For two decades, blue zones — known for having the most people in the world who live to age 100 or older — have held the interest of longevity-seekers. Now, researchers are questioning the idea of blue zones and whether or not places like Okinawa, Japan and Loma Linda, California actually have the number of centenarians that has been reported.

In 2005, Dan Buettner, who at the time was a reporter for National Geographic, wrote about visiting and studying areas around the world with abnormally high concentrations of centenarians. He deemed them blue zones — a term that first appeared in an academic paper about longevity in Sardinia, Italy published in 2004.

The five blue zones are:

• Okinawa, Japan
• Ikaria, Greece
• Nicoya, Costa Rica
• Loma Linda, Calif., U.S.
• Sardinia, Italy

Across these areas, residents seemed to have common practices like eating a mostly plant-based diet, having a purpose, prioritizing relationships and belonging to a faith. Buettner believed these behaviors could explain their longevity, and declared them the Power Nine.

But researcher Saul Justin Newman believes the large number of centenarians that are reported in blue zones are a result of something less remarkable: pension fraud and bad record keeping.

In 2019, Newman, a senior research fellow at the University College London Centre for Longitudinal Studies, shared a preprint paper about his findings. The paper, which has yet to be peer-reviewed, took a closer look at databases for the areas in the world with the longest-living people in countries like the U.S. and Japan. Newman found something interesting while working to identify the places with high concentrations of centenarians and supercentenarians.

"At the time, I had mapped more than 80% of the world's 110-year-olds. And what was striking is that those 110-year-olds were not falling in regions with good health," Newman said in a recent episode of Vox Media podcast, "Today, Explained."

"They were falling in regions that had terrible health, and surprisingly that was the shared characteristic of most blue zones. I found that the centenarians in the blue zones were missing or dead when the study was conducted."

Newman discovered that pension fraud and reports of "missing" centenarians were common in certain countries with blue zones like Japan and Greece: "They were getting older on paper, but they were already dead."

There's also the matter of using birth certificates to determine centenarians. Some birth certificates are missing, and others could be incredibly inaccurate, Newman told NPR, which is a common issue in demography when studying older populations.

"If you go into a hospital without paperwork, there's no machine in there that says, 'Oh, bing!' and knows your age," he told NPR. "So if you get consistently wrong errors in your paperwork, they're undetectable."

Buettner and his fellow researchers responded to Newman's claims in a rebuttal published to Buettner's website. In the letter they state that Buettner's claims about four of the blue zones, Okinawa, Sardinia, Ikaria and Nicoya, "have been fully validated by strict demographic criteria."

Buettner told The New York Times that he frequently visits the blue zones to make sure birth records are accurate. In Sardinia, he and his team compared birth records to "civil databases, handwritten church archives and genealogical reconstructions," according to the publication.

But Buettner did admit to The New York Times that Loma Linda was a bit of an "outlier." The California city was added to the list of blue zones in Buettner's initial article in National Geographic because his editor told him, "you need to find America's blue zone."

'I agree with the philosophical idea of people who live that way,' longevity expert says

Whether or not blue zones actually are what they have long been touted to be, longevity researchers still endorse the practices Buettner's research helped identify.

"I think people who adhere to that type of lifestyle generally do well. And whether the actual blue zones are really blue zones, I'm not sure. [They] could be," Dr. Frank Lipman, a longevity researcher and doctor of functional medicine, tells CNBC Make It.

"It's pretty well known that the way, supposedly, people live in blue zones with community and moving their bodies and having some meaning in life, that all makes sense. And yes, those people do well."

Since Buettner first conducted his research in blue zones, he's witnessed many of their habits change which could negatively affect their longevity, he told The New York Times. "I think you're going to expect to see all these blue zones gone within a generation or even a half a generation," he said.

Still, Lipman's longevity research supports the practices that align with Buettner's list of nine common denominators of the world's longest-living people, which include keeping family close and drinking a little bit of alcohol each day.

Lipman says that his patients who follow these behaviors tend to be healthier, and can live longer:

• Having a purpose
• Surrounding themselves with community and love
• Practicing gratitude
• Eating a healthy diet
• Avoiding overeating
• Moving their bodies often
• Prioritizing their sleep

"All the factors that come up in Blue Zones are correct. I see the same factors affecting my patients," Lipman says.

"I like the concept. I agree with the philosophical idea of people who live that way. Whether that actually happens in those areas, I'm not sure."

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