In 1949, American occupation authorities ran a food-intake survey across Okinawa and recorded something that would occupy nutritionists for the next fifty years: the islanders were getting roughly 69 percent of their calories from a single root crop, on a total intake of about 1,785 calories a day. The crop was the sweet potato, known locally as imo. Animal foods of every kind — pork, fish, eggs, dairy — supplied less than four percent.
And for decades, the villages that ate the most imo produced the highest concentration of centenarians ever documented in a peer-reviewed population study.
The story is stranger, and shakier, than the popular version suggests.
What the villagers actually ate
The dietary reconstruction comes from wartime and post-war archives, applied to Okinawans born in the 1890s and early 1900s. Alongside the tuber sat leafy greens, soy, seaweed, bitter melon and, occasionally, pork. Fish was less common than mainland stereotypes suggest. Rice was a luxury food, eaten at festivals. Meat appeared perhaps a few times a month.
The published analysis of those records — Willcox and colleagues in the Journal of the American College of Nutrition — put the macronutrient split at roughly 85 percent carbohydrate, 9 percent protein and 6 percent fat. That ratio, close to ten parts carbohydrate to one part protein, is what later drew the attention of ageing researchers, and it anchored the BBC’s account of Okinawan nutritional history. It is close to the inverse of every popular high-protein diet of the past thirty years.
One detail gets flattened in the retelling. The beni-imo that appears on tourist menus is purple all the way through, coloured by anthocyanins — the pigment class that darkens blueberries and red cabbage. But the historical staple was a mix of varieties, and the Willcox analysis describes the calorie-bearing crop as orange-yellow root vegetables as much as purple ones. The vivid purple tuber is partly a modern brand applied backwards onto a more ordinary field crop.
In laboratory work, extracts from purple sweet potato have shown antioxidant and anti-inflammatory activity. Whether that translates to a measurable effect on human lifespan is a much harder question, and one the Okinawa research never fully answered.
The tuber was not the whole answer. It may not even be most of the answer. But it was the calorie floor on which the rest of the diet sat.
How the story became a brand
The Okinawan finding did not become a global phenomenon on its own. The term “Blue Zone” was coined by Belgian demographer Michel Poulain and Italian physician Gianni Pes, who were validating longevity claims in the Ogliastra region of Sardinia in the early 2000s and marking confirmed cases on a map in blue ink. Science’s account of the field’s origins traces the name to those converging dots. Journalist Dan Buettner, who had joined the project, carried the concept into a 2005 National Geographic cover story and later trademarked the term.
The framing was tidy. Five regions — Okinawa, Sardinia, Nicoya in Costa Rica, Ikaria in Greece, and the Seventh-Day Adventist community in Loma Linda, California — each with its own signature food, and each with a population that seemed to live remarkably long.
Space Daily has written before about what those five regions actually share, which turns out to be less about food than about daily physical work into the ninth decade.
For a period of about two decades, the Okinawan cluster was the densest documented population of people past 100 on Earth. That distinction has since blurred as centenarian numbers climb everywhere: Technology Networks’ survey of the demographic data records more than 90,000 centenarians in Japan alone, with the US figure nearly doubling in twenty years.
The Okinawa Centenarian Study, running since 1975, is the longest-running centenarian cohort in the world and has examined more than three thousand people. It tracked cardiovascular markers, cognitive function, hormone levels and diet, and compared the imo generation with their children and grandchildren. The comparison was where the story turned.
The generational collapse
In 2000, Okinawan male life expectancy fell to 26th out of Japan’s 47 prefectures, after nearly thirty years at the top. Obesity rates rose faster than anywhere else in the country. The reversal has a name in the Japanese press: the 26 shock.
The proximate cause was dietary. American military bases established after 1945 brought canned meat, wheat flour, sugar and fried food. The sweet potato receded fast — consumption fell by roughly 90 percent between 1950 and 1960 — and rice, bread and pork took its place.
Diet was not the only variable. Okinawan elders famously practise hara hachi bu, eating until around 80 percent full, and combined with a low-calorie-density staple the result was a population in mild, chronic caloric restriction — the one intervention that has consistently extended lifespan in laboratory mammals. Whether the mechanism translates to humans remains contested. The correlation, in the Okinawan data, was hard to miss.
The centenarians of the 1990s were people who had eaten imo as children in the 1900s and 1910s. Their grandchildren, raised on Spam and white rice, are a different population with a different health trajectory. The natural experiment ran in both directions.
What 55 years of centenarian research actually found
A 2026 scoping review in Discover Public Health mapped more than half a century of centenarian studies. Its conclusion was blunt: no single dietary factor, gene, or lifestyle habit explains exceptional longevity.
The review found that centenarians tend to share certain patterns — plant-forward diets, physical activity built into daily life, strong social networks, low neuroticism, high conscientiousness — but that the effect of any one factor is modest. Researchers estimate genetics accounts for as much as half of the ability to reach extreme old age. The rest is a scatter of small advantages accumulated over decades, plus luck.
The review also flagged a survivorship problem that haunts all longevity research: studying centenarians means studying only the people who reached 100, not the many more who ate the same food, walked the same paths, and died at 72.
Strip the story down and what survives is narrow. Okinawans born between roughly 1890 and 1920 ate a diet in which sweet potato supplied the majority of daily calories; that cohort produced a per-capita centenarian rate several times the Japanese average; when the diet shifted, the advantage disappeared within about forty years. What remains uncertain is causation. The imo-heavy diet was also low in calories, low in refined sugar, low in animal fat, high in fibre, and embedded in a culture of physical work and modest portions. Isolating the tuber from the context is not something the existing data can do.
The data-quality problem
There is a further complication. In 2024, University College London researcher Saul Justin Newman released a preprint — not yet peer-reviewed — arguing that a substantial portion of documented supercentenarian records worldwide are the product of clerical error, pension fraud, or missing birth certificates. His analysis, summarised by Al Jazeera, found that regions with the highest concentrations of people claiming to be over 100 tended also to be regions with the poorest vital-statistics infrastructure. The work won the first Ig Nobel Prize in Demography.
Okinawa was not spared. Japan’s 2010 pension audit, triggered by the discovery of a man in Tokyo who had been dead since 1978 while his family collected three decades of payments, found roughly 230,000 registered centenarians who were missing or already dead. In Okinawa specifically, Newman argues, the best predictor of where the centenarians cluster is where the halls of records were destroyed during the 1945 fighting.
The sharper problem for the sweet-potato story is contemporary. Reporting by The World from PRX lays out Newman’s objection alongside the researchers who dispute it: in Japan’s national nutrition survey, which has run since 1975, Okinawa ranks last among prefectures for vegetable consumption and first for body mass index. The prefecture built into the global imagination as a vegetable-eating paradise has, for the entire period anyone has measured it, eaten fewer vegetables than anywhere else in Japan.