In recent years, particularly in Japan, spending time in forests has become the wellness trend du jour. A recent meta-analysis investigates whether so-called forest bathing really can significantly reduce levels of stress.
Forest bathing — a translation of the Japanese term shinrin yoku — is not a new idea, of course; ramblers have enjoyed forest walks for generations.
However, in 1982, the Japanese Ministry of Agriculture, Forestry, and Fisheries first coined the term shinrin yoku.
The act goes beyond walking through nature, according to experts who explain that shinrin yoku "can be defined as making contact with and taking in the atmosphere of the forest."
The primary benefit that proponents assign to forest bathing is a reduction in stress levels. However, others go even further.
For instance, the authors of a 2017 review concluded that "forest therapy is an emerging and effective intervention for decreasing adults' depression levels." Other researchers have investigated whether forest bathing might also help prevent lung and heart disease.
The power of nature
Aside from forest bathing, the psychological benefits of being around green spaces, in general, are also receiving growing attention from scientists. Similarly, there is some evidence, although it is low-quality, that exercising in natural environments, as opposed to indoors, improves mental well-being.
Weaving these threads together, it seems that the overall effect of nature on our mental health is a topic worthy of study.
Recently, a group of researchers from Italy set out to develop a clearer picture of forest bathing's effect on stress levels. They published their findings in the International Journal of Biometeorology.
The team designed a review and meta-analysis of relevant studies, focusing on levels of cortisol as a biomarker for stress.
Cortisol is a steroid hormone, the production of which increases during times of stress. It is possible to measure these increases in samples of saliva or serum, making it a relatively easy way to assess an individual's levels of psychological stress at any given time.
The authors screened almost 1,000 articles but selected only 22 to include in their systematic review and eight to use in their meta-analysis.
Reviewing forest bathing
The authors explain that for their analysis, forest bathing "was defined as staying in a forest, either walking or simply resting and watching it, and taking in its air for a specified amount of time."
Some of the studies that the scientists included used a control group where there was no intervention, while others compared forest bathing with other activities, such as walking through an urban area.
The authors found that all but two of the studies reported a benefit: Cortisol levels were significantly lower in the forest group compared with the control or comparison group.
The authors also noticed an "anticipatory effect" of shinrin yoku — individuals experienced a drop in cortisol just before beginning their forest session. For instance, in one study, participants' cortisol levels dropped once the researchers had informed them that they were going to take part in forest bathing. The authors explain:
"Forest bathing is considered an anti-stress practice, and planning to visit a forest seems to positively influence cortisol levels, even before physically interacting with it; therefore, watching a forest, and possibly even the sole mental visualization of a forest, may have a role in triggering anticipated placebo effects."
Although scientists do not fully understand the placebo effect, they know that it is powerful.
The placebo effect and beyond
Having assessed the available research, the authors of the current analysis conclude that "the anticipated placebo effect related to planning and visualizing the intervention may play a more important role in influencing cortisol levels [...] than the actual experience of shinrin yoku."
The researchers believe that this topic is worth following up. After all, visiting a forest is, generally, cost-effective and free of side effects, so if it can impart any physical benefits, it may be a useful tool.
However, there are significant issues with the current state of research into forest bathing, not least the small size of most of the relevant studies. The published experiments also vary greatly in quality and methodology.
Also, as the authors note, "publication bias cannot be excluded." In other words, a journal might be more inclined to publish a paper with a positive finding than one that reports no significant effect. Publication bias tends to skew the available data to appear more positive than they truly are.
It is worth noting that systematic reviews from 2012 and 2017 found no significant benefits of forest bathing. The authors of the latter conclude that "the lack of high-quality studies limits the strength of results, rendering the evidence insufficient to establish clinical practice guidelines for its use."
Another concern is that it might not be the forest itself producing the drop in stress levels. Instead, it might be the absence of urban environments that drives the effect.
Often, our stress has its roots in modern life, including work, school, or home life. Therefore, anything that reminds us of these places, even subliminally — such as buildings, cars, familiar faces, traffic fumes, or perhaps the odors that we associate with our workplace — might drive or maintain our stress levels. Simply removing these reminders of the pressures of life might reduce stress.
It is clear that scientists need to carry out much more investigation into forest bathing before doctors can begin prescribing it for stress. However, because the placebo effect appears to play an active role, it wouldn't hurt to imagine a forest stroll during your next stressful day at the office.