In the early years of this century, demographers studying populations with unusually long life spans mapped out places where longevity correlated with certain lifestyles, marking those areas in blue.

Dan Buettner, a National Geographic fellow, has been studying these “blue zones” for more than a decade, talking to centenarians, studying their genetics, examining their diets and habits, and writing several books.

His latest is “The Blue Zones Solution: Eating and Living Like the World’s Healthiest People.”

Buettner starts by introducing the reader to five blue zones: Okinawa, Japan; Sardinia, Italy; the Greek island of Ikaria; the Nicoya peninsula of Costa Rica; and Loma Linda, Calif.

“Okinawans have long told their children to eat something from the land and the sea every day,” he writes in one chapter; in another, he lays out a “biblical diet” — based on grains, fruits, nuts and vegetables — followed by Seventh-Day Adventists in Loma Linda and says it is “demonstrably yielding the healthiest Americans.”

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The book’s main focus is on what people in these various cultures eat — there are 60 pages of recipes — but includes other lifestyle advice as well.

“Blue zone your bedroom” by setting the thermostat at 65 at night (temperatures below 54 degrees or above 75 can interfere with sleep, Buettner writes) and dim the lights all over your house an hour before bedtime.

Get more exercise by “deconveniencing” your home: Ditch the riding lawn mower, shovel your own snow.

Eat with friends and family — never alone, never standing up. And “hara achi bu” — Japanese for “stop eating when you’re 80 percent full.”