Why Does Forest Bathing Boost Natural Killer Cell Function?

“Why Does Forest Bathing Boost
Natural Killer Cell Function?” Studies on the effects of forest bathing—
a traditional practice in Japan of visiting a forest and breathing
its air—have found it can induce significant increases in the number
and activity of natural killer cells that can last for as long as a month. And because natural killer cells are one
of the ways your body fights cancer by killing off tumor cells, the
findings suggest that forest visits may have a preventive effect on
cancer generation and progression. OK, but how? Why did the
forest environment increase natural killer cell activity? What is
it about the forest environment? One thought is that the boost may
be related to a reduction in stress. If you measure the amount of adrenaline
flowing through people’s systems, did hanging out in a forest, but not
in a city, drop adrenaline levels down? Yes, so that checks out, but drip some
adrenaline on human blood cells in a Petri dish, and there does
not appear to be any effect. The stress hormone, cortisol,
on the other hand, dramatically suppresses
natural killer cell activity. So maybe the forest led to less
stress, less cortisol, which released the natural killer cells under its
thumb, and you get that big boost? We know being surrounded by nature can
decrease levels of cortisol in our saliva, but what about our bloodstream?
A significant drop after a single day trip to the forest. But a week later
the cortisol was normalizing, and the forest effects sometimes
appeared to last an entire month. Anything else that could cause a
longer-term immune system change? Maybe we’ve been missing
some of our old friends. If you sample outdoor air, you can pick
up an abundance of microorganisms floating around from the soil or water,
which are absent in indoor air (which is dominated by organisms that
either live on us or trying to attack us). So maybe on a day-to-day basis,
in terms of keeping our immune system on ready alert, it might not be sufficient
to encounter only the biased microbes of the modern synthetic indoor environment
that lacks some of the old friends, and probably bears little resemblance
to the microbes we evolved to live with over millions of years. Or maybe it’s the plants themselves.
Maybe it’s the aroma of the forest. Trees produce aromatic volatile
compounds called phytoncides like pinene, which you can breathe into
your lungs in the forest. But do these compounds actually
get into your bloodstream? One hour in the woods and
you get like a 6-fold increase in circulating pinene levels
circulating throughout your system. OK, but to fully connect all the
dots, the phytoncides like pinene, these tree essential oils
would have to then induce human natural killer cell activity.
And guess what? Phytoncides induce human
natural killer cell activity. If you stick natural killer
cells in a Petri dish with some unsuspecting leukemia
cells, your killers can wipe out some of the cancer cells. But add
a whiff of cypress, white cedar, eucalyptus, or pine and the
cancer cells don’t stand a chance. A combination of wood aromas
improved the recovery of mice put through the wringer, but this
is the study I was looking for. If we want to know
if the magic ingredient is the fragrance of the forest, then let’s see if we can get that same
boost in natural killer cell activity just vaporizing some essential
oil from one of the trees into a hotel room overnight.
And it worked! A significant boost in natural killer cell activity, though
it just boosted their activity rather than their number, and being
in the actual forest can do both. So maybe it’s a combination of the tree fragrance and the lower
cortisol levels working together? Ironically, these phytoncide compounds
are part of the tree’s own immune system, which we may be able to commandeer. The researchers speculate these
compounds may be playing some role in the fact that more heavily forested
regions in Japan appeared to have lower death rates from breast
cancer and prostate cancer. Being out in nature has been found
to be an important coping strategy among cancer patients. It turns out
this could potentially be helping more than just with the coping,
thanks to the fragrance of trees.
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