How Hiking In Forests Can Boost Your Immune System By 50%

The concept of forest bathing first emerged as the Japanese art called shinrin-yoku. Forest bathing is the act of being in nature and connecting with it using all of our senses — sight, hearing, taste, smell, and touch (Li, 2018). The awareness that spending time in nature improves human health is certainly not new. The scientific study of this concept, however, is relatively novel — and very neat.

The saying that something good is “a breath of fresh air” is not without substance. There is a reason that breathing in fresh forest air, listening to breezes ripple through leaves, and touching the chilled waters of a bubbling stream make us feel happier, relaxed, and rejuvenated. Spending time in nature physically affects our bodies. It improves their ability to produce effective immune responses.

Leader in forest medicine, Dr. Qing Li, and colleagues conducted a study that showed that forest bathing can lead to a ~50% increase in the activity of natural killer (NK) cells. These are white blood cells that control tumors and microbial infections by limiting their spread.

I’ve summarized Dr. Li’s study regarding these cells with an intimidating name below. You’ll also find science-backed information about how hiking in forests serves as a booster for both your mental health and your immune system.

The Study

Summary

Dr. Li and his colleagues led participants of their study on a forest bathing, or shinrin-yoku, trip. They investigated the effects that walking or hiking through a forest has on human immune function. They found that over 90% of the participants showed significantly higher NK activity after the trip compared with before. This demonstrates that soaking up Mother Nature’s gifts improves the body’s ability to fight disease.

Methods 

The participants consisted of 12 men between the ages of 37 and 55. They each had their blood sampled three days prior to their forest bathing journeys to serve as a control group. 

Participants walked through a forest for two hours on Day One, then for two hours in the morning and two hours in the afternoon on Day Two. The number of daily steps for participants during the study was not significantly different from what they experienced in their typical daily routines. 

Results

Key Takeaways

• Significant increases in the percentages of white blood cells
• An increase in both the number and percentage of NKs
• Participants had increased vigor & lower anxiety, depression, and anger

The study found that these nature walks led to significant increases in the percentages of white blood cells that respond to tumors and microorganisms. These specific cells are lymphocytes and monocytes. The forest bathing experience also increased both the number and percentage of NKs. This enhances the body's ability to fight tumors and microbial infections.

In a past study, Dr. Li found that the presence of environmental phytoncides also contributes to increased NK activity in humans. Phytoncides, which have antimicrobial properties, were present during the forest bathing trip. This is likely to have been a factor in the enhanced NK presence following the nature walks.

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In addition to testing for the cells involved in the immune response, the study tested the participants’ Profile of Moods States (POMS). Following the forest bathing, the POMS test showed a significant increase in the participants’ scores for vigor and significant decreases in their scores for anxiety, depression, and anger. 

Discussion

Dr. Qing Li’s study showed that spending time in the forest leads to improved immune function, as well as elevated moods and mental states. This means a person is more likely to produce a healthy, effective immune response if they allocate time to spend in nature. Those who spend time hiking, walking, or simply sitting in nature are more likely to be happier, healthier, and less anxious.

This past winter, the record high snowfalls where I live in northern Arizona resulted in very few hikeable trails. I spent a lot of time indoors feeling stressed, anxious, and lonely. The first day that trails were not obscured by snow, I went on a seven mile hike. Within minutes, the low feelings that I had been experiencing dissipated. Being amongst trees and grand red rocks made me feel bright and limitless. 

The Science Behind The Benefits

Increasing white blood cell and NK cell activity is not the only way that nature improves immune system function. Simply being amongst the trees or beside a stream is known to reduce stress and anxiety.

High stress contributes to high blood pressure and increases the risk of heart disease. Chronically high levels of cortisol — the stress hormone — can also lead to resistance in the body’s natural immune response.

Previous studies by Dr. Qing Li have linked forest bathing to reduced levels of cortisol in saliva, lower blood pressure, and more stabilized nervous systems. Being in a natural environment will lessen your risk of developing these conditions. Diminish your stress and anxiety levels by soaking up sunshine, touching a tree, or listening to the birds sing. 

The Connection Between Hiking & Immune System Function

If you’re ready to spice things up a bit, and move from simply being in the forest to hiking in the forest, you’ll gift your body even more health benefits. Just sitting in nature improves immune and mental health. Moving in nature, however, elevates your cardiovascular health and increases your body’s strength.

Hiking doesn't have to be strenuous or technical. Hiking can be as simple as walking on a forest floor through vibrant vegetation next to a meandering stream. It may be cruising through an open, breezy meadow surrounded by the wildflowers and butterflies of a warm spring day.

Whatever hiking looks like to you, engaging in physical activity can lower blood pressure, decrease cholesterol, and improve sleep. The healthier your body is overall, the stronger your immune system is likely to be. Let your body move through trees and bathe in fresh air while your immune and mental health thrive.

Other Health Benefits Of Hiking In Forests

In addition to improving your physical health, hiking outdoors can also have a drastic positive impact on your mental health. Furthermore, recent studies highlight the importance of mental health on the body’s immunity.‍

One study assessed the effects of walking in a natural environment on the area of the brain linked to mental illnesses. The study states that over 50% of the American population lives in urban areas, and urbanization is linked to increased levels of mental illness. With the average American spending a whopping 93% of his or her time indoors, it’s hardly a surprise that mental illness rates are rising.

During this study, researchers found that those who went on 90-minute walks among trees and grasses experienced lower levels of rumination — repetitive negative thoughts about the self. They also experience reduced neural activity in the brain area linked to mental illnesses like  depression.

Being in a forest feels freeing, enlightening, and mystical. I have never been upset in the presence of 500 year old trees. Being at the top of a 14,000-foot peak makes me feel invincible. Adhering to the path of a steady creek feels like I’m sharing its journey.

Conclusion

Bathing in the sights, scents, and sounds of a forest results in improved physical health. In turn, this leads to an elevated immune response and increased cardiovascular strength. Being in nature also enhances mental health, relieving stress and anxiety and boosting moods. As humans spend more and more time indoors and in urban settings, it is absolutely essential to prioritize connecting with nature.

Let Mother Nature fulfill her role in a world that is fostering a loss of connection between humans and their natural environment. Leave the laptop and TV behind. Feel the sun on your skin, breathe in the fresh air, and hear the crunch of the earth underneath your boots.

Resources

• American Heart Association - What's the link between physical activity and health?
• Harvard Health - Health benefits of hiking: Raise your heart rate and your mood
• Nature News - A Resource for assessing exposure to environmental pollutants
• Time - The benefits of 'forest bathing'
• NIH - Current Directions in Stress and Human Immune Function
• NIH - Forest Bathing Enhances Human Natural Killer Activity and Expression of Anti-Cancer Proteins
  
• NIH - Mental Health & Immunity Review
• DOI - Functions of natural killer cells