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.
