Getting Back to Nature Boosts Natural Killer Cells

Analysis by Dr. Joseph Mercola Fact Checked

Story at-a-glance -

  • Spending time in the woods, called forest bathing, may boost your ability to fight viruses and infections
  • Data revealed the same effects are not experienced by being outdoors in an urban environment; diffusing cypress stem oil indoors produced similar but weaker results
  • Spending time in green spaces has additional benefits, including blood pressure reduction, cortisol secretion and lowering the incidence of diabetes
  • Other healthy habits that support your immune health include taking care of your gut microbiome, exercising, getting quality sleep and maintaining adequate vitamin D levels

During the COVID-19 pandemic, many are asking how they can strengthen their immune system to reduce the potential of getting sick, and how they can support their body if they have a viral infection. As Dr. Roger Seheult explains in this video, natural killer cells are part of your immune response.

While there is no known drug or medication that kills a virus, there are nutraceuticals that have an effect on them. Some of the nutraceuticals support your immune system, while others have the ability to reduce your viral load, as discussed by Mark McCarty and James DiNicolantonio.1

Solar ultraviolet-B radiation2 and supplemental vitamin D3 have also been shown to reduce pandemic fatality rates; this makes sense considering how important vitamin D is for controlling infections and lowering your risk for influenza and the common cold.

How Your Body Uses Natural Killer Cells

Natural killer cells are part of a larger group of cells called lymphocytes. These play a key role in your immune response and defense against viral infections. Inside the cytoplasm of natural killer cells are lytic granules,4 such as granulysin, perforin and granusyme A and B.5

Natural killer cells work by recognizing stressed cells and then using cytotoxic abilities to kill the abnormal cell.6 Once recognized, natural killer cells secrete the granules into a synapse delivering the granules to a target cell. These then trigger apoptosis in the infected cell.

Defects in this system are related to fatal diseases,7 such as Griscelli syndrome type 2.8 This rare, inherited condition affects the immune system, leaving individuals prone to recurrent infections.

Natural killer cells control infection by limiting tissue damage and spread and they help control tumor growth by limiting the spread of abnormal cells.9 They also regulate cells engaged with macrophages, T cells and endothelial cells. Additionally, they can mediate the immune response.

As Seheult explains in the video, natural killer cells are part of the innate immune system, which is the first line of defense against pathogens.10 The next step is adaptive immunity, also called acquired immunity. This response is specific to the pathogen that enters your body. When you build up an immunity to a pathogen, it is the function of your adaptive immune response.

A Forest Walk May Boost Your Body’s Ability to Fight Viruses

Scientific evidence shows that walking in a forest, also called forest bathing, can help enhance your natural killer cell activity, including the number of cells and the length of activity. Research has been done in men and women involving field trips to the forest. In this first study,11 researchers engaged 12 healthy men who normally worked at large companies in Tokyo.

The participants were taken on a three-day, two-night trip to three different forests. On the first day, they walked for two hours; on the second day they walked for two hours both in the morning and afternoon; and on the third day they left for home. Blood was drawn in the morning before any intervention and after the first and second days of walking.

The study team analyzed natural killer cell activity and the proportion of lytic granules and natural killer cells. On the last day, 11 out of 12 of the subjects had 50% higher activity compared to their levels before entering the forest. The researchers believed:12

“Taken together, these findings indicate that a forest bathing trip can increase NK activity, and that this effect at least partially mediated by increasing the number of NK cells and by the induction of intracellular anti-cancer proteins.”

Later, the same team performed the same intervention on 13 healthy women.13 Blood and urine samples were also collected seven days and 30 days after the trip was completed. The team also measured concentrations of phytoncides in the forest, which are airborne chemicals emitted by the plants.

The results of this study showed natural killer cell activity remained raised for more than seven days, as did the number of natural killer cells and the “levels of intracellular anticancer proteins.”

The team questioned whether the effects on the natural killer cells was representative of being outdoors, due to exercise or related to the trees. They designed and executed another study14 with two groups of men who visited two different forests or a city for three days and two nights.

The intervention was the same with blood drawn and urine samples collected before the study began, each day and on Days 7 and 30 after the intervention was complete. During this study they also measured the phytoncides in the forest and the city. As you might expect, measurements in the forest were high, but in the city, most levels were so low they couldn’t be measured. The researchers concluded:15

“The present study confirmed that a forest bathing trip enhances the immune response as measured by human NK activity and the numbers of NK cells, as reported previously. In contrast, a trip to places without forests (city tourist visit) had no effect on NK activity or the numbers of NK cells, indicating that forest bathing does indeed enhance human NK activity.

Moreover, we also found that the increased NK activity and numbers of NK cells induced by a forest bathing trip lasted more than 7 days, even 30 days, after the trip. This suggests that if people visit a forest once a month, they may be able to maintain increased NK activity. This may be important in health promotion and preventive medicine.”

Can You Repeat These Results at Home?

Once the researchers had identified phytoncides as the probable reason individuals were experiencing a higher measure of natural killer cell activity, they designed a follow-up study.16 In this they attempted to recreate the boost in natural killer cells using essential oils from trees.

They engaged 12 healthy men who stayed in a city hotel for three nights between 7 p.m. and 8 a.m. In the morning they returned to work in the city. During the night, researchers ran a humidifier with stem oil from the hinoki cypress tree. “Blood samples were taken on the last day and urine samples were analyzed every day during the stay,” they reported.

Phytoncide measurements in the room were also taken. The study team found there was a statistically significant jump in natural killer cell activity after sleeping in the hotel room, despite the fact they returned to work in the city each day. Urine was analyzed for adrenaline and noradrenaline, since the presence of adrenaline reduces the number of natural killer cells.

Although there was a statistically significant difference in the measurement of adrenaline in the urine of those who stayed in the forest,17 the same difference did not occur in all who stayed in a hotel with stem oil diffused in the room.18

When the data were analyzed together, there wasn’t a statistically significant drop in the excretion of adrenaline in the urine. However, when the researchers separated the samples into those where the adrenaline increased and those where it decreased, there was a statistically significant difference in the reduction of adrenaline in the urine.

They concluded that, while there was beneficial activity to short-term exposure, prospective studies in people exposed to phytoncides long-term would be needed to confirm the effect.

You Also Benefit From Green Spaces

If you don't have a local forest, you can still benefit from exposure to green spaces. In one meta-analysis of 143 studies,19 scientists noted statistically significant reductions in blood pressure, heart rates and levels of cortisol secretion measured in saliva. They also identified decreased incidences of diabetes and all-cause and cardiovascular mortality.

The researchers from the University of East Anglia20 analyzed studies looking at postoperative recovery time. In all, there were 290 million participants across all 143 studies from 20 different countries.

When they correlated the amount of time individuals had spent in green areas with 100 health measurements, they also found that women who were pregnant had a reduced risk for preterm birth and a lower risk of delivering a baby who was small for gestational age.

The relationship may be related to a higher level of physical activity that often comes in green spaces. It could also have something to do with social interaction or greater exposure to sunlight. The researchers concluded:21

“Green prescriptions involving greenspace use may have substantial benefits. Our findings should encourage practitioners and policymakers to give due regard to how they can create, maintain, and improve existing accessible greenspaces in deprived areas.”

Develop Habits to Support Your Immune System

Developing healthy habits is important to support your immune system, reduce the potential for viral infections and lower your risk of chronic disease. While there are multiple choices you make each day, the following factors are foundational to creating a strong health plan.

Gut microbiome — Your gut microbiota is key to your immune system and digestive health. Fermented food and fiber are vital to repopulating your gut with beneficial bacteria and providing the necessary nutrients for them to thrive.

Exercise — Research evidence underscores the benefits of exercise to your immune health. Seek to raise your heart and breathing rate for 30 minutes each day in addition to moving out of your chair frequently. Your body requires both exercise and movement to stay healthy.

Sleep — The quality and quantity of your sleep has a regulatory effect on your immune system.22 Developing good sleep habits requires just a little effort. If you struggle with getting to sleep or staying asleep, consider the tips I share in “Top 33 Tips to Optimize Your Sleep Routine.”

Vitamin D — This is another powerful component to supporting your immune system and the prevention and treatment of viral infections, including COVID-19. To read more about maintaining appropriate levels and how it may help reduce your risk of COVID-19, see “Vitamins C and D Finally Adopted as Coronavirus Treatment.”