Randomized controlled trial on the efficacy of forest walking compared to urban walking in enhancing mucosal immunity.

The short version

This randomized controlled trial with 78 healthy men aged 40 to 70 explored whether walking through a forest affects immune function differently than walking through a city. The researchers focused on secretory immunoglobulin A (sIgA), an antibody found in saliva that forms part of the body's mucosal immune system — the first line of defense lining the nose, mouth, and airways against viruses and bacteria. They also tracked cortisol, the body's primary stress hormone, and mood, on the idea that stress and immune function are closely linked through the body's hormonal stress-response system. Eighty-four participants were randomly assigned to a 90-minute guided walk, either along a certified forest-therapy trail in a Tokyo forest park or an equivalent route through Tokyo's Ginza shopping district, walked in two 45-minute segments with a shared lunch. Guides kept pace, distance, and rest breaks consistent between groups and encouraged participants to engage their senses with surroundings appropriate to each setting. Saliva and blood were sampled before and after walking to measure sIgA, cortisol, and related markers, and mood was measured with the POMS2 questionnaire. Forest walking significantly increased salivary sIgA compared with urban walking (adjusted difference +32.1 µg/mL, 95% CI 2.3–62.0, meaning a real, though modest, immune-marker boost; p = .035). Cortisol dropped significantly more after the forest walk (p = .007, indicating a stronger stress-hormone reduction), and dopamine — linked to pleasure and motivation — rose more as well (p = .048). Mood benefited too: participants felt more energized (higher Vigor-Activity, p = .001) and less generally distressed (lower Total Mood Disturbance, p = .007) after the forest walk than after the urban walk. Interestingly, white blood cells and certain immune cell counts (CD4+ and CD8+ T cells) rose more after the urban walk, which the researchers link to the body reacting to city air pollution rather than a benefit of urban walking. No one experienced side effects, and everyone who started the walks finished them. These results suggest a single, unhurried forest walk can meaningfully support immune defenses and lower stress compared with an identical walk in the city, at an activity level gentle enough for most adults. The study only tested one walking session in healthy middle-aged and older Japanese men, so it can't say whether the benefits build with regular practice or extend to other groups. Participants and staff also knew which environment they were in, which could shape self-reported mood. Still, it adds solid evidence that time in nature is a low-risk way to support wellbeing.

Key findings

• A 90-minute guided walk through a forest significantly raised levels of secretory immunoglobulin A (sIgA), an antibody in saliva that helps defend the body's mucous membranes against infection, compared to an identical walk through a city.
• The forest walk also significantly lowered cortisol, the body's main stress hormone, while urban walking did not produce the same reduction.
• Mood improved more after the forest walk than after the urban walk, with participants reporting higher vigor and lower overall mood disturbance on a validated mood questionnaire (POMS2).
• The benefits appeared after a single walk in healthy men aged 40 to 70, suggesting forest walking is a low-intensity, low-risk activity that can be incorporated into everyday life for immune and stress support.
• Because the study was not blinded to participants and examined only a single session, more research is needed to confirm whether these benefits hold up with repeated, habitual forest walks and in other populations.

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