Thumb Suckers and Nail Biters May Develop Fewer Allergies – New York Times
Babies have been seen sucking on their fingers in utero weeks before birth. But the sight of an older child with his fingers constantly in his mouth, sucking her thumb, biting his nails, can drive parents crazy, bringing up fears about everything from social stigma to germs.
A new study suggests that those habits in children ages 5 to 11 may indeed increase exposure to microbes, but that that may not be all bad.
In a study published Monday in the journal Pediatrics, researchers drew evidence from an ongoing study of New Zealand children to show those whose parents described them as thumb-suckers and nail-biters were less likely to have positive allergic skin tests later in life.
The question of such a connection arose because of the so-called hygiene hypothesis, an idea originally formulated in 1989, that there may be a link between atopic disease — the revved-up action of the immune system responsible for eczema, asthma and allergy — and a lack of exposure to various microbes early in life. Some exposure to germs, the argument goes, may help program a child’s immune system to fight disease, rather than develop allergies.
From Wikipedia: In medicine, the hygiene hypothesis is a hypothesis that states that a lack of early childhood exposure to infectious agents, symbiotic microorganisms (such as the gut flora or probiotics), andparasites increases susceptibility to allergic diseases by suppressing the natural development of theimmune system. In particular, the lack of exposure is thought to lead to defects in the establishment of immune tolerance.
The study found that children who frequently sucked a thumb or bit their nails were significantly less likely to have positive allergic skin tests both at 13 and again at 32. Children with both habits were even less likely to have a positive skin test than those with only one of the habits.
These differences could not be explained by other factors that are associated with allergic risk. The researchers controlled for pets, parents with allergies, breast-feeding, socioeconomic status and more. But though the former thumb-suckers and nail-biters were less likely to show allergic sensitization, there was no significant difference in their likelihood of having asthma or hay fever.
“The hygiene hypothesis is interesting because it suggests that lifestyle factors may be responsible for the rise in allergic diseases in recent decades. Obviously hygiene has very many benefits, but perhaps this is a downside. The hygiene hypothesis is still unproven and controversial, but this is another piece of evidence that it could be true.”
Dr. Hancox pointed out that the study does not show any mechanism to account for the association. “Even if we assume that the protective effect is due to exposure to microbial organisms, we don’t know which organisms are beneficial or how they actually influence immune function in this way.”
Dr. Sears said, “My excitement is not so much that sucking your thumb is good as that it shows the power of a longitudinal study.” (A longitudinal study is one that gathers data from the same subjects repeatedly over a period of time.) And in fact, as researchers tease out the complex ramifications of childhood exposures, it’s intriguing to look at long-term associations between childhood behavior and adult immune function, by watching what happens over decades.
So perhaps the results of this study help us look at these habits with slightly different eyes, as pieces of a complicated lifelong relationship between children and the environments they sample as they grow, which shape their health and their physiology in lasting ways.