Japan’s average life expectancy rose by 4.2 years over the past quarter-century to 83.2 but a regional disparity has widened for as yet unknown reasons, a study led by a Japanese researcher showed Thursday.


Factors other than the health care system and lifestyle, including diet, may have played a role in expanding the gap between the prefectures with the highest and lowest rates, according to the research published in British medical journal The Lancet.


The research team led by Kenji Shibuya, professor of international health policy at the University of Tokyo, said it plans to find out what lies behind the disparity by looking at local government health budgets and health awareness of residents.


The study group compared data on deaths and diseases in 1990 and 2015. While the country’s overall life expectancy rose, the biggest gap between prefectures grew from 2.5 years to 3.1 years.


Nagano and Shiga prefectures in central and western Japan came first in the list of average life expectancy in 1990 and 2015, respectively, while Aomori in the northeast ranked the lowest in both years.


Japan’s healthy life expectancy, or the average number of years that a person can live in full health, rose to 73.9 from 70.4 but the gap between the prefectures with the highest and lowest rates widened to 2.7 years from 2.3 years.


Thanks to a decline in the mortality rates attributable to heart disease or cancer, the age-standardized death rates fell by 29 percent.


Life expectancy is the average time a person at birth is expected to live, calculated from demographic data, while the mortality rate is the number of deaths per 100,000 people.


Japanese women have long enjoyed the world’s longest average life expectancy, although they relinquished the top spot in 2015, overtaken by women from Hong Kong. Japanese men ranked fourth that year, according to the welfare ministry’s data.


The Kyushu region in southwestern Japan and the Kinki area in western Japan logged notable mortality rate declines, while the Tohoku region in Japan’s northeast and the southernmost main island of Okinawa saw moderate falls.


Mortality rates by disease type also showed regional differences. In 2015, the Tokyo metropolitan area saw higher death rates related to ischemic heart diseases such as angina and heart attack, while lower rates were registered in the Hokuriku region on the Sea of Japan coast and Kyushu.


The research team analyzed medical fees, the number of doctors and nurses, salt consumption and smoking habits in each area, but found no major correlation with the regional disparity.


Meanwhile, the researchers also looked into factors triggering diseases in Japan. They linked an unhealthy diet or smoking, among other habits, to 34 percent of deaths in 2015 and high blood pressure or high cholesterol to 25 percent.


Although no major differences between prefectures have been detected, excessive salt consumption or a lack of vegetables and fruit in the diet aggravated health conditions, according to the findings.


The study also showed smoking was the biggest risk factor for deaths among men.