Japanese citizens are still the healthiest people in the world, a new study found. (Image credit: Tomohiro Ohsumi/Getty Images)

Reported by Dr. Lauren Browne:

No one knows whether it's their great diet, good health care or just great genes, but after two decades Japanese citizens are still the healthiest people in the world, according to a decades-long study on population health published today.

The Global Burden of Disease Study 2010, a large-scale collaboration funded by the Bill and Melinda Gates Foundation to assess trends in health and disease in 187 countries, has released its rankings of the top 10 countries with the highest healthy life expectancies.

Healthy life expectancy is the number of years that a person can expect to live in good health. It takes into account number of deaths as well as years of health lost to disability.

The United States tied Norway for 29 th place in male healthy life expectancy and tied Estonia for 33 rd place in highest female healthy life expectancy.

Japan is the healthiest nation for men and women, a title it was first given by same study in 1990. For men, Singapore, Switzerland, and Spain ranked second, third, and fourth. South Korea, Spain, and Singapore ranked highest for women.

Highest Male Healthy Life Expectancy:

Japan Singapore Switzerland Spain Italy Australia Canada Andorra Israel South Korea

Highest Female Healthy Life Expectancy:

Japan South Korea Spain Singapore Taiwan Switzerland Andorra Italy Australia France

Click here to see the healthiest U.S. counties.

"It's hard to pinpoint one thing that makes certain countries healthier," said Joshua Salomon, a professor at Harvard School of Public Health and one of the lead investigators on the study. "It's likely a combination of factors, a combination of genetics and of healthy behaviors, including diet."

Despite major disparities in health among nations - female healthy life expectancy in Haiti was 37.1, while in Japan was 71.1 - the researchers found one common trend across the globe.

Lowest Male:

Burkina Faso Chad DRC Malawi Zimbabwe Mozambique Swaziland Central African Repbulic Lesotho Haiti

Lowest Female:

Liberia Burundi Afghanistan Malawi Zimbabwe Mozambique Swaziland Lesotho Central African Republic Haiti

The world may be living longer, but it is also getting much sicker.

"I was surprised to see that over the last 20 years we have given up more healthy life years. That's true for countries with high life expectancy and countries with low life expectancy," said Salomon. "This is not a problem of only rich countries or only poor countries. It's a problem of all countries around the world."

And women seem to be giving up more healthy life years to disability than are men. On average, men live 9.2 years of their life with a disability, while women on average live 11.5 years with one.

This could in part be explained by the fact that women live to older ages and consequently are more susceptible to disability. Yet the exact reason for the increased burden on women is still unclear.

Mental illness and musculoskeletal disorders, like multiple sclerosis, were among the most disabling illnesses identified.

For national health systems, the costs associated with caring for increasingly large numbers of people are likely to rise. Countries looking for solutions might benefit from examining more successful health systems like Japan.

"Japan give us something to aspire to, but there's more work to be done to figure out what their formula really is," said Saloman.