More trees in your area, more Natural Killer Cells in your body
The most underestimated part of your immune system is that of the innate immune system: the part of your immune system that reacts almost reflexively when a germ enters your body, or if a cell turns into a cancer cell. Natural Killer Cells are key players in this innate immune system. According to an older Japanese human study, the body makes more and more aggressive Natural Killer Cells during a stay in a natural environment where there are many trees.
The researchers had 12 healthy men aged between 35 and 65 spend 3 days in the woods [Forest bathing trip]. On another occasion the men visited for three days a city [City tourist trip]. The researchers ensured that the men's daily routines were practically identical on both occasions.
Already on the first day of their stay in the woods, in the men's blood the amount of Natural Killer Cells increased. Four weeks after they had returned to their hometown, the number of Natural Killers Cells in their system was still increased. The stay in the city, on the other hand, had no positive effect on the number of Natural Killer Cells.
The stay in the woods had the same positive effect on the activity of the Natural Killer Cells.
The natural environment of the forests decreased the concentration of adrenalin in the subjects' blood. This suggests that a natural environment reduces stress. In theory, this decrease of hormonal stress could have a positive effect on the Natural Killers Cells.
The researchers suspect that phytoncides also play a significant role in the immunostimulating effect of trees. Pytoncides are volatile substances which are released by trees and, according to fundamental research, may have a positive effect on health. The Japanese found significantly higher concentrations of these phytoncides in the air of the forests that the test subjects visited than in the city air.
"A forest bathing trip can increase human Natural Killer Cell activity, [and] the number of Natural Killer cells", the researchers write. "Forest bathing may contribute to decreased stress and improved immunity, and phytoncides from trees may partially contribute to this effect."
Int J Immunopathol Pharmacol. 2008 Jan-Mar;21(1):117-27.
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