About that pacifier study: What it might mean to swap spit with your baby

Or, what it might mean to swap spit with your baby

by Tara Haelle

You've probably seen the buzz on the recent study on pacifier "cleaning" and allergy risk that came out in Pediatrics, but, as often happens with these studies, the conclusion(s) in the popular media have tended to be oversimplified and overhyped. Those that were nuanced and good (like the NPR piece) skipped entirely over an interesting part of the study related to birth methods.

The study, from a couple Swedish researchers, is fascinating, and I covered the basics of it in my dailyRx piece.

However, we need to keep a couple caveats in mind: First, the study is very small. Second, the associations the authors identifed were very small, and several disappeared as the kids grew older. Third, multiple possible mechanisms could explain or contribute to the findings, so jumping to conclusions about what it "means" or what pacifier "cleaning" methods do or don't "cause" is definitely premature.

First the study: The researchers recruited 184 women within a few days of giving birth to participate in the study if they gave birth at full term and their children did not receive neonatal intensive care. Most of the babies (80%) had at least one parent with allergies, which theoretically should have made the children more prone to allergies as well.

After an initial interview, the parents kept a diary of the babies' first year of life, recording "food introduction, weaning, diseases, medications and other significant events." The parents provided this info in a 6-month interview where they were also asked if their child used a pacifier (74% said they did) and whether it was "cleaned by boiling, rinsing in tap water or by the parents sucking on it?"

Let's stop there for a moment. While the parents could choose more than one option here, that's a really open question. When my son's pacifier fell on the floor at a restaurant, sometimes I picked it up, popped it in my mouth and then popped it in his. Sometimes I dipped it in my ice water at the table and popped it in his. And sometimes I tossed it in my bag, later running it through the dishwasher. How would I answer that question? I guess all three? (But even answering all three hardly captures how often I did each of those, and whether my frequency of each made a difference. I very rarely put it in the dishwasher, I never boiled it, and I frequently "cleaned" it with my own mouth - or not at all.)

In any case, the answers were that 83 percent of the parents rinsed the pacifier with tap water, 54 percent boiled it, and 65 cleaned it by sucking on it themselves.

Next, the researchers took saliva samples from the kids at 4 months old (to investigate the bacteria makeup in them) and then assessed them for allergy symptoms when the kids were 18 months old and 3 years old. They looked for symptoms of asthma and eczema and then tested the kids' sensitivity to a combination of food allergens (milk, egg, soy, fish, wheat and peanut) and another combination of environmental allergens: birch, timothy grass, mugwort, cat, dog, horse, mold and house dust mites.

Here's what the researchers found:

  • Kids whose parents boiled the pacifiers were very slightly at higher risk for asthma – but the findings were not statistically significant. That means they could have been due to chance. Given how small the study population was and how few kids had asthma at all (10 kids, or 5%, had symptoms at 18 months old, and 15 kids, or 8%, had symptoms at 3 years old), this finding isn't much of a finding.
  • The kids whose parents sucked on the pacifiers were very slightly less likely to have asthma (12% less likely) or eczema (37% less likely) at 18 months old. But by age 3, the asthma link vanished (and again, remember how few kids in whole study had asthma at all). The eczema link remained: Kids were half as likely to have eczema if their parents sucked on their pacifiers, but the statistical significance was getting close to the realm of possible chance. Overall, about a quarter of the kids had eczema, but that's still fewer than 50 kids in the whole study. Again, these are very tiny numbers for an epidemiological study.
  • There was no difference in how frequently the kids had respiratory infections, regardless of what the parents did with the pacifiers.
  • There was a slight tendency toward less sensitivity to the environmental allergens combination in kids whose parents sucked their pacifiers, but it could have been due to chance, and there was no link for the food allergens combo.
  • When the researchers compared the kids' mouth bacteria, those whose parents sucked on the pacifiers had a different makeup of microorganisms than the kids whose parents never used their mouths to clean the pacis. "It is possible that the observed effect of parental pacifier sucking is due to the transfer of oral bacteria from the parents, via the pacifier, to their infant," the researchers wrote. "By no doubt, this habit allows for a close oral contact between parents and child, facilitating bacterial transfer at a very young age, before the child starts to use spoons and so forth."

So the link between pacifier "cleaning" and allergies may have to do with getting the parents' bacteria into the kids' mouths (not so much about whether the pacifiers are "properly" cleaned or "too clean" or anything else). In other words, if a parents just swapped spit with their kids in other ways (they could try the baby bird regurgitation thing – just sayin'), the same effect might be achieved. Or, it could be a combination of a parents' contribution of bacterial critters and whatever critters on the pacifier were not eliminated by the parents' mouth "cleaning" that made a difference in the kids' allergy risk – if the risk difference is related to bacterial differences at all. These possibilities all touch on the hygiene hypothesis: the idea that more exposure to more germs at a younger age keeps the immune system busy enough that it doesn't try attacking itself (autoimmune diseases like lupus) or overreacting to otherwise harmless visitors (food and/or environmental allergies).

Related to all this bacteria talk, there's an interesting part of the study that most news sites have overlooked: More of the kids whose parents used their mouths to clean the pacifier were born vaginally. That was the only major difference between the mouth-cleaners and the paci-boilers in terms of factors that might strongly influence allergies. Otherwise, they were similar in the rates of parent allergies, how long the children were breastfed, whether a parent smoked, and whether the family had pets.

Why is that important? There's been more research about the different bacteria a baby is exposed to exiting the birth canal than if the baby is born by C section. Some studies have found reduced risks of allergies and other health issues (mostly autoimmune conditions) among vaginally born babies compared to C section-born babies. The reasons, it is thought, go back to the microbiomes that babies (all humans) have. We all have trillions of critters living in us (one figure says we're 90% bacteria), and the make-up of those critters is thought to influence all sorts of health-related things, from obesity risk to HIV risk to allergies.

So, the researchers made adjustments to their analysis to account for the fact that more of the pacifier-sucking moms gave birth vaginally. There was still a lower risk of eczema among the pacifier-sucking parents' kids, but there also was a lower risk of eczema among those born vaginally, regardless of pacifier cleaning methods:

  • 20% of the kids born vaginally whose parents cleaned pacifiers with their mouths had eczema
  • 31% of the kids who were born vaginally or had mouth-cleaned pacifiers had eczema
  • 54% of the kids born by C section whose parents never used their mouths to clean the pacifier had eczema

Again, though, this is a small study. I want to keep emphasizing that because it's so small that it's really hard to draw any conclusions about trends here. It is more of a proof-of-concept study, a tease, where researchers uncover an interesting finding or trend – and I do find all these findings fascinating, especially since I've been trying to follow the research on the hygiene hypothesis – but a lot more work is necessary to really understand what's going on here.

So, for now, here's what we know: Giving your kids your mouth bacteria might may have a small connection to reducing their risk for eczema. (Hence the conclusion: "Parental sucking of their infant’s pacifier may reduce the risk of allergy development, possibly via immune stimulation by microbes transferred to the infant via the parent’s saliva.") A baby born vaginally is less likely to get eczema. That's it. That's all this study tells us, and it needs to be replicated with much larger populations and perhaps more detailed questionnaires.

All that said, for fun, I'm going to quote a friend of mine when she saw this study, "Nice to know, once again, that science supports my laziness."

A version of this post originally appeared at Double X Science and Red Wine & Apple Sauce.

[Image credit for homepage image and thumbnail, Microsoft stock images.]

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