XI. 5. Nature Exposure Therapy (“Green Prescriptions”)

Time spent in nature—from forest bathing to ‘green prescriptions’—helps restore the body’s balance and nourishes the diversity of your gut microbiome.

Why Nature Matters for Your Gut

Spending time in natural environments restores balance in the body and nurtures microbial diversity [286] [286].

Nature exposure therapy – the formal medical practice of prescribing time in natural environments as a health intervention – gained scientific credibility through a convergence of epidemiological, physiological, and immunological research beginning in the 1980s. The Japanese practice of Shinrin-yoku (forest bathing), documented in the scientific literature by Qing Li and colleagues at Nippon Medical School Tokyo, provided the most systematic human evidence for the physiological effects of time in natural forest environments. [257] Li's studies, conducted between 2005 and 2010, brought participants to forest environments for two- to three-day excursions, with physiological sampling before, during, and after. Salivary cortisol fell, sympathetic tone (measured through salivary amylase) decreased, natural killer cell activity increased, and anti-cancer proteins perforin and granzyme B were elevated for up to 30 days after the forest visit. Li attributed these effects primarily to phytoncides – volatile compounds released by trees, particularly cedar and pine, with documented antimicrobial and immunomodulatory properties. [287] The gut microbiota connection to nature exposure operates through multiple routes: inhalation of diverse airborne organisms from natural environments, skin contact with environmental microbiota on plant surfaces and soil, reduced cortisol-mediated intestinal permeability and gut microbiota disruption associated with stress reduction, and the direct effects of phytoncides on intestinal epithelial cells and microbiota composition documented in animal models. [288] A green prescription meta-analysis published in Environmental Health Perspectives in 2018, synthesizing 143 studies, found that living in greener urban environments was associated with lower cortisol, lower inflammatory markers, lower cardiovascular disease risk, and lower rates of metabolic syndrome – all outcomes with documented gut microbiota correlates. Nature exposure is not an alternative to dietary microbiota management, but a physiological complement that operates through overlapping pathways.

The concept of prescribing time in natural environments as a medical intervention was formalized in New Zealand in the late 2000s under the term "green prescriptions," following evidence from Japanese research programs on shinrin-yoku – forest bathing – that had been developing since the 1980s. A series of studies by Qing Li at Nippon Medical School documented immune effects of forest walking compared to urban walking in controlled crossover designs. [257] The most cited finding was that forest walking increased the activity of natural killer (NK) cells in peripheral blood, with elevated NK cell number and cytotoxic activity persisting for seven days after a three-day forest visit. The effect was not seen with urban walking matched for distance and exertion. Qing Li proposed a mechanism: phytoncides – antimicrobial volatile organic compounds released by trees, particularly conifers – were inhaled during forest walking and modulated immune function. [287] The gut microbiota component of nature exposure therapy is less directly documented but mechanistically plausible. Forest environments expose participants to high densities of airborne soil and plant-associated microorganisms not present in urban air. Inhalation and incidental ingestion during outdoor activity represents a microbial input that differs qualitatively from indoor or urban air. Studies of gut microbiota in populations with regular outdoor nature exposure show higher diversity and enrichment of environmental taxa compared to exclusively indoor populations. [288] The clinical translation has reached primary care in several countries. New Zealand, Canada, and Scotland have implemented green prescription programs in which clinicians formally recommend hours per week in natural outdoor environments for patients with inflammatory conditions, metabolic disease, depression, and chronic fatigue. The microbiota rationale is one component of a broader evidence base including cardiovascular, neuroendocrine, and immune effects of nature exposure [288].

When people ask whether being in nature can matter for gut health, I usually start with a simple point: it rarely changes the gut overnight, but it can change the conditions that the gut depends on. The microbiota lives inside the body, yet it is influenced by how we sleep, how stressed we feel, how we move, and what we eat—factors that often shift when we spend time outdoors [288].

Modern life has reduced day-to-day contact with natural settings. Many hours are spent indoors with filtered air and relatively uniform microbial exposure. Built environments still have microbes, but the mix is often less diverse than what is found in soil, plants, and outdoor air. This difference is one reason researchers have become interested in how environmental biodiversity may relate to immune regulation.

The biodiversity hypothesis proposes that reduced contact with natural biodiversity can affect immune tolerance. The strongest supportive data come from population studies, especially in childhood, where living closer to farms or greener environments is linked with lower rates of some allergic conditions. These findings do not prove that nature prevents disease, but they do suggest that repeated environmental contact may be one piece of immune “calibration.”

For adults, the most consistent pathway is often stress and sleep. Natural settings are commonly associated with lower perceived stress and improved sleep quality. That matters for the gut because stress physiology can change motility, sensitivity, and barrier function. In other words, nature may help the gut indirectly by stabilizing the host signals that shape the intestinal environment.

Environmental microbial contact is real, but it is important to describe it accurately. Studies show that green space exposure can change the microbial patterns on the skin and in the upper airways, likely through direct contact with environmental microbes. Whether these exposures lead to stable, long-term colonization in the gut is less certain. Still, transient exposure may influence immune signaling even without permanent gut colonization.

Behavior also changes in nature. People tend to walk more, sit less, and spend time in daylight. Regular movement supports metabolic regulation and can influence gut function, including transit time and appetite. Daylight exposure helps anchor circadian rhythms, and circadian stability is relevant because gut motility and microbial activity follow daily patterns.

It is also worth being practical. Nature is not sterile, and it is not automatically beneficial in every context. Allergens, ticks, and pollution can be relevant depending on the setting. The goal is not extreme exposure, but regular, sensible contact with green spaces that fits a person’s health status and lifestyle.

Clinically, the most realistic message is that nature supports gut health through small, repeatable influences. Better sleep, lower stress load, more movement, and broader environmental contact tend to work together. Over time, those conditions support a more resilient gut ecosystem and a more balanced interaction between the microbiota and the immune system.

So nature does not “fix” the microbiota in a single step. It changes the background in which the gut and immune system operate, and that background matters—especially when it is repeated week after week, rather than treated as a one-time intervention.

Structuring Nature Exposure for Gut Stability

Regular time spent in green environments is most beneficial when it becomes part of a consistent routine rather than an occasional intensive experience.

Short, repeated outdoor exposures—such as daily walks in parks or time in a garden—appear more physiologically stabilizing than rare, prolonged excursions.

Direct contact with natural elements, including soil and plant surfaces, may broaden environmental microbial exposure; this is most relevant for immune signaling rather than permanent gut colonization.

Replacing some indoor sedentary time with light outdoor movement supports metabolic regulation and circadian alignment, both of which influence gut function.

Exposure to natural daylight helps anchor circadian rhythms, indirectly supporting microbial diurnal patterns and gastrointestinal motility.

Stress reduction associated with quiet natural settings may attenuate sympathetic activation, which is known to affect gut permeability and sensitivity.

Travel or holidays in biodiverse environments can provide extended restorative periods, though benefits likely derive from combined effects of rest, movement, daylight, and environmental contact.

Excessive sterilization practices in low-risk outdoor settings may unnecessarily limit environmental microbial interaction; balanced hygiene remains appropriate without striving for sterility.

The overall goal is cumulative ecological exposure integrated into daily life rather than sporadic “microbiota-focused” interventions.

Microbiota Effects

• Regular exposure to biodiverse natural environments is associated with broader environmental microbial contact, with measurable effects most consistently observed in skin and airway microbiota; effects on stable gut colonization remain less clearly defined [257] [257].
• Contact with soil- and plant-associated microbes may transiently influence immune signaling pathways, supporting regulatory immune responses rather than guaranteeing long-term gut microbial integration [287] [287].
• Natural environment exposure is linked to reduced physiological stress markers; lower stress signaling can indirectly stabilize gut microbial composition by influencing motility, permeability, and mucosal immune activity.
• Improved sleep and circadian alignment during regular daylight exposure may support microbial diurnal rhythmicity, which interacts with host metabolic and immune pathways.
• Early-life exposure to environmentally diverse microbial ecosystems is associated in epidemiological studies with lower prevalence of certain allergic conditions; this relationship appears mediated by immune education rather than by a single microbial species.
• Increased outdoor activity often accompanies nature exposure and may influence gut microbial metabolic function, including short-chain fatty acid–related pathways, primarily through host-mediated mechanisms.
• Environmental microbial transfer may include bacteria from taxa such as Pseudomonadota (formerly Proteobacteria), Actinomycetota (formerly Actinobacteria), and soil-derived organisms; these are more consistently detected on skin surfaces than as persistent gut residents.
• The overall microbiota impact of nature exposure appears cumulative and modulatory rather than curative, contributing to ecological resilience rather than abrupt compositional shifts.

Patient Guidance

• Spend time outdoors most days, even if only for a short walk in a park or green area.
• Include regular contact with natural surfaces such as grass, soil, or plants when appropriate.
• Replace some indoor sitting time with light outdoor movement.
• Maintain consistent sleep and daylight exposure to support circadian rhythm.
• Use normal hygiene outdoors, but avoid unnecessary overuse of sanitizers in low-risk settings.
• Combine nature time with relaxed breathing and reduced screen exposure.
• After stressful periods, prioritize quiet time in green environments to stabilize routine.
• Integrate outdoor time into weekly habits rather than relying on occasional intensive trips.

References

[257] Rook, G. A. Regulation of the immune system by biodiversity from the natural environment. Proc Natl Acad Sci USA. 2013. Link

This review summarizes evidence that proximity to natural environments associates with reduced mortality, cardiovascular disease and psychiatric morbidity. The authors highlight that rising chronic illness in high-income countries is associated with failing immunoregulation and persistent low-grade inflammation, partly attributable to lost exposure to evolutionarily co-adapted Old Friends microorganisms. The hypothesis links biodiversity-rich environments to immunoregulatory training that protects against chronic inflammatory disease. The findings reframe green-space exposure as immunological rather than purely psychological intervention.

[286] Flies EJ, Skelly C, Negi SS et al. Biodiverse green spaces: a prescription for global urban health. Front Ecol Environ. 2017. Link

This entry is the same Flies et al. 2017 perspective ('Biodiverse green spaces: a prescription for global urban health') as ref-266, here indexed with a different DOI variant. The paper argues that urban biodiversity supports human immune training, mental health, cardiometabolic outcomes and microbiome diversity, with mechanisms including soil/plant microbial transmission, stress reduction, physical activity and air-quality improvements. The authors call for planning-policy integration of biodiverse green-space provision as a high-leverage, low-cost public-health intervention. See ref-266 for the full summary; the duplicate citation reflects parallel use across chapters.

[287] Li Q, Morimoto K, Nakadai A et al. Forest bathing enhances human natural killer activity and expression of anti-cancer proteins. Int J Immunopathol Pharmacol. 2007. Link

This study examined the effects of a three-day forest bathing trip on natural killer (NK) cell activity in 12 healthy male subjects aged 37-55 years from Tokyo. Subjects walked in three different forest fields during a two-night trip, with blood sampled on days 2 and 3 and compared with a control workday baseline. NK activity, NK cell number, and perforin, granzyme A/B and granulysin expression in peripheral blood lymphocytes were measured. Forest bathing significantly enhanced NK activity and cytotoxic mediator expression. The findings support forest exposure as an immune-enhancing intervention.

[288] Song C, Ikei H, Miyazaki Y. Physiological effects of nature therapy: a review of the research in Japan. Int J Environ Res Public Health. 2016. Link

This review provides objective evidence for the physiological relaxation effects of natural-environment stimuli. The authors note that humans spent over 99.99% of their evolutionary history in natural settings, and the gap between this adapted state and the modern urban environment contributes to chronic stress. Reviewed studies demonstrate consistent reductions in stress-system activity (sympathetic nervous system, HPA axis) and improvements in mood and physiological markers following nature exposure. The findings establish nature therapy as a measurable physiological intervention.