By culturing samples taken from the participants' armpits over the course of the study, the researchers found that using antiperspirant and deodorant appears to completely rearrange the bacterial ecosystem of our skin.
The study, led by the North Carolina Museum of Natural Sciences in Raleigh, is published in the journal PeerJ.
The authors explain that their findings lay the groundwork for future studies to investigate whether changes in armpit bacteria are good or bad.
Lead author Julie Urban, assistant head of the genomics and microbiology laboratory at the museum, says:
"Within the last century, use of underarm products has become routine for the vast majority of Americans. Yet, whether use of these products favors certain bacterial species - be they pathogenic or perhaps even beneficial - seems not to have been considered, and remains an intriguing area needing further study."
For the purpose of the study, the researchers distinguish between antiperspirants (which reduce sweat by blocking sweat glands with aluminium-based salts) and deodorants (which kill off odor-producing bacteria with ethanol or other antimicrobials).
Study compared users and non-users of antiperspirant and deodorant
The researchers recruited 17 men and women and put them in three groups. One group had three men and four women who were regular users of antiperspirant; another group of three men and two women were regular deodorant users; and the third group of three men and two women did not use either.
The authors note that while not all the participants who regularly used a product used the same brand, "all antiperspirant users used products containing aluminum zirconium trichlorohydrex Gly as the active ingredient."
The study took 8 days. On day 1, all three groups followed their normal hygiene routine. On day 2-day 6, none of the groups used any deodorants or antiperspirants.
On day 7 and day 8, all participants were asked to use an antiperspirant-deodorant product. The researchers supplied one brand for the women and another for the men - in both cases, the products contained aluminum zirconium trichlorohydrex Gly as the active ingredient.
On each day - between 11 am and 1 pm - the scientists took swabs of the participants' armpits. They cultured the daily armpit samples to see what types and amounts of bacteria were growing on each participant and how they varied from day to day.
Antiperspirant-deodorant dramatically reduces bacteria growth
The results showed that on day 1, the regular antiperspirant users had less bacteria in their armpits than the group that did not use either antiperspirant or deodorant. However, the authors note there was a lot variability, which made it difficult to draw any firm conclusions.
Also, from the day 1 samples, the researchers observed that the deodorant users - on average - had more armpit bacteria than the group that did not use either antiperspirant or deodorant.
By day 3 - the second day of no product use at all - the antiperspirant group armpit samples were starting to show more bacteria. By day 6, all participants were showing roughly similar amounts of bacteria in their armpit samples.
However, on days 7 and 8, when all the participants were using an antiperspirant-deodorant, the researchers found very few bacteria in the armpit samples, showing that the product dramatically reduces microbe growth.
After ceasing use, dominant bacteria were different to non-use group
From genetic tests on samples from days 3 and 6, the team found the bacteria in armpit samples from participants that were not regular users of antiperspirant or deodorant were 62% Corynebacteria, 21% Staphylococcaceae bacteria and the remainder was a random assortment of other bacteria.
Corynebacteria are the type of bacteria largely responsible for producing body odor. However, they are also considered "friendly" or "commensal" microbes that are important for defending against infection.
Staphylococcaceae are among the most common bacteria on human skin. Most types are thought to be beneficial, but there are also some that cause harm.
When the team ran genetic tests on the day 3 and 6 armpit samples of the groups that were regular antiperspirant or deodorant users, they found a completely different picture. Their armpit bacteria were dominated by Staphylococcaceae.
Senior and corresponding author Julie Horvath, head of the genomics and microbiology research laboratory at the museum, and an associate research professor at North Carolina Central University in Durham, concludes that using antiperspirant and deodorant appears to completely rearrange the microbial ecosystem of our skin. She adds:
"And we have no idea what effect, if any, that has on our skin and on our health. Is it beneficial? Is it detrimental? We really don't know at this point. Those are questions that we're potentially interested in exploring."
Perhaps one area that might warrant further investigation is the effect that wearing these products might have on the skin's ability to heal itself. In 2014, Medical News Today described a study that showed the type of bacteria living on skin can affect wound healing.