It might not be a bad thing. (Justin Sullivan/Getty Images)

People slather on antiperspirant and deodorant to keep from stinking. But could the simple act of swiping some on change the delicate ecosystem beneath our pits? New research suggests that the answer is yes — and there may be significant differences between the armpits of people who use these products and those who don’t.

The thought of spending months researching the creatures found under people’s arms isn’t every scientist’s cup of tea. But it had a particular appeal to Julie Horvath, an evolutionary genomicist who tracks the genomes of humans and other primates.

“I’m excited about armpits,” Horvath said. In humans and apes, armpits (or, to get technical about it, the much less stinky-sounding axilla) are a real hot spot of evolutionary potential. Because they’re shielded from the elements, they’re in a place where microbes can freely propagate. And, conveniently, they’re home to the glands that produce the most body odor, which can affect how animals mate and pass on genetic material.

In a new study published in the journal PeerJ, Horvath and colleagues analyzed the microbes living inside the armpits of a group of 17 citizen-scientists. Over the course of eight days, they worked with three groups — antiperspirant users, deodorant users and those who don’t use any products under their arms. They swabbed the pits of each participant twice a day, then tweaked the product use of each participant. On days 2 to 6, participants were asked to go without any products under their arms. Then, all participants were instructed to use an antiperspirant on days 7 and 8.

Armed with a boatload of armpit-soaked swabs, researchers then set about culturing them and analyzing their DNA. They found that the colony of microbes living under the arms of people who stopped using deodorant and antiperspirant increased dramatically the longer they went without products. By Day 6, the number of microbes was comparable to what they found in the control group’s pits. Once the entire set of subjects put on antiperspirant, microbes all but disappeared.

The effect of deodorant on microbe communities was negligible in the study. In fact, some of the deodorant users started out with more microbes than their au naturale counterparts. But when most folks talk about deodorant, they really mean antiperspirant. Antiperspirants did a much better job of killing off microbes in the study. Given that antiperspirant actually blocks sweat glands (deodorant simply masks bacteria’s stinky byproducts), that finding wasn’t earth-shattering. But scientists were surprised at just how different the underarms of different users were mere days after they stopped using product.

The armpits of the control group contained a mix of about 62 percent of Corynebacteria, a mostly harmless form of bacteria that can be found almost everywhere. These bacteria are responsible for most body odor — they eat human sweat and metabolize it, creating a gas that the human nose tends to interpret as stink. Twenty-one percent of the microbes in control armpits were from the Staphylococcaceae family, and the rest were other types of bacteria.

In contrast, the armpits of antiperspirant users contained more than 60 percent of Staphylococcaceae once they stopped using products, just 14 percent of Corynebacteria, and more than 20 percent of other types of bacteria. Deodorant users may have had microbial populations comparable in size to those with stinky pits, but the perfumed product seems to turn the bacterial ratio on its head: Those subjects had an average of 61 percent Staphylococcaceae and 29 percent Corynebacterium, with 10 percent left to other opportunistic microbes.

DNA analysis wasn’t able to home in on which types of Staphylococcaceae were under the arms of any participants — types of such bacteria responsible for infections such as MRSA are just a fraction of the variations of the family of bacteria, and many of the related bacteria are actually beneficial to human health — so you shouldn't assume your pits are full of flesh-eating superbugs. It's possible that these microbial changes do no harm at all or that they're even good for us. And it goes without saying that a study with 17 subjects is on the small side. More pits are needed to confirm the findings.

But the takeaway, Horvath says, is that “your daily habits influence the microbes living on you.” Now that scientists have shown how much a person can change just by swiping on some deodorant, they can start to look at how those products might influence things like disease, antibiotic resistance, or even the ways humans evolve. Things like grooming habits, bathing and length of time without product could also influence the microbial communities.

Next, Horvath plans to tackle a study comparing the microbes found in earwax and underarm microbes (the same gene that produces different consistencies of earwax also influences the amount people sweat). Her goal isn’t to win an award for nastiest research ideas — it’s to find out how much human genes influence microbes. After all, she says, “everybody has an armpit,” even if everybody also thinks they’re disgusting. Beyond the ick factor lies a body part that is as promising as it is putrid. Next time you swipe on some product, remember: That stinky pit could one day change science.

Erin Blakemore (@heroinebook) is a freelance journalist from Boulder, Colo. She is the author of "The Heroine’s Bookshelf" (Harper). 

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