No exercise, no supplements, just 'Shinrin-yoku': In 1982, Japan introduced ‘forest bathing’, now scientists find a 3-day forest trip may boost immune cells for more than 30 days

Have you ever imagined what is one of the simplest ways to support your immune system? You would be surprised to know it does not involve any supplement, no complicated workout and no prescription. All you have to do is just spending time among trees.

A fascinating line of research from Japan has found out what happens to the body's natural killer (NK) cells after people spend several days in a forest. The findings around 'forest bathing' and immune function are particularly interesting because the effects were not limited to the time people spent among the trees. Researchers found that increased NK cell activity remained measurable for more than 30 days after a forest-bathing trip.

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That does not mean a walk through the woods can prevent cancer or viral infections. But the research offers an intriguing look at how the natural environment may influence the body's immune system.

What is forest bathing or Shinrinyoku?

In Japan, the practice is known as “Shinrin-yoku”, which translates roughly to forest bathing. It is essentially a leisurely visit to a forest for relaxation and recreation, involving slow walks, quiet surroundings and breathing in the forest air.

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The practice was first proposed as part of a healthy lifestyle by Japan's Forest Agency in 1982 and has since become associated with relaxation and stress management. Unlike intense exercise, forest bathing does not require a set route, specific pace or breathing technique.

The idea is simply to slow down and immerse yourself in the forest environment.

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Japanese study found changes in natural killer cells

Qing Li, an immunologist at Nippon Medical School in Tokyo, has conducted research examining how forest environments affect human immune function.

In one study, participants went on a 3-day, 2-night forest-bathing trip. Researchers collected blood and urine samples during the trip and again after participants returned home.

The scientists measured natural killer cell activity, along with the number of NK cells and immune cells expressing proteins including granulysin, perforin and granzymes A and B.

The results were commendable. NK cell activity increased during the forest trip, while the number of NK cells and the levels of several immune-related proteins also rose. More importantly, the increase in NK activity remained measurable more than 30 days after the forest trip.

City walks did not produce the same result

One of the most interesting parts of the research was the comparison with urban environments. Participants who visited a city as tourists did not experience the same increases in NK activity, NK cell numbers or intracellular granulysin, perforin and granzymes.

That distinction suggests the effect was not simply because people were taking a break from their normal routine or walking around somewhere new.

What are natural killer cells?

Natural killer cells are part of the body's innate immune system. They play an important role in identifying and destroying certain abnormal cells, including virus-infected cells and some cancer cells. That makes them an important component of the body's first-line immune defenses.

However, one must understand that study is not a proof that forest bathing prevents cancer, cures infections or guarantees better health. The research examined specific immune markers, not whether participants actually developed fewer diseases.

Still, the persistence of the changes is what makes the findings particularly fascinating.

Why could trees affect the immune system?

Researchers have proposed that phytoncides may play a role. Trees release these volatile organic compounds as part of their natural defense mechanisms against insects, bacteria and other threats.

Forest air can contain compounds associated with tree essential oils, including alpha-pinene and limonene.

When people walk through a forest, they naturally inhale these compounds. The research suggests that exposure to the forest environment, including these airborne substances, could contribute to changes in immune activity.

Could one forest trip a month be enough?

The researchers found that increased NK activity lasted for more than 30 days following the forest-bathing trip. Based on this finding, the review suggests that a forest-bathing trip once a month could potentially help maintain a higher level of NK activity.

That does not mean one monthly forest visit will keep your immune system permanently “boosted,” and the research should not be treated as a prescription. But it does offer a remarkably simple idea: regular exposure to natural environments may have measurable effects on the body.