A study by Q Li, K Morimoto, A Nakadai et al., published in the International Journal of Immunopathology and Pharmacology provided perhaps the first evidence that a forest bathing trip is able to increase human natural killer (NK) cell activity.
Background
a) Forest Bathing
According to forest-healing.co.uk the practice of forest bathing, also known as Shinrin-yoku, originated in Japan in the 1980s. The term ‘Shinrin-yoku’ was coined by the Japanese Ministry of Agriculture, Forestry, and Fisheries in 1982.
Shinrin-yoku is translated into forest bathing in English. ‘Shinrin’ in Japanese means ‘forest’, and ‘yoku’ means ‘bath’. As defined by Qing Li Shinrin-yoku means bathing in the forest atmosphere, or taking in the forest through our senses. This is not exercise, or hiking, or jogging. It is simply being in nature, connecting with it through our sense of sight, hearing, taste, smell and touch.
The development of forest bathing as a therapeutic practice was a response to the rapid urbanisation and technological advancements in Japan, since the 1970s, which contributed to the substantial increases in anxiety and stress-related illness.
It appears that the concept of forest bathing drew inspiration from ancient Shinto and Buddhist practices that emphasize the spiritual and healing qualities of nature, as well as Japanese cultural traditions and beliefs that highlight the restorative powers of nature. Also, in accordance with Thursd.com this remedy, based on these ancient Shinto and Buddhist practices, means to let nature into your body through the five senses.
In 1988 Prof. Yoshifumi Miyazaki the author of Shinrin-yoku – The Japanese way of forest bathing for health & relaxation, was employed by Japan’s Forestry and Forest Products Research Institute, where he started his research into ‘Why we feel relaxed when we encounter nature’. And, the first forest bathing experiments were carried out on an island called Yakushima, where Prof. Miyazaki researched the effects of cedar on stress hormones in the human body. And that’s how the concept came into being.
To date, several studies indicated that Shinrin-yoku or forest bathing decreased blood glucose levels in diabetic patients, and reduced blood pressure and heart rate. Forest bathing has also been shown to reduce stress hormones such as cortisol, epinephrine (adrenaline), and norepinephrine (noradrenaline), including urinary adrenaline and noradrenaline.
One study found that salivary cortisol levels were significantly lower in participants who spent time in forest environments compared to urban settings. In addition, it increased the activity of parasympathetic nerves and reduced the activity of sympathetic nerves.
Moreover, systematic reviews have shown that forest bathing can reduce anxiety, depression, anger, and fatigue while increasing vigor, suggesting it may be especially effective for short-term relief of anxiety symptoms.
b) Natural killer cells
NK cells are a type of cytotoxic lymphocytes that belong to the rapidly expanding family of known innate lymphoid cells (ILC). NK cells represent a distinct lineage of lymphocytes, separate from T and B cells, and represent 5–20% of all circulating lymphocytes in humans.
First identified in the 1970s, their name derives from their intrinsic ability to recognize and eliminate target cells, such as those infected with viruses or undergoing malignant transformation, without the need for prior sensitization or antigen-specific priming. Morphologically, they are often characterized as large granular lymphocytes (LGLs) due to their size and the presence of cytoplasmic granules containing cytotoxic effector molecules like perforin and granzymes.
While cytotoxic T cells can only activate by detecting the antigen presented on major histocompatibility complex class I (MHC class I) molecules on infected cell surfaces, NK cells recognize and kill ‘stressed’ cells that are lacking the MHC class I molecules. They were named “natural killers” because of the notion that they do not require activation to kill cells that are missing ‘self’ markers of MHC class I.
These cells play a pivotal role in immune surveillance, constantly monitoring the body for unhealthy cells. NK cells swiftly identify and eliminate virus-infected or abnormal cells without prior sensitization. By lysing transformed or infected cells, they limit tumor growth and viral infections.
Of note, stress hormones such as cortisol and catecholamines, in general, suppress NK cell activity. It appears that NK cells are ‘the most sensitive’ cells to the suppressive effect of stress, and not surprisingly, in the past, NK cell activity has been used as a bona fide index of stress-induced suppression of cellular immunity (for review see M. Irwin, 1994). The potent suppressive effect of CAs on NK cell activity is probably due to the fact that NK cells possess the highest number of β2-adrenergic receptors among all lymphoid cells.
Synopsis
In the study by Q Li, K Morimoto, A Nakadai et al. twelve healthy male subjects, aged 37-55 years were selected from three large companies in Tokyo. The subjects experienced a three-day/two-night trip at three different forest fields; On the first day, subjects walked for 2 hours in the afternoon in a forest field; On the second day, subjects walked for 2 hours in the morning and afternoon, respectively, in two different forest fields.
Blood was sampled on the second and third days and three days prior to the trip as a control. Since it has been reported that human NK cell activity shows circadian rhythms, all samples were obtained at 8:00 am. And, NK activity; proportions of NK, T cells, granulysin, perforin, and granzymes AlB-expressing cells in peripheral blood lymphocytes (PBL) were measured.
The authors reported that eleven of twelve subjects displayed increased NK activity during and after the trip as compared to three days before. Significant differences were observed both before and after the trip and between days I and 2 in NK activity. In addition, the investigators found that Forest bathing also significantly increased NK cells in all subjects, with significant differences before and after the trip and between days I and 2 in both the percentage and total number of NK cells.
Interestingly, they also found that the forest bathing trip significantly increased the score for vigor and decreased the scores for anxiety, depression and anger. And, notably, the hours of sleep were increased during the trip compared with the control day.
Conclusion
Thus, the study by Q Li, K Morimoto, A Nakadai et al. demonstrated that a forest bathing trip can enhance the human NK activity, and the percentage and absolute numbers of NK cells. As mentioned above, this is conceivably the first report to investigate the direct effect of forest bathing on human NK activity and numbers.
As per the mechanism of enhancement of NK activity by forest bathing, the authors investigated the effect of forest bathing on the intracellular levels of perforin, GRN, and GrAi/B in PBL. They found that the forest bathing trip significantly increased both the proportion and number of perforin, GRN, GrA/B-expressing cells in PBL. Of note, these cytolytic molecules are known to contribute to NK and anti-tumor activity.
The authors also discussed that it has been reported that dominance by the parasympathetic nervous system causes an increase in circulating lymphocytes and decrease in granulocytes in human peripheral blood. The investigators found that forest bathing significantly increased the proportions of lymphocytes and monocytes and decreased the proportions of granulocytes in human White Blood Cells (WBC), suggesting a hyperactive or dominant parasympathetic nervous system in these subjects.
Overall, as discussed by the authors, their study indicated that forest bathing may have a beneficial effect by increasing human NK cell activity, an effect in part mediated by the induction of intracellular perforin, GrA/B and GRN and increased number of NK cells.
Cover Image Credit (Left Panel): Photo by @ricky_saldivar22 on Instagram; (Right Panel): Natural killer cell https://en.wikipedia.org/wiki/Natural_killer_cell
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