Japan's longevity rate is the envy of many countries. But what factors combine to make Japanese live so long?
That is a question that has intrigued Ichiro Kawachi, a professor of social epidemiology at Harvard University.
Japanese men can expect to live 79.44 years, ranked eighth in the world, while Japanese women have an expected lifespan of 85.9 years, the second highest after Hong Kong, according to data for 2011. But Japan would appear to have all the potential drawbacks to have shoter lifespans.
The average Japanese has a relatively high intake of salt due to the widespread use of soy sauce in the diet. Alcohol consumption is also high. The nation has an army of smokers, compared with other developed countries. To top it off, people put in notoriously long hours, which should be a source of stress.
Kawachi suspects that the tightly knit aspect of Japanese society plays a key role in longevity.
"As is shown in the Japanese expression 'otagaisama,' a sense of mutual trust and support in Japanese society must have contributed to the longevity of Japanese," said Kawachi, 50.
He is prominent in the field of social epidemiology, a science that tries to find a statistical correlation between people's health and the makeup of their society.
Kawachi, who was born in Japan, moved to New Zealand when he was 12 as his father, a geologist, found work there.
After receiving a medical degree from the University of Otago in New Zealand, Kawachi became a physician and treated patients with heart disease and stroke for two years.
He detected a common thread in the patients he treated. They generally had an unbalanced diet and were smokers.
Kawachi came to believe that conventional treatments amounted to nothing more than "simply attaching a band-aid to their injuries," unless their lifestyles are altered.
He decided to switch to preventive medicine in the hope of engineering genuine change in public health.
Kawachi then published a research paper on the health hazards of passive smoking, which helped prod New Zealand into tightening regulations on smoking.
The experience opened his eyes to the potential of epidemiology, which offered the prospect of prolonging tens of thousands of lives.
He was 31 when moved to the United States, a frontrunner in epidemiology, to continue his research. There, he discovered that the United States has lower longevity ratings than elsewhere.
It may be the world's wealthiest country, but an enormous economic inequality exists between the rich and the poor.
That prompted Kawachi to look at Japan, where the gap between the haves and the have-nots is narrower and people live longer.
He acknowledges, however, that income disparities have widened in Japan in recent years. That inevitably has had an impact on public health.
In its policy for public health for the coming decade, released in July, the government set a goal of reducing factors that affect overall health.
"People are stressed out due to economic inequality, and that is eroding a sense of mutual trust and social norms," Kawachi said. "The strength of a tightly-knit Japanese society is invisible to see, but if it is the key to (the nation's) longevity, then people need to keep those values alive and protect them."
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