Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Previously, I showed how exposure to nature can have self-reported psychological benefits, but there was a dearth of data on changes in objective measurements. So, I was excited to see this paper on the effects on levels of the stress hormone cortisol in the saliva of those partaking in “forest bathing”––which just means visiting a forest and surrounding yourself by trees.

The level of cortisol in your saliva is considered an indicator of your stress level, and after walking in a forest, compared to walking in a city, or even after just hanging out in a forest compared to a city, people’s salivary cortisol levels were significantly lower. But wait a second, the same effect was found before they went to the forest. Huh? Forest bathing was associated with significantly lower salivary cortisol both before and after, compared with visiting an urban area. Therefore, it appears that just the thought of going to spend the day in the forest relieved stress. So, when comparing the effects of forest bathing versus urban visiting, the anticipation placebo effect may play a more important role in influencing stress levels than the actual experience of being in the forest. So, I was ready to dismiss it as just another nebulous psychological effect until I read this. Studies on the effects of forest bathing on the immune function showed that visiting a forest can induce a significant increase in the number and activity of natural killer cells––one of the ways our body fights off cancer. That got my attention.

It all started with this study. Twelve men were taken on a long weekend to walk in some forests, and almost all of the subjects (11 out of 12) showed higher natural killer cell activity after the trip––and not just a little; about a 50 percent increase compared to before the trip. Now, just exercise can affect immune function, but they weren’t walking any extra; they were just walking in a forest instead. Yeah, but they also were taken on a trip somewhere, introducing other variables. So, how about randomizing them to go on some city trip versus the forest trip? And if there is some special forest effect, how long does the effect last? Do you have to, like, walk in the forest every day? Before jumping into all that, how about we first see if it works in women too?

Same kind of setup, and same results: a significant boost in natural killer cell activity walking around in the woods. And this time, they went back a week later to retest them, and they were still up––though after a month, they came back down. But hey, once a week should do it. But it was a multiple-day trip. Who has time to hang out in forests all weekend, every weekend? How about just a little day trip? The title gives it all away. Boom! Same thing! The same big jump measured the day after the trip, compared to before, and with the same staying power. Natural killer cell activity still boosted a week later. This suggests that if people visit a suburban forest park once a week on a day trip, they may be able to maintain the increased anti-cancer immunity.

Okay, but I’m still not convinced. How can you attribute the benefit to the forest itself, when all you have is before and after data? To make the case that nature had anything to do with it, you’d need a control group that took the same kind of trip but went to somewhere else instead. And…here we go. It turns out visiting a forest, but not a city, increases human natural killer activity. Here’s the forest data, just like before, but nothing on a trip to go walking in a city. By the end of the forest trip, 80 percent of the subjects experienced a boost, compared to only 1 in 10 of the city walkers.

And, both trips were matched for physical activity, and alcohol, and sleep—other things that can affect immune function. And so, here we go. Confirmation of boosted immunity––but only on the forest trip, “indicating that forest bathing does indeed enhance [natural killer cell] activity.” Moreover, they found that the increased activity lasted up to 30 days after the trip. Check it out. Still up a week later, and even a bit up a month later. “This suggests that if people visit a forest once a month, they may be able to maintain increased [natural killer cell] activity.”

Okay, so, now that we know that it’s a real effect, the next question is, why? What is it about forests that gives us the boost? And (you can imagine Big Pharma thinking), can you make it into a pill? We’ll find out, next.

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