By Amy Norton
TUESDAY, Feb. 2, 2016 (HealthDay News) -- That antiperspirant may keep you dry, but it might also disrupt the bacterial "community" that resides in your armpits, a new, small study suggests.
Researchers said it's not clear whether that disruption has any dire effects -- or whether it could even be beneficial. But the findings, published online Feb. 2 in the journal Peer J, add to questions about the ways in which modern lifestyles could be altering the human "microbiome."
The term refers to the trillions of bacteria and other microbes that inhabit the human body, inside and out. The skin is covered in a range of microbes --- most of which are either harmless or beneficial, according to the U.S. National Institutes of Health (NIH).
Some microbes, the NIH says, protect the skin from invasion by harmful bugs, and may also play a role in "educating" the immune system cells that dwell in the skin.
"We know that these skin microbes interact with the immune system," said lead researcher Julie Horvath. "So it's important to consider what our daily habits do to the skin's microbiome."
The point of this study was not to demonize deodorant, according to Horvath, who heads the genomics and microbiology research lab at the North Carolina Museum of Natural Sciences, in Raleigh.
For starters, anything put on the skin -- from lotion to makeup to soap and water -- might change the microbial community. There are other factors, too, according to the NIH -- such as age, gender and sun exposure.
Horvath became interested in antiperspirants' effects after she and her lab colleagues ran an experiment among themselves: They took swabs of their armpits, then cultured the samples to see what microbes were dwelling there.
At the time, Horvath was dealing with some public-speaking jitters, with the help of a clinical-strength antiperspirant. And it turned out that her armpit swab was free of microscopic organisms.
"I thought, 'Where are my microbes?' " she said. "And then I remembered the clinical-strength antiperspirant."
To dig deeper, Horvath's team recruited 17 volunteers for an eight-day experiment. Seven of the men and women regularly used antiperspirant, five used deodorant, and five used neither product.
On day one, all volunteers followed their normal hygiene routine. On days two through six, they refrained from all underarm products. On the final two days, all used antiperspirant.
On the first day, the researchers found, armpit swabs from the antiperspirant users tended to show far fewer bacteria, compared with both nonusers and deodorant users. Deodorant users actually had the most bacteria.
Horvath said it's not surprising that antiperspirant and deodorant users would differ from each other: Deodorants do have antimicrobial ingredients that fight odor, but antiperspirants actually prevent sweating -- and bacteria like to feed on sweat.
Things got more complicated when the whole study group stopped using all underarm products: By day six, all volunteers showed similar amounts of bacteria in their armpit swabs -- but the type and diversity of those bacteria varied widely.
Among the people who usually used no products, the most common bacteria belonged to a group called corynebacteria -- accounting for 62 percent of the microbes in their armpit swabs. Staphylococcaceae bacteria made up another 21 percent.
That pattern was reversed among people who normally wore antiperspirant or deodorant, with staph bacteria dominating.
Corynebacteria are partly responsible for body odor, Horvath said, but they also help defend the body from harmful bacteria. Staph bacteria have a bad reputation, but most strains are beneficial. Horvath said her team did not determine the types of staph study participants carried.
Pieter Dorrestein is a professor at the University of California, San Diego's Skaggs School of Pharmacy and Pharmaceutical Sciences. In a recent study, he found that "a lot of personal care products" -- from deodorant to lotion to shampoo -- remain on the skin, even after a few days' break.
"The data also suggested that personal care and lifestyles may influence the microbes that reside on the skin surface," said Dorrestein, who was not involved in the new study.
He said he was happy to see other researchers taking a "detailed" look at one personal care product. But he also pointed to some limitations of the study, including the small number of volunteers, and the "arbitrary" choice of an eight-day experiment.
Plus, Dorrestein said, it's not certain that the changes in people's armpit swabs actually reflect a dramatic shift in their skin bacteria. An alternative explanation, he noted, is that when people were using underarm products, the swabs just weren't reaching as many bacteria through the barrier.
Dorrestein pointed to the dramatic drop in swabbed bacteria on day seven -- just the first day that all volunteers starting using (or reusing) antiperspirant.
Still, he said, the study hypothesis was good, and he'd be "surprised" if personal care products did not change the skin's microbial community.
The big question is what that means.
"We know a little bit about the skin microbiome," study co-author Horvath said. "But we have so much left to learn."