I was 32 when I first used a rake, and dug my hands deep into soil. In 2016, three weeks after my daughter was born, we moved out of London to a town in the British countryside and, for the first time in my adult life, I lived in a house with a garden. Soon after, by some luck, a nearby allotment became free.
Starting off easy, we planted some manky old potatoes from our vegetable bag that had started sprouting green nodules. A few months later, I wrenched up the plants that had grown half a meter high and started digging. Avenues of black soil revealed moon after moon of luminous potato. So I planted more: an abundance of acid-green parsley, a blush of bright pink radishes, trumpets of butternut squash flowers spilling over and tumbling here and there.
As my time outside increased, I quickly noticed two things. First, my baby daughter seemed to like eating soil. Second, during and after time spent in the allotment or the garden, I felt happy, upbeat, less stressed and generally more positive. Initially, I put it down to the physical exercise, time to myself, the curious magic of botany, another date venue for my rekindled relationship with the natural world. In fact, there was likely to be a biological reason too, at least in part. I saw a poster on a Facebook parenting group. “Get Dirty,” it proclaimed. “Exposure to soil bacteria Mycobacterium vaccae is like a natural antidepressant, activating brain cells that improve mood, reduce anxiety and facilitate learning.” “True or woo?” a user had asked the group. Most responded with anecdotal stories, and though there was a good deal of skepticism, someone wrote that, as far back as the 1760s, soil was thought to have a curative effect on the mentally ill.
In 2004, Mary O’Brien, an oncologist at the Royal Marsden in Surrey, England, discovered something fascinating by accident. She created a serum that contained M. vaccae, a species of bacteria found in soil. In close-up photographs, colonies look like moldy, spotty, yellow growths. She wanted to see if the bacterium could boost the immune systems of her lung cancer patients and thus prolong their lives, because of its immunoregulatory effects, which were discovered in the 1990s. It didn’t help them to live longer, but, strangely, those who received the immunization reported feeling happier.
Separately, a neuroscientist called Dr. Christopher Lowry was working at the University of Bristol, studying the antidepressant-like effects of the bacterium M. vaccae. He heard about O’Brien’s findings and began to hypothesize that an immune response to M. vaccae stimulates the brain to create more serotonin, the happy chemical that antidepressant pills are designed to boost. He immunized mice with M. vaccae to find out more and reported that the immunized mice had a response to the bacterium, which could communicate to the brain and activate a group of serotonin neurons in the dorsal raphe nucleus, a structure in the midline of the brainstem. Inside this nucleus, serotonin-releasing cells are linked directly to the limbic system, where emotions are generated. This system is thought to play a crucial role in coping with stress. He tested the mice’s stress levels by dropping them in a little swimming pool. Happy mice swim; stressed mice don’t, according to previous research—and these M. vaccae mice enjoyed a dip. At the time Lowry told the BBC, “These studies help us understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health . . . They also leave us wondering if we shouldn’t all spend more time playing in the dirt.”
Since then, Lowry has spent years studying the impact of M. vaccae as a clinical application. Shifting his focus from allergies to psychological disorders, Lowry and his team wondered if M. vaccae could suppress inappropriate inflammation within cells, thus preventing negative outcomes of stress and post-traumatic stress disorder (PTSD)–like syndromes.
And it did. Mice injected with the bacterium exhibited fewer anxiety-or fear-like behaviors and were 50 percent less likely to have stress-induced colitis. Currently Lowry is trialling the effects of immunoregulatory bacteria on people with PTSD, to see if it could buffer the side effects from high-stress situations such as combat.During and after time spent in the allotment or the garden, I felt happy, upbeat, less stressed and generally more positive.
The 64,000-dollar question, as Lowry put it when we spoke, was exactly how bacteria like M. vaccae impact the brain to increase stress resilience. Scientists were studying sensory pathways to find out more. One possibility is that M. vaccae changes the phenotype (the physical properties and characteristics) of immune cells which migrate to the brain and regulate emotional behavior. Lowry and his team also identified a small molecule in the bacterium which could prevent allergic asthma when injected. “We suspect that this is just one molecule out of hundreds,” he said. “If you think about the whole scale of our microbiome it’s unimaginable how many molecules like this there must be.”
Inside your body, there are probably more microbial cells than human cells. Symbiotic organisms colonize various areas of the body—the mouth, skin, vagina, pancreas, eyes and lungs—and many reside in the gut microbiota. You almost certainly have microscopic mites living on your face in the hundreds, or even thousands—mating, laying eggs and, at the end of their lives, exploding, unbeknown to you.
You may have heard the incredible fact that the resident microbes in your body outnumber your own human cells ten to one. That figure has been downgraded to three to one or an equal number, which is still astonishing. They mostly resemble mini jumping beans or Tic Tacs on a much smaller scale. These organisms aren’t simply parasitic freeloaders: they are intricate networks that intertwine and interconnect, influencing our health and well-being through complex ecological processes. They are involved in the workings of the immune system, the gut-brain axis, protection against harmful organisms and, indirectly, they have some relationship to our mental health.
When we breathe, we suck different species of microorganisms into the body. Studies suggest 50 different species of mycobacteria would be normal in the upper airways of healthy individuals, making their way into the teeth, oral cavity and pharynx. The environment around you might look clear and empty, but it will be swarming with microscopic organisms, depending on where you are.
Our microbiota are healthiest when they are diverse—and a diverse microbiota is influenced positively by an environment filled with organisms, which are found more abundantly in outside spaces than inside. We imagine our skin and our bodies to be armored, or a shell impenetrable to the outdoors, that we have somehow transcended our biological origins. But the human epidermis is more like a pond surface or a forest soil, as Paul Shepard, the late American environmentalist, suggested. Even if we don’t yet understand or know exactly how many of the abundant microorganisms in our bodies arrived with us through exposure to nature—and, indeed, how they affect our mental and physical health—we are woven into the land, and wider ecosystems, more than we realize.
Crucially, these “old friends” that we have evolved with are able to treat or block chronic inflammation. There are two types of inflammation: the good, normal, protective type, whereby the immune system fires up to respond to an injury, with fever or swelling or redness; then there is the chronic, systemic kind you don’t want. This is the simmering, low-level constant inflammation within the body which can lead to cardiovascular disease, inflammatory disorders, decreased resistance to stress and depression. This kind of raised, background inflammation is common in people who live in industrialized, urban environments and is associated with the unhealthy habits of the modern world: our diets, poor sleep, smoking and alcohol consumption, stress and sedentary lifestyles. As we age, our bodies become more inflamed. Scientists can measure levels of inflammation by looking at biomarkers such as proteins in the blood.
It should be no surprise, then, to learn that the gut microbiota of people who live in urban areas and developed countries are less biodiverse than those who still have profound contact with the land, such as hunter-gatherers and traditional farming communities.
Scientists are starting to understand more deeply the role inflammation may also play in our mental health. Evidence that bodily inflammation can affect the brain and have a direct effect on mood, cognition and behavior is relatively new. But it is strong and compelling. Depression may well be all in the mind, the brain and the body. This view runs counter to the dominant view of Western medicine that our bodies and minds are separate and thus should be treated apart from each other, a view dating back to 17th-century French philosopher René Descartes’ concept of dualism. As the neuropsychiatrist professor Edward Bullmore has said, “In Britain in 2018, the NHS is still planned on Cartesian lines. Patients literally go through different doors, attend different hospitals, to consult differently trained doctors, about their dualistically divided bodies.”
But perhaps we are not as dualistically divided as the Cartesian orthodoxy our health systems are still built on would lead us to believe. A study of 15 thousand children in England found that those who were inflamed at the age of nine were more likely to be depressed a decade later, as 18-year-olds. People with depression, anxiety, schizophrenia and other neuropsychiatric disorders have been found to have higher levels of inflammation biomarkers. European people have higher levels of cytokines in the winter months, which is also a time of increased risk of depression. Levels of cytokines are higher in sufferers of bipolar disorders during their manic episodes, and lower when they’re in remission. Early findings suggest anti-inflammatory medicines may improve depressive symptoms. People with a dysregulated immune system are more likely to have psychiatric disorders.
In his book The Inflamed Mind, Bullmore argued that some depressions may be a symptom of inflammatory disease, directly related to high levels of cytokines in the blood, or a “cytokine squall,” as he puts it.
Could our lack of contact with the natural world be a contributing factor to high levels of inflammation, which could be related to depression and other mental health disorders? Studies show that just two hours in a forest can significantly lower cytokine levels in the blood, soothing inflammation. This could partly be caused by exposure to important microorganisms.
There are multiple reasons why babies born in the rich, developed world have a less diverse population of mycobacteria—for example, the use of antibiotics, diet, lack of breastfeeding and reduced contact with the natural environment. We live inside, often in air-conditioned buildings cleaned with antibacterial sprays, with reduced exposure to organisms from the natural environment via plants, animals and the soil. Our food is sprayed and wrapped in plastic. We don’t live alongside other species of animals, as we did for millennia. The opportunities to be exposed to diverse microorganisms are much fewer—which might explain why my daughter liked to eat soil.
“All babies left to their own devices will eat soil,” said Graham Rook, Emeritus Professor of Medical Microbiology at University College London, and a distinguished expert on microbes and the immune system. “It’s something that all vertebrates do.” Rook has spent much of his long career studying how much the beneficial effect of green spaces on long-term human health outcomes might have to do with microbial biodiversity driving immunoregulatory mechanisms. We spoke over Skype and he was animated, owlish and quick to laugh. I enjoyed talking to him a lot.
Currently, scientists don’t know exactly how many of the microbes in the soil also live within us, or which strains. Spores—reproductive cells that can develop into new bacteria—are also important. If you’ve got a low level of diverse gut microbiota, you can replenish them if they’re encountered as spores in your environment, Rook told me. A third of all organisms in the gut are spore-forming. They’re small, tough and can stick around in the environment for tens of thousands, maybe even millions of years. “Julius Caesar passed through this area,” said Rook, speaking from his study in Islington, North London, “and perhaps there are spores from an organism from Julius Caesar’s gut wafting around. Some of them might have colonized my gut. The imperial organisms,” he joked.
Studies show that contact with natural environments during pregnancy or the neonatal period results in a lower prevalence of allergic disorder, which is connected with regulation of the immune system. Most of these studies have compared people who live on farms with those who don’t. “Not only are they less likely to get allergic disorders, they’re less likely to get some of the other chronic inflammatory disorders and probably less likely to have psychiatric problems as well,” said Rook.
I don’t think it was a coincidence that during the writing of this chapter I acquired a couple of backyard chickens.
The Amish and the Hutterites are traditional farming communities in the United States and Canada. The Amish population emigrated from Switzerland and the Hutterites from South Tyrol in Italy during the eighteenth and nineteenth centuries.
The Hutterites’ belief in absolute pacifism and refusal to take part in military activities or pay war taxes has led to their persecution and banishment from various countries since their formation as a religious group in the 16th century. In the 1870s, the remaining population of around four hundred found a new home in Dakota County, Minnesota. In contrast to the Amish, the Hutterites live on large, industrialized farms. They don’t shun technology, and farm with huge air-conditioned tractors. Because the barns are full of machinery, the children don’t run around inside them.
The Amish, however, still live on traditional farms, using horses and ploughs, and the children play in the barns, spending time with the livestock, exactly as they did in the 19th century. They live across the United States but with large populations concentrated in Pennsylvania and Ohio.
A study of the innate immunity of children in the Amish and Hutterite communities found that the Amish had lower levels of chronic inflammation (the type you don’t want, the subtle, long-term activation of the immune system). Just the dust around the Amish homes and farmyard areas was enough to protect against allergic asthma and other allergies. The prevalence of asthma in Amish versus Hutterite schoolchildren was 5.2 percent versus 21.3 percent and the prevalence of allergic sensitization was 7.2 percent versus 33.3 percent. It was a significant difference. The study also found that cytokines were much lower in the Amish, who were exposed to natural environments more than the Hutterites.
The study validates Lowry’s earlier statement about us all needing to spend more time in the dirt. Perhaps, instead of teaching children that soil and the bugs that live in them are dirty and gross, we should encourage them, from a young age, to explore and play and get muddy, to enable them to benefit from the complex ecological networks—and rewards in terms of health—that exist within them. “Relating to depression, all we can say is that if you’ve got persistently raised background inflammation, you’re more susceptible to it,” said Rook. “So, for instance, I wouldn’t mind betting that the Hutterites have more trouble with depression than the Amish. A psychologist would say, ‘Oh, it’s because of the wonderful relaxing effect of sitting on a plough watching a horse’s arse in front of you for hours a day.’ And they might be right, there may be some element of that, but I think that there are also these molecular mechanisms at work.”What does it mean, then, to live in a city, as the vast majority of us do? By 2050, 68 percent of the world’s population will live in urban areas.
In affluent, developed countries, rates of allergies, autoimmune disorders and inflammatory bowel disorders have increased, suggesting that our immune systems are attacking targets they should not attack. Also, there are permanently raised levels of inflammation—seen in higher levels of the molecule C-reactive protein, a ring-shaped protein found in plasma which rises in response to inflammation—without evidence of inflammatory disorder.
What does it mean, then, to live in a city, as the vast majority of us do? By 2050, 68 percent of the world’s population will live in urban areas.
In 2018, a study group that included Lowry and Rook compared people who grew up in the countryside around farm animals with those who grew up in cities without pets. The hypothesis was that growing up in a rural environment with greater microbial diversity would decrease vulnerability to stress-associated mental and physical health problems in later life.
The scientists gave twenty healthy young men from the two very different environments the Trier Social Stress Test to activate a stress and anxiety response. The procedure is used to induce a psycho-physiological stress response in a laboratory setting. Participants were asked to prepare a speech for a simulated job interview. In the test room, they were given no instructions about how long it should be. Then there was a mental arithmetic test, where the participant was asked to count backwards from 3,079 by subtraction of 17. If they made a mistake, they’d have to start from the beginning.
In the results of the test, those who grew up in the city had a higher social stress response, which was seen in exaggerated numbers of white blood cells and pro-inflammatory cytokines in the blood. This greater number suggests an increased risk for chronic, low-grade inflammation. “These data are in line with the biodiversity, missing-microbes, and old-friends hypotheses, which propose that the rapid rise in inflammatory physical and mental diseases in modern societies is due in part to a lack of exposure to immunoregulatory microorganisms,” the study group concluded.
It made playing in the dirt sound even more critical, and I asked Lowry how confident he was in saying that spending time in nature, in the garden, with soil, could be both a treatment and a preventive measure for people with mental health problems. “I think that’s very likely to be true,” he said. “Then the question is, what’s the effect size? How big is the effect? It would be hard to argue that that kind of exposure would not be beneficial, but how beneficial compared with other interventions? I think we might be surprised how effective that might be.”
And it is not a quick, temporary fix. In animal studies, T-cells (cells which modulate the immune system) have a surprisingly long half-life of 27 days. This suggests that your post-gardening high could keep you buzzing for days, or even weeks.
Excerpted from Losing Eden by Lucy Jones. Reprinted with permission of the publisher, Pantheon Books. Copyright © 2021 by Lucy Jones.