Visiting a Forest, but Not a City, Increases Human Natural Killer Activity and Expression of Anti-Cancer Proteins

Abstract

We previously reported that a forest bathing trip enhanced human NK activity, number of NK cells, and intracellular anti-cancer proteins in lymphocytes. In the present study, we investigated how long the increased NK activity lasts and compared the effect of a forest bathing trip on NK activity with a trip to places in a city without forests. Twelve healthy male subjects, age 35-56 years, were selected with informed consent. The subjects experienced a three-day/two-night trip to forest fields and to a city, in which activity levels during both trips were matched. On day 1, subjects walked for two hours in the afternoon in a forest field; and on day 2, they walked for two hours in the morning and afternoon, respectively, in two different forest fields; and on day 3, the subjects finished the trip and returned to Tokyo after drawing blood samples and completing the questionnaire. Blood and urine were sampled on the second and third days during the trips, and on days 7 and 30 after the trip, and NK activity, numbers of NK and T cells, and granulysin, perforin, and granzymes A/B-expressing lymphocytes in the blood samples, and the concentration of adrenaline in urine were measured. Similar measurements were made before the trips on a normal working day as the control. Phytoncide concentrations in forest and city air were measured. The forest bathing trip significantly increased NK activity and the numbers of NK, perforin, granulysin, and granzyme A/B-expressing cells and significantly decreased the concentration of adrenaline in urine. The increased NK activity lasted for more than 7 days after the trip. In contrast, a city tourist visit did not increase NK activity, numbers of NK cells, nor the expression of selected intracellular anti-cancer proteins, and did not decrease the concentration of adrenaline in urine. Phytoncides, such as alpha-pinene and beta-pinene were detected in forest air, but almost not in city air. These findings indicate that a forest bathing trip increased NK activity, number of NK cells, and levels of intracellular anti-cancer proteins, and that this effect lasted at least 7 days after the trip. Phytoncides released from trees and decreased stress hormone may partially contribute to the increased NK activity.

Trends in research related to “Shinrin-yoku” (taking in the forest atmosphere or forest bathing) in Japan

“…This enhancement in NK activity was at least partially mediated by an increase in the expression of the three cytolytic molecules mentioned above. These researchers also found that the effect of the forest environments on the NK activity lasted for more than 7 days following the trip, whereas a corresponding trip to an urban area did not cause any changes in NK activity [45]. Based on their most recent study, they report that similar results of increased and lasting NK activity after ''Shinrin-yoku'' were confirmed in female subjects [46].…”

Two Weeks of Permanence in Negatively-Charged Air Conditions Causes Alteration of Natural Killer Cell Function

“…The surrounding conditions of a forest, particularly near waterfalls, are thought to include plenty of negatively-charged particles. However, forested areas also include other substances such as phytoncides (a-pinene and~-pinene) that may affect the human immune system (41)(42)(43). In this study, there were also many factors surrounding the subjects, such as mood and acclimatization to the dormitory and training activity.…”

“…These repeated conditions may cause the steady stimulation of NK activity. Under similar circumstances, forest bathing trips are reported to enhance NK activity for approximately one month after people visit a forest area and stay for 3 days with a few hours of walking twice a day (41)(42)(43). The surrounding conditions of a forest, particularly near waterfalls, are thought to include plenty of negatively-charged particles.…”

The physiological effects of Shinrin-yoku (taking in the forest atmosphere or forest bathing): evidence from field experiments in 24 forests across Japan

“…Further, cortisol concentration also holds great significance in terms of human immunological activity. Furthermore, the study of Li et al [34][35][36] reported that forest surroundings could aid in the recovery of the human immune system, as determined from the perspective of NK activity.…”