Blood tests showed their natural killer cells had increased 40 percent

Li brought a group of middle-aged Tokyo businessmen into the woods in 2008. For three days, they spent a couple of hours each morning hiking. By the end, blood tests showed their natural killer cells had increased 40 percent. Moreover, the boost lasted for seven days. A month later, their NK count was still 15 percent higher than when they started. In contrast, during urban walking trips of the same duration, NK levels didn’t change. Since then, Li has published results from similar studies with male and female subjects in half a dozen peer-reviewed journals. In one, Li was curious to know if a one-hour trip to a city park would have a similar effect, since most of us can’t spend three days a week walking in the woods. It did, although the immune surge didn’t last quite as long.

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