Men in same-sex marriages are living longer, according to Danish researchers, but mortality rates among married lesbians have begun to rise after a long period of decline.

The study, published Tuesday in the International Journal of Epidemiology, used Denmark's civil registry to follow 6.5 million adults from 1982 to 2011. The study is the first of its kind to examine mortality -- the risk of death during a specific period of time -- and relationship status for an entire nation.

"Our study expands on century-old knowledge that married people generally have lower mortality than unmarried and divorced persons," wrote the lead author, Dr. Morten Frisch, a professor of epidemiology at Aalborg University. "From a public health viewpoint it is important to try and identify those underlying factors and mechanisms."

Researchers found that marriage in and of itself did not ensure low mortality during the period studied. For instance, opposite-sex married couples who lived apart faced a two-fold increase in their mortality rate.

Also, heterosexual men and women saw a steep jump in their mortality rate during the study period if they were married two or more times. The rate increased 27% for women with each successive marriage, and it increased 16% for men.  

Same-sex unions have been legal in Denmark since 1989. Since that time, however, mortality rates have changed greatly among homosexual men and women.

The mortality rate for married gay men plummeted during the study period and is now just slightly lower than the mortality rate observed for unmarried or divorced heterosexual men. Authors speculated that this steep decline was due to advances in the treatment of HIV/AIDS.

Surprisingly, researchers found that the risk of death during the study had grown worse for lesbian couples. After dropping for many years, mortality rates had begun to rise slightly, so that they were now higher than those of married gay men and cohabiting heterosexual couples.

"Lesbians may constitute a largely unnoticed high-risk population for suicide and breast cancer," the authors wrote.

It remains unclear precisely why heterosexual married couples have the lowest risk of death during the years studied, although researchers suspect it has to do with income, health care, increased social support and other factors.

Authors noted that homosexual couples represented less than 1% of the study sample, and that this might limit its findings.

"Future studies of health patterns among homosexual persons will gain statistical power, and presumably be more representative of the target population," researchers wrote.

Follow me on Twitter @montemorin