dinner table, my oldest daughter shouted “YES!” pumping her fist in the air. Four years previously, she was a nail-biter. “Mama, you MADE me stop biting my nails!” Well, honey, that’s the process of science. New data makes us re-examine our old ideas.


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