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.