Why do Japanese people live so long?
The Japanese dietary pattern, rich in vegetables and fish and low in fat, is clearly associated with a longer and healthier life; anyone adopting it at home would do best to combine it with adequate protein and physical activity in later life.
Japanese people eat plenty of vegetables, legumes, fish and grains, and relatively little fat, sugar and calories. In a study of nearly 93,000 people followed for an average of almost nineteen years, those who stuck most closely to this pattern were less likely to die from heart and blood vessel disease and also tended to live longer overall. A modified version of the pattern, with even less salt and with coffee and dairy added, was linked to the lowest death rates among men.
Beyond a lower chance of dying early, the Japanese dietary pattern is also associated with a meaningfully lower risk of physical limitations in old age and of dementia. In both cases, the risk for people who followed the diet most closely was roughly one fifth lower than for those who followed it least. Whether the dietary pattern itself is the full cause of this difference has not been proven; other explanations may also play a role.
Okinawa is a special case within this picture. The island is known as a place where people reach exceptionally old ages: life expectancy there is demonstrably higher than the Japanese average, which is itself among the highest in the world. Researchers attribute this to a combination of diet, physical activity, climate, genes and geographic isolation. Studies looking at diet combined with a broader healthy lifestyle suggest that healthy life expectancy, that is, years lived free of disease, may be eight to ten years longer as a result.
Seaweed is a typical part of the Japanese diet and is linked to lower rates of heart and blood vessel disease in two large Japanese studies. A note of caution applies here: seaweed also contains iodine and traces of heavy metals, which can be harmful in very large quantities. The evidence for seaweed on its own is still too limited to draw firm conclusions.
Living this long also has a downside. Japan is ageing rapidly, and older people are at risk of muscle loss and frailty if they do not get enough protein, calories and vitamin D. Living longer is therefore not the same as ageing well: the quality of nutrition in later life remains crucial.
This answer draws mainly on two large studies involving tens of thousands of Japanese people followed for many years, supplemented by a broad survey of research into so-called Blue Zones such as Okinawa. In almost all cases the findings are associations measured in populations that were not randomly assigned, which means cause and effect cannot be established with certainty. The findings on healthy life expectancy carry a somewhat stronger suggestion of causation, but have not yet been definitively confirmed in controlled studies.