Is dirt really a dirty word? Or could it be something else, something more powerful? Full disclosure: I am a mom and I work with small children, so my tolerance for dirt is pretty high. But studies have found dirt doesn’t have to be gross-dirty. Instead, it can just be dirt-dirty, and it can pack a positive punch. 

Over the past year, health and exercise professionals have transformed what we do and how we do it. In response to pandemic-induced gym closures, we had to find alternatives to traditional group fitness studios and squat racks. And, for many of us, that meant taking fitness sessions outside and utilizing outdoor spaces in creative ways. 

But let’s be honest. Many people shy away from outdoor training because of the inherent mess that comes with it, preferring instead to sweat in a climate-controlled, artificially lit, sterilized environment. But I will argue that using outdoor spaces enhance the benefits of a workout. A good old-fashioned messy, muddy, dirty workout can bring benefits beyond the obvious physical ones by improving cognition and reducing stress, anxiety and depression.

First, letting go and getting dirty can be exhilarating and downright fun. And fun, in and of itself, improves overall outlook and mood. As a high school field hockey player, my daughter played in many highly competitive games. In her senior year, her team played their archrival in the final match. Prior to the tournament, we had two solid days of rain, leaving the field sodden. As the first half progressed, the field conditions prompted each team to play tentatively, rather than the aggressive style of play they typically showed. That is, until one player fell—splat—while reaching to put her stick on the ball. Her white, home-team uniform was soaked and dripping with mud., but that tumble was a game changer.

It opened the floodgates (so to speak) for the team to resume its normal, all-in aggressiveness. Another teammate immediately charged for the ball, slipping in the mud as well. Others followed, and soon a match of white v. green uniforms became brown v. green. The dirt and mud shifted the mental grit of her team, reducing their anxiety, releasing their stress and allowing them the freedom to play to their fullest. The mental block of defeating their rivals was unlocked, and these girls played with ferociousness, ultimately winning the match and the tournament. Their outlook, their mood and their physicality shifted through the freedom of letting go and getting dirty.  

Most adults don’t get a chance to purposefully roll around on a field hockey pitch in the mud. In fact, we are not exposed to a lot of dirt in our modern, sanitized world. Most of us live and work inside, which is very different from the hunter-gatherer and agrarian world from which we evolved. But what happens when we get back to our evolutionary roots and expose ourselves to dirt? As it turns out, some good stuff.

First, there is the immune-boosting power of dirt. We now know that the health of our gut microbiome has a big effect on our immune health. Jack Gilbert, director of the Microbiome Center at the University of Chicago, is the author of Dirt is Good: The Advantage of Germs in Your Child’s Development. In that book, Gilbert recounts studies showing that kids who play outdoors in the dirt and mud have stronger immune systems than their peers who shy away from mud pies and digging for worms. Exposure to the naturally occurring microbes in the environment helps build a good microbiome.  

But the benefits of getting dirty are not limited to the immune system. Instead, the positive effects extend to our brains, specifically the regions that govern mood, stress, and cognition.  

Dirt contains a bacteria called Mycobacterium vaccae (M. vaccae). In 2004, researchers stumbled upon some of the beneficial properties found in this harmless and common soil-dwelling bacteria. Recognizing the immune-boosting properties of dirt, they hypothesized that injecting killed M. vaccae into cancer patients undergoing chemotherapy might help them fight the disease. Sadly, that aspect of the experiment was unsuccessful, but the patients did report dramatic increases in their quality of life after the injection. They were happier, less likely to be depressed, had better cognitive function and felt more vital despite the devastating toll of cancer. 

In 2007, a study found that M. vaccae plays a role in how the body responds to stress. When mouse subjects were injected with the bacteria and placed in stress tests, they responded almost as if they were on antidepressants, quite unlike their counterparts who were not injected with the bacteria and were highly stressed during the same tests. Researchers discovered that M. vaccae causes the brain to release serotonin which helps regulate anxiety, along with stimulating the neurons that govern the immune response. This study further proves the link between stress and the immune system.

Dirt can also spark mental acuity. In another study, researchers fed mice M. vaccae-laced peanut butter sandwiches and set them loose in mazes in a race against mice who had not eaten “dirty” snacks. The bacteria-dosed mice successfully completed the mazes more rapidly than their peers. Scientists believe the bacteria stimulates the hippocampus, which is tasked with spatial memory, while also stimulating the executive brain function, which sharpened the rodents’ focus. 

We have long known that training outside provides a host of benefits including increased Vitamin D levels from sun exposure and reduced stress levels from being in green spaces. Now we can add yet another benefit. When we are active in outdoor spaces, we stir up the dirt, inhaling all of that M. vaccae goodness that can improve mood and reduce incidence of depression, anxiety, and stress. So instead of heading back inside to the well-scrubbed group exercise studio, keep offering exercise opportunities in those outdoor spaces.  Oh, and maybe throw in a free trial-sized bottle of high powered laundry detergent to ease the transition for those who might be more hesitant!