CUriosity: Why does playing in the dirt feel so good?
In our new, bi-weekly series CUriosity, experts across the CU Boulder campus answer pressing questions about humans, our planet and the universe beyond.
This week, Integrative Physiology Professor Christopher Lowry, answers: “Why does playing in the dirt feel so good?”
Sink your hands into the soil to plant some fall bulbs or harvest that last summer crop, and, chances are, you’ll feel a sense of calm wash over you.
It makes sense: You’re not at work. The sun is shining. And there’s something inherently gratifying about watching things grow.
But as Integrative Physiology Professor Chris Lowry explains, there’s a more scientific explanation for why getting dirty makes you feel so good.
That dirt is often rich with beneficial bacteria which can slip into our bodies via our mouths, lungs or the plants we eat, dampening inflammation that can fuel illness—including mental illness.
“These bacteria we co-evolved with have a trick up their sleeve,” explains Lowry, kneeling in front of a pile of dusty, freshly-dug potatoes in his Boulder garden. Beside them are juicy cherry tomatoes and a few poblano peppers nurtured by thick black soil.
He grabs a handful and continues.
“When they get taken up by our immune cells, some of these microorganisms release fat molecules that bind to our cell receptors and shut off this inflammatory cascade.”
This mechanism, which Lowry’s lab discovered in a bacterium called Mycobacterium vaccae, is just one of many ways our “old friends,” as scientists call these microbial companions, support mental health.
His research has shown they also activate brain cells that produce the feel-good chemical serotonin. And when mice are given a shot of M. vaccae—essentially a stress vaccine—they act less anxious when faced with a challenge.
The trouble, says Lowry, is that we aren’t getting dirty often enough.
“If you think about evolution, the original mammals were burrowing animals,” he says. “They literally lived in the soil, and for a long time we existed in harmony with it.”
According to the Old Friends Hypothesis, the more we remove ourselves from these microbes—via urbanization and replacement of fresh fruits and vegetables with ultra-processed food— the more vulnerable we become to inflammatory diseases. The more we nestle up to them, the better we feel.
Take farm kids.
Some research suggests they have greater resilience against asthma and allergies.
In , Lowry and colleagues also found that children who grew up in rural areas, surrounded by animals, gardens and plenty of dirt, had a healthier immune response to stress in adulthood.
But what explains that short-term bliss that seems to overcome a kid making mud pies or a university professor digging potatoes?
Sunlight, fresh air and companionship, of course, play a role.
But Lowry also points to another soil study. Researchers had one group of people sow seeds into soil rich with beneficial bacteria and another into inert dirt. Meanwhile, the scientists measured electrical activity in the gardeners’ brains.
“You could tell within 10 minutes which group was which,” Lowry says. “That’s pretty spectacular isn’t it? There’s something going on here that we don’t quite understand yet.”
Until we do? Get out in nature. Eat fresh fruits and vegetables (even if you wash them, they still retain some "old friends" soaked up from the soil).
“And everyone should take time to play in the dirt,” he says.