How Does Nature Benefit the Brain?

Post by Lani Cupo

Why is nature “good” for you?

Many people around the world have the intuition that spending time outside, especially in nature, is good for you. There is evidence to support that this is indeed true - research studies provide a wealth of data linking exposure to the natural environment with improved health outcomes. For example, in a recent systematic review, the authors found that exposure to nature promoted social behavior and physical activity, reduced stress levels and heart rate, and reduced exposure to traffic-induced air pollution. These beneficial relationships were reported in about 70% of papers, while a vast minority (~3%) reported negative effects. Exposure to green space during childhood has further been associated with decreased levels of psychiatric disorders later in life, including mood disorders. Other research shows that nature can have an effect on brain development as well - it has been shown that lifetime exposure to green space is associated with increased gray matter volume in the prefrontal cortex. 

One factor that could potentially explain how some of these studies arrived at different conclusions is that “natural spaces” themselves differ. Researchers often measure exposure to natural environments as exposure to “green space” in urban environments. While the word “nature” may conjure images of sweeping mountain landscapes or the wild seashore, 55% of people worldwide lived in cities as of 2018, and in 2050, 68% of the projected 9.7 billion people are expected to live in urban areas. Therefore, urban green space must be considered a significant opportunity for nature exposure in the increasingly urbanized world. Urban green space is defined by the United States Environmental Protection Agency (EPA) as the “land that is partly or completely covered with grass, trees, shrubs, or other vegetation” which can include “parks, community gardens, and cemeteries.”. In the previously mentioned systematic review, some studies included only farmland or forests, while others examined parks as well. Even restricting comparisons to those examining urban parks, for example, might yield drastic differences, as parks differ on important characteristics, such as the biodiversity of flora and fauna they sustain. One recent paper examining urban biodiversity from 15 parks in Portland, Oregon found significant differences in species richness and biodiversity indices based on the purpose of the park (whether they were geared more towards formal recreation, such as sports, or passive use, such as walking). Factors such as the shape and size of a park, as well as distance to water and connectivity to other green spaces, can also impact biodiversity. These differences could contribute to the mixed results found in studies that synthesize multiple experiments. But why would greater biodiversity improve human health?

How does urban biodiversity relate to mental health?

A multitude of factors could contribute to associations between exposure to green space and improved mental and neurological health. In recent years, however, one set of hypotheses has risen in prominence. As humans evolved, a subset of microbes developed symbiotic relationships with us, inhabiting our bodies (think intestinal tract and skin) and priming our immune system to respond to external threats. The “old friends” hypothesis explains that some microbes co-evolved with the human immune system to establish a defense system against invading microbes. The relationship between the human gut microbiome and the brain was explored in a previous BrainPost by Elisa Guma. As the world undergoes increasing urbanization and the biodiversity of flora and fauna decreases, the beneficial microbes we are exposed to suffer as well. The “biodiversity hypothesis” holds that reduced exposure to the diverse “old friends” we evolved with can increase inflammation, contributing to the myriad diseases that are increasing globally, such as asthma, obesity, allergies, and autoimmune disorders. Additionally, inflammation has been implicated in many psychiatric disorders as well, such as depression, schizophrenia, bipolar disorder, and chronic stress. This relationship may be bi-directional (chronic inflammation can contribute to psychiatric disorders which may, in turn, increase chronic inflammation through lifestyle factors), however, a healthy microbiome can reduce chronic inflammation.

What evidence is there for the Biodiversity Hypothesis?

Evidence from humans and the laboratory supports the link between exposure to a biodiverse environment and a rich gut microbiome. Many human studies are observational, finding differences in microbiomes across gradients of rural-urban, industrialization, and land use. However, with all observational studies, there can be confounding factors, such as socioeconomic status (SES) that could also impact the microbiome. Intervention-based studies can increase experimenter control. For example, one study in Finland introduced forest floor and sod into four urban daycares for children to play with. After 28 days, the authors compared skin and gut bacteria as well as markers of inflammation in the blood between children in these daycares with those from three urban daycares without intervention and three nature-centered daycares. The authors found increased diversity of bacteria on the skin and in the gut of the intervention daycares, comparable to those in nature-centered daycares. This study provided the first interventional evidence in humans supporting the biodiversity hypothesis. Animal models suggest that even exposure to a diverse “aerobiome”, or airborne microbiome, can improve gut microbiome and potentially reduce anxiety-like behavior: Using fans, the authors exposed mouse cages to dust from either a no-soil control, a low diversity soil, or a high-diversity soil. Importantly, the mice did not directly interact with the soil but were exposed to low levels of dust, as might occur if someone commutes through green space to and from work. The authors found exposure to high-biodiversity soil increased the diversity of the gut microbiome and reduced anxiety-like behavior in the most stressed mice. Together, this evidence provides strong support for the biodiversity hypothesis as a means of associating external biodiversity and mental health.

What else mediates the relationship between nature and health?

While the biodiversity hypothesis presents a compelling, interdisciplinary approach linking human and environmental health, there are other important factors linking the two. The human microbiome itself is also influenced by factors like delivery method at birth (natural delivery or Cesarean section), diet, antibiotic use, and age. Diet and access to green space are heavily influenced by socioeconomic status as well. Furthermore, the quality of green space depends on land use and pollutants, which can differ based on a neighborhood’s community SES. There is some evidence to suggest the most economically disadvantaged might benefit the most from exposure to biodiverse natural environments. More intervention-based research in humans could help further develop evidence for the impact of biodiversity and provide recommendations for public policy.

In addition to exposure to a diverse microbiome, exposure to green space can benefit people in a number of other ways. For example, those who access green spaces may spend more time physically active, or socializing with community members and friends, which can improve mental health outcomes as well. Since these factors can work together, it is important not to over-simplify the effects of environmental exposures to the microbiome alone. Therefore, public policy should focus on an integrative approach to human health, intrinsically linked to our environment. 

References +

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