Forest bathing, an ancient Japanese medicinal treatment used to reduce stress and anxiety, has recently been studied for its therapeutic effects on the immune system. Researchers have determined that spending time in nature effectively improves immune response by increasing natural killer cell production

Forest bathing, also known as shinrin-yoku, is the ancient practice of visiting a forest and breathing in its air.^[i] A Japanese nature therapy practice used as a natural remedy for stress relief and mental fatigue, forest bathing has received much scientific attention in recent years, with many studies exploring the physiological and psychological benefits of spending time in nature.^[ii]

Urbanization is a growing global trend, and 68% of the world's population is projected to live in urban areas by the year 2050.^[iii] Urban living environments are associated with increased anxiety and mental health concerns, and urbanicity also has numerous negative impacts on physical health, including increased weight gain, poor food quality, and increased risk of cardiovascular problems when compared to those living in rural areas.^[iv],[v]

For these reasons and others, researchers have begun tracking and comparing the effects of nature excursions such as forest bathing on human health. In addition to psychological benefits, there is now evidence that forest bathing may improve immunological function.

Immunological Benefits of Forest Bathing

A study conducted in Japan explored the effects of forest bathing on immune function. Healthy male participants between the ages of 35 and 55 years were selected to participate in a three-day nature trip that involved hiking in the woods.^[vi]

Natural killer (NK) cell levels were measured in the men before and after the trip, and nearly all participants experienced an increase in natural killer cell activity after the trip.^[vii] Researchers also measured perforin, granzymes and granulysin-expression in peripheral blood lymphocytes and found that the trip dramatically increased the production of these anti-cancer proteins, signifying that forest bathing may indeed increase immunological function.^[viii]

This wasn't the first study to find shinrin-yoku beneficial for stimulating immune function, however. A study of healthy young females found similar results in natural killer cell production and anti-cancer proteins after a three-day nature excursion, and found that the results lasted at least seven days after the trip had ended.^[ix]

Researchers believe that phytoncides, a type of aromatic compound released from trees and plants, may be responsible for the decrease in hormone stress levels and increase in NK production.^[x],[xi] Other studies have backed these results and found that NK production levels were still increased even 30 days after such trips, suggesting forest bathing once a month may drastically improve immunological function.^[xii]

Other Researched Benefits of Forest Bathing

In addition to its benefits on the immune system, forest bathing has been studied for its positive effects on a variety of ailments including:

• Stress. Forest bathing has been shown to effectively alter cortisol levels, a biomarker of stress.^[xiii] Additionally, the placebo effect of forest bathing is quite strong and reduced the salivary cortisol levels of participants before they had even experienced the forest bathing intervention.^[xiv]
• Depression and anxiety. Anxiety disorders are the most common form of psychiatric disorder and affect approximately one-third of the population. Further, about 10% of those who experience mild forms of depression are at risk for later developing clinical depression.^[xv],[xvi]

Researchers have demonstrated that forest bathing effectively lowered heart rate and systolic blood pressure, indicating an increase in parasympathetic nervous activity and a decrease in sympathetic nervous activity, even when the subject's exposure to natural settings was as short as 15 minutes.^{[xvii],[xviii]}

Participants also experienced decreased scores for anger, depression, fatigue and confusion, while urban walkers experienced higher scores in these areas.^[xix] These findings suggest that even brief walks in nature can significantly reduce anxiety and enhance psychological health, and could present a potential treatment for anxiety disorders and depression.^[xx],[xxi]

• Cardiovascular disease. Researchers studied the effects of forest walks on healthy individuals in a study involving 48 young adult males.^[xxii] The researchers measured heart rate variability, systolic blood pressure and diastolic blood pressure and compared results in participants who walked in the forest as opposed to those who walked in an urban setting.^[xxiii]

In addition to these physiological measurements, participants also reported their feelings in questionnaires. Forest walkers reported feeling "refreshed" after their walk, while urban walkers scored significantly lower in this category.^[xxiv] Overall, researchers suggested that the positive effects on cardiovascular response after forest bathing may reduce the risk of cardiovascular disease and improve heart and mental health.^[xxv]

• Poor mood, anger and fatigue. In a study involving 128 middle-aged or elderly participants, researchers demonstrated that forest walks may improve mood, and found that feelings of anger, fatigue and bewilderment were significantly lowered after nature walks.^[xxvi] These results were consistent even when the walks were kept to a one- or two-hour period and a short 2.5 km (1.5 mile) distance.^[xxvii]

The benefits of spending time in nature are not new to anyone who has recently visited the forest. It's clear that the physical and psychological benefits of these nature excursions are vast, and researchers are only just beginning to understand the myriad of health conditions that could be improved by forest bathing.

If you'd like more information on the immune system or other ways to naturally stimulate your immune response, please visit the GreenMedInfo.com research database on immunostimulatory agents. You can also find more studies on the practice of forest bathing and its research-backed benefits.

References

[i] Int J Biometeorol. 2019 Aug;63(8):1117-1134. doi: 10.1007/s00484-019-01717-x. Epub 2019 Apr 18.