LOS ANGELES - Here's a scientific finding that may knock you off your feet: At least 80 types of fungi reside on a typical person's heel, along with 60 between the toes and 40 on the toenail.
Altogether, the feet are home to more than 100 types of fungus, more than any other area of the body, according to a study published this week by the journal Nature. And those fungi are in constant motion as we walk through life.
It may sound icky, but many of the fungi on our skin serve a useful purpose, said study leader Julie Segre, a geneticist at the National Human Genome Research Institute in Bethesda, Md.
"One of the major functions of healthy fungi is to prevent pathogenic fungi from adhering to our skin," where they can cause athlete's foot, plantar warts and stubborn toenail infections, she said. "There is something about toenails that fungi just love."
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Segre is at the forefront of research on the human microbiome - the collection of bacteria, viruses, fungi and mites that live in or on our bodies. The work increasingly shows that a thriving microbiome is essential to our health by helping us digest our food, fight disease and generally keep our systems in good working order.
In previous studies, Segre and her colleagues from various branches of the National Institutes of Health conducted a genetic analysis of bacteria that populate the different regions of our skin. In the study released Wednesday, they turned their attention to fungi.
The results revealed that an individual's fungal biome looks different from place to place. The forearms had 18 to 23 types of fungi; the forehead and chest had two to 10.
But the feet stood out, with the most variety from heel to toe. Among the denizens were Saccharomyces, the kind of yeast that ferments bread and beer; Penicillium, which is used to make penicillin; and Malassezia, which can cause dandruff.
The findings also showed that our feet are like a bustling fungi hotel, with different types moving in and out at a fast clip.
Segre said she doesn't know why the fungi on our feet are so diverse and unstable, but she has a few ideas.
For starters, while skin temperature on our core remains a pretty stable 92.3 degrees, the temperature of our feet fluctuates wildly. Cold feet may be hospitable to some strains of fungi, while hot feet may be attractive to others.
Since they're so close to the ground, our feet are probably also exposed to more fungal diversity than other parts of our body.
And there's one more thing: "People are fastidious about washing everything off their hands, but people don't really specifically wash their feet," Segre said. "For many people, standing in the shower seems good enough."
Elizabeth Grice, assistant professor of dermatology at the University of Pennsylvania, said experts are just beginning to get their feet wet when it comes to understanding fungi.
