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Want to boost your immune system? Eat some dirt.

As we reflect back on our childhood days, many of us dream fondly of simpler times spent running outdoors and rolling in the grass, completely uninhibited by the worry of getting a little dirty. We’d return to the house, wiping our face with our muddy fingers, and wash up for dinner. As it turns out, this outdoor playtime was its own vital probiotic boost.

In the not-so-distant future, your doctor might recommend supplementing your immune-boosting routine with spending time going back to your roots and getting dirty. It’s not as outlandish as it sounds.

In a number of studies around the world, researchers are proving that the bacteria Mycobacterium vaccae found in soil could be vital to our health. For most of history, humans and bacteria have had a symbiotic relationship; bacteria in our gut help us process our food, among many other jobs. Unfortunately, modern lifeespecially our cleanliness compulsionis reducing the diversity of our gut microbiome. Getting microbes like M. vaccae back into our microbiome could have wide-ranging health benefits, from fighting depression to boosting our immune systems.

 

Reuniting with old friends

America’s’ obsession with cleanliness (and killing bacteria) began during the Civil War when doctorswho treated 10 million cases of injury and illness over just 48 monthsgained a better understanding of the relationship between cleanliness, dirt, and disease. The obsession accelerated with the invention of modern advertising at the end of the 19th century, when the original Mad Men discovered they could make a fortune convincing consumers that being  super clean meant being healthy. Their motives may not have been pure, but the public’s embracement of personal hygiene, along with public health efforts, drastically reduced death and disease. Between 1900 and 1999, US infant mortality decreased 90 percent, and maternal mortality decreased 99 percent, a drop partially attributed to better hygiene.

From an evolutionary perspective, however, we may have gone too far and tipped that all-important symbiotic balance. Many scientists believe that all this washing has thrown our microbiome out of whack. Because humans evolved right alongside parasites, fungi and bacteria, our symbiotic relationships with these organisms are actually vital to our immune system’s functioning.

According to the hygiene hypothesis, children who are exposed to viruses, bacteria and parasites at an early age have healthier immune systems and develop asthma less than other children. The prevailing theory was that children in underdeveloped countries actually had fewer allergies and illnesses, such as asthma, precisely because they did not live in sterile environments.

However, the hygiene hypothesis couldn’t fully explain why high-income countries have seen a rise in diseases where the immune system attacks systems it should not attack, including our own tissues  (multiple sclerosis, Type 1 diabetes), harmless molecules in the air we breathe (hay fever, allergic asthma), or the contents of our gut (inflammatory bowel diseases), Graham Rook, a University College London microbiologist, wrote in an introduction to his paper on the topic. He ultimately developed the “old friends” hypothesis to explain how modern human lifestylesindoor living, pasteurized food and obsessive cleanlinessand medical practices, particularly the overuse of antibiotics, have upset the gut’s natural diversity.

According to Rook, because humans have less contact with the natural environment and, therefore, fewer microbial inputs in early life, our immune, endocrine and metabolic systems do not develop correctly and can malfunction. Thus, it’s imperative that we reintroduce these “old friends”microbes found in dirtback into our systems.

 

How dirt affects mood

Up until that point, researchers were primarily interested in studying how reintroducing environmental microbes could affect physical disease. Along the way, however, scientists discovered that the gut microbiome could actually play a role in our mental health, as well, and research into this connection is yielding some very promising results.

In a study from 2004, Mary O’Brien, an oncologist at the Royal Marsden Hospital in London, injected cancer patients with M. vaccae to see if survival rates would improve. They didn’t, but the experiment yielded a surprising result.

O’Brien wrote in her paper that patients showed improvement in cognitive functioning and vitality and a reduction in adverse effects, including nausea and vomiting. In short, the dirt-borne bacterium reduced the emotional toll of advanced cancer and improved quality of life.

The link between M. vaccae and our mood was further investigated in a study conducted in 2007 by Christopher Lowry,  a neuroscientist at the University of Bristol. In his research, Lowry injected mice with the bacterium and put them through a series of stress tests. The results showed that neurons that synthesize serotonin, a neurochemical whose impacts include reducing depression and regulating anxiety, were activated.

Even more promising, the mice that were injected also had activated neurons related to immune response. That finding reinforced the results of a 2003 study on humans conducted by Richard Davidson at the University of Wisconsin, Madison, which found a connection between our emotions and our immune systems. When asked to dwell on unhappy episodes, human participants had much lower antibody levels after an influenza injection, while those who thought of happy times had higher levels.

In 2016, Lowry, now at the University of Colorado, Boulder, again injected mice with heat killed M. vaccae and put them through a series of tests to measure the effects on their anxiety levels, which included placing them with a large aggressive maie for 19 days. Not only did those injected show 50 percent less of the “flight or freeze” behavior associated with anxiety, but they behaved more proactively around the aggressive male.

Lowry and Lisa Brenner, a psychiatry professor at the CU School of Medicine, are currently conducting human trials on veterans suffering from PTSD with a probiotic that has immunoregulatory properties similar to M. vaccae.

Dirt sandwiches and M. vaccae showers

Lowry’s study caused a sensation in the scientific community, and other researchers took his results with mice one step further. In 2010, Dorothy Matthews of the Sage Colleges fed mice peanut-butter laced with live M. vaccae. Those mice completed mazes twice as fast as the control group and also showed fewer anxiety-related behaviors. She concluded that the results show the “positive benefits of naturally-delivered live M. vaccae…” In other words, you don’t have to inject microbes from dirt to enjoy their potential antidepressant-like effectseating them can work, too.

Not surprisingly, there are now numerous M. vaccae supplementscalled psychobiotics, which are probiotics that specifically bestow mental health benefitsmarketed to boost brain power and suppress depression. It’s probably not necessary to shell out the cash for a pill, however. All you have to do to harness the positive impact of the dirt’s microbiome is to step outside, go to a park, work in a garden, or, better yet, grow your own vegetables.

If getting dirt under your fingernails isn’t your thing, you might be able to get your dose of M. vaccae from your daily shower. Lowry and University of Colorado-Boulder microbiologist Noah Fierer, have undertaken an unusual experiment: they are collecting water from shower heads in the US and Europe to determine how many mycobacteria, including M. vaccae, live there. Through their Showerhead Microbiome Project they hope to establish what factorsfrom the type of shower head to the municipal water supplyimpact the type of microbes present.

Take advantage of the summer weather, go back to your childhood roots, and roll around in the dirtor at least take a walk. And maybe leave the hand sanitizer at home.

 

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