Long before Jenny Craig, the Atkins diet and that gym membership you never use, there was Confucius. "Eat not until you are full," advised old wispy beard, "but until you are eight parts out of 10 full".

This teaching, known as hara hachi bu, fell out of favour in most of the Confucian realm long before the appearance of the first Sizzler all-you-can-eat salad bar.

But there is one place where it is still strictly followed. The results are astounding.

Okinawa, the string of 160 subtropical islands east of Taiwan that constitute Japan's southernmost prefecture, contains more centenarians per head of population than anywhere else.

Five times as many Okinawans live to be 100 years old as their compatriots elsewhere in Japan - and Japan has the highest life expectancy of any country.

There are several possible explanations. Okinawans are famously laid-back; the local business uniform of a Hawaiian shirt and slacks is certainly a world away from Tokyo's suited salarymen. Then there's the stress-free nature of the place itself: a subtropical archipelago of coral reefs, warm seas, lush jungles and NapiSan-white beaches.

But nutritionists speculate that diet - and how their food is consumed - has much to do with it. Not only are Okinawans thought to be the only culture with a tradition of self-imposed calorie restriction (hara hachi bu), but their traditional diet has few calories to start with.

Pork may be acidic, but it is high in protein and its acidity is neutralised by lashings of mozoku, a local seaweed that is alkaline, calorie-free and constitutes the other staple of the traditional Okinawan diet. These ingredients are complemented by seafood, tofu, vegetables and a traditional intake of grains, salt and sugar that is much lower than in most cultures.

Like Indiana Jones in The Last Crusade, I'm on a quest for the holy grail of eternal life - or at least a clue as to how to get one of those letters from the Queen for reaching triple figures.

Okinawa may contain slices of paradise, but its capital, Naha, certainly isn't one. Flattened by Allied bombs during World War II and hastily rebuilt without much care for aesthetics, its low-rise, Soviet-sur-mer architecture reminds me of Noumea, the capital of New Caledonia: the same grimy, brutalist apartments and concrete canals; the same reclaimed mangrove-swamp suburbia.

Thankfully, the bombs and talentless architects didn't destroy the city's gastronomic reputation; hidden among its back alleys are eateries that are said to have changed little since this was the seat of power of the Ryukyu Kingdom, a court that took much of its culinary and cultural influences from nearby Thailand and southern China, not faraway Japan.

At Yunangi, tucked off Kokusai Dori, Naha's glitzy main drag, I sit at a bar decorated with traditional Okinawan pottery and order a saucer of mimiga (pickled pig ears) washed down with a cold beer. They eat everything but the oink in Okinawa: sliced pig ears, as fine as wood shavings and surprisingly crunchy, are pickled in vinegar for a tangy bar snack; trotters, bellies and offal are thrown into stir-fries; while peeled-back pig faces resembling those rubber Richard Nixon masks garland butcheries like Halloween decorations.

Next comes a steaming bowl of Okinawa-soba, the ubiquitous local comfort food of udon noodles flavoured with chillies, swimming in a broth of pork ribs and lurid-pink chunks of shima dofu, a local tofu variant that is pickled and fiercely spicy. The servings are tiny, Confucian style, and as I don't yet feel 80 per cent full, I round off the meal with a bowl of goya champuru.

The prefecture's signature dish, this stir-fry contains tofu, pork and goya, a bitter, super healthy gourd vegetable the size of a corncob and as scaly as a shingleback lizard. The dish is an acquired taste, though I enjoy its strange combination of sweet, bitter and savoury.

When I step out into the night I do notice an unusually high proportion of old people. Some are walking their dogs; others pacing the streets for an evening stroll, but they all look good for their age: ruler-straight postures and glowing brown skin. Perhaps there's something in the (fire) water?

To find out, I buy a bottle of awamori, a rice-distilled liquor with the kick of Godzilla that was introduced from Thailand more than 500 years ago, and is Japan's oldest distilled drink. Okinawans can't get enough of the stuff: Mizuho, the most famous awamori distillery, produces 200,000 1.8-litre bottles of the alcohol annually, with the vast majority knocked back in the prefecture.

It only takes a few shots of the liquor - which has an alcohol content of 30 per cent and is traditionally kept in clay pots with a snake in the bottom - to feel like it's putting hairs on my chest and, hopefully, years on my life.

I'm still feeling the effects of a boisterous drinking session with staff members of my guesthouse when I trudge to Naha's port the next morning and board a ferry to the island of Zamami. I don't have time to explore Okinawa's more remote islands - which range from deserted atolls to rainforest reserves teeming with wildlife - so I hope Zamami, a day trip from Naha and reputed for its beauty, will offer a taste of Okinawa's laid-back soul.

Within five minutes of arriving I am left alone. I walk through the tiny township and over a hill fringed with jungle and offering views of a sole gardener hunched over a field below. On the other side of the hill I leave my clothes on a stretch of sand deserted by all but a few hermit crabs and go skinny-dipping in the warm East China Sea for half an hour. Could this be the answer to long life?

Before leaving Zamami (and after putting my clothes back on) I feast on a lunch of sashimi prepared before my eyes from tropical fish and garnished with sea grapes, a mouth-popping seaweed variety that tastes like a rock-pool. The meal is the most invigorating I eat in Okinawa.

That night back in Naha, I see signs of the American presence I'd read about but not yet seen. Okinawa was returned to Japanese rule only in 1972 after being governed by US forces following World War II. Today, more than 20,000 US Marines are still controversially stationed in what is increasingly a point of strategic interest.

This cultural influence is changing the Okinawan diet. Spam is now sometimes added to stir-fries, Blue Seal ice-cream is an Okinawan institution (try the green tea and sugar cane varieties) and the prefecture's youth are said to be more interested in eating burgers and fries than pig ears and goya. What this means for Okinawa's reputation for longevity remains to be seen, but the signs are ominous: diabetes and childhood obesity, once unknown problems, are now present and increasing.

There is a succinct display of this transformation as I sit down for dinner at Paikaji, a traditional Okinawan affair of tatami mats and paper walls. My meal is great: a dish of worm-like mozoku seaweed served with a blob of wasabi followed by a rich bowl of ikasumi-jiru, a pork and vegetable stew blackened with squid ink to the point that the whole dish resembles an oil spill.

But I can't help but notice the golden-arched establishment across the road is doing better business. While I slurp on my seaweed in near-empty Paikaji, the distinctly young crowd outside the building opposite slurp on thickshakes; while I try in vain to mop squid ink from my lips, the kids reach for napkins to wipe spilt mustard and ketchup. And no, they don't seem to be observing hara hachi bu.

Were he alive today, Confucius would be tearing his beard out.

Three other things to do

1 Okinawan ceramicists are renowned throughout Japan, and Tsuboya, an inner-city neighbourhood of Naha, has been a centre of ceramic production since kilns were consolidated here by royal Ryukyu decree in 1652. Typical pieces to look out for are cups, awamori jars and the ubiquitous shesa, dragon-lion-dog mascots that adorn every Okinawan roof. Tsuboya-yachimun-dori, a lane of crumbling houses, has several pottery shops and studios, while nearby Garb Domingo is a great gallery to buy local works.

2 Most visitors to Okinawa expect to spend their time face down on the beach, not on a safety mat. But martial arts enthusiasts shouldn't pass up the opportunity to train with a karate master, or sensei. Okinawa is considered the birthplace of modern karate and Murasaki Mura cultural park in Yomitan, an hour's drive from Naha, has one-hour karate classes for 3500yen ($43) including park entry and uniform rental.

3 Diving in Okinawa may be more expensive than in south-east Asia but the standard of guiding and equipment is higher and the seas often healthier. As the scene of some of the fiercest fighting of the Pacific campaign, Okinawa's waters are home to several World War II wrecks, as well as tropical fish and coral reefs. Piranha Divers offers full-day dives off Okinawa's main island from 17,000yen with equipment rental for 5000yen.

Trip notes

Getting there
JAL flies from Sydney to Tokyo (Narita). JAL has several flights daily from Tokyo's Haneda airport to Naha. 1800 802 228, jal.com.

Getting around
Zamami Sonei ferry line operates two high-speed ferries daily (3140yen ($40) one-way, 50 minutes) and one regular ferry (2120yen, 90 minutes) between Naha and Zamami. +81 98 868 4567.

Staying there
Despite its Western name, Stella Resort is a guesthouse with a distinctly Okinawan flavour. Rooftop showers, an aquarium and loft rooms cooled by the tropical breeze are testament to the fact that Naha is closer to Taipei than Tokyo. Private rooms from 2500yen a person are a bargain. +81 98 863 1330, stella-cg.com.

For high-end beach bums, in June the Hoshinoya hotel group opened a resort on the island of Taketomi-jima. Villas, with all meals included, are from 54,000yen a night. + 81 3786 0066, hoshinoyataketomijima.com.

Eating there
Yunangi, open Monday to Saturday for lunch and dinner, 3-3-3 Kumoji. +81 98 867 3765.
Paikaji, open daily for dinner only, 1-1-7 Uenoya. +81 98 866 7977.

More information
jnto.org.au