(CNN) You have probably heard that Japan consistently has among the highest life expectancy in the world and that the island of Okinawa has the greatest concentration of centenarians (people who live to at least 100).

I have long been fascinated by centenarians and the life lessons they have to share. But the truth is, I have never been sure I would want to live that long unless I could somehow be certain I would have a strong mind and body.

That is why this next statistic really caught my attention: Nearly two-thirds of the residents of Okinawa are still functioning independently at age 97. That meant they were in their own homes, cooking their own meals and living their lives fully -- at nearly 100 years old!

For too long, we have too strongly identified age as a number. And yet, most doctors don't focus that much on age. They really want to know more about the physiological age of their patients than the chronological one . They want to know more about their patients' lives, rather than the length of time they have been alive. After all, there are plenty of 50-year-olds whose health is more like that of a 70-year-old. But the opposite is also often true, and that is what I wanted to find when I decided to travel to Okinawa last summer.

Okinawa resident Umeto Yamashiro, 97, with CNN's Dr. Sanjay Gupta.

Okinawa is a chain of islands about 400 miles southwest of mainland Japan. The name means "rope in the open sea" because Okinawa is plopped in the middle of the oceans, with the Pacific to the east and the East China Sea to the west. The water is emerald blue, the beaches are sandy white, and the weather is tropical. In many ways it seems the perfect place to house the Land of Immortals, as the island has long been called.

After landing on the main island of Okinawa, I could immediately tell that something was different there. Although I didn't see people running outside or going to gyms, I did see plenty of active elderly people: gardening in their yards, doing tai chi in the park, riding bikes and playing a croquet-like game called gateball with their friends.

The elders there are less likely than their counterparts in the United States to have heart disease, dementia or certain cancers. Their bones are even stronger than those of similarly aged people around the world. Many of these residents, I learned, are the subjects of one of the largest studies of centenarians ever conducted. Since 1976, nearly a thousand centenarians have been studied, and their wonderful secrets are revealed in the first episode of my docuseries " Chasing Life ."

If you ask anyone in Okinawa why they live so long, you will doubtlessly hear two words: ikigai and moai.

Ikigai, loosely translated, means sense of purpose in life . And in Okinawa, a person's ikigai often grows as they get older. It is their reason for living, that thing that propels them out of bed in the morning. In the United States, people often retire in their mid-60s, but there isn't a similar word in Japanese because the concept of retirement doesn't even exist.

Moai is an informal social group of people who have common interests and look out for each other. Your moai is your "tribe" and another reason Okinawans believe they live so long.

But, more than anything, the Okinawa diet has long captured the headlines, and for good reason. Before I tell you what the Okinawans eat, there is a valuable lesson in how they eat.

Remember this term: hara hachi bu. Translation: Stop eating when you are 80% full. With all the talk about calorie restriction, this notion is often hard to incorporate into your life, especially in a "clean your plate" culture.

With hara hachi bu, the philosophy is that you should still be a little hungry when you push the plate away. Having adopted this practice myself, I more often skip dessert, reduce my portion sizes, use smaller plates and eat more slowly. While the average calorie consumption for an American man is 2,500 calories a day, in Okinawa, it is closer to 1,900 calories.

There is a basic biological reason this works. It takes about 20 minutes for the stomach to send signals to the brain that it is full. Unfortunately, most people can shovel down another several hundred calories in that short time. Instead, if you push the plate away and just wait, you will have eaten less and still feel satisfied.

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Most everyone in the scientific community promotes the idea that eating less is associated with longevity, but of course, that also depends on what you eat. In Okinawa, I shared a meal with Craig Willcox, the author of the book " Okinawa Program ," to learn more about the local diet.

We received our food on six compartment trays, known as bento boxes, which was already radically different than how we typically receive our lunch in the United States. Eating bento box-style meant we ate smaller portions and a wider variety of foods. Our six compartments came with rice (which is served at most meals, including breakfast), sweet potatoes, a bitter melon dish known as goya, a very small slice of fish, root vegetables and a portion of fruit. There was also a small bowl of miso soup and green tea to cap it off.

Dr. Sanjay Gupta joins a family meal in Okinawa, Japan.

Willcox told me that Okinawans typically eat seven different fruits and vegetables and 18 different foods a day, and more than 200 different foods and spices regularly in their overall diet. In the United States, we are lucky to consume a dozen different foods in our regular daily diet, total.

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To be fair, even in Okinawa, things are starting to change -- and not for the better. The younger generations are eating more meat and fast food instead of fish and soy, and the locals tell me they are dealing with traffic jams regularly as people have increasingly traded their bikes for cars.

The elderly there are still widely revered, but there are fewer of them, and they are less often living to 100 than in decades past. Still, the Land of Immortals reminds us of what is possible, whether you live on Okinawa or anywhere else in the world.