FRIDAY, March 8 (HealthDay News) -- The health benefits of marriage seem to fade when you need them most, according to a large new study.

Although marriage does seem to benefit those who are in good health, it provides less protection against death as health declines, the Ohio State University researchers found. The researchers also discovered that married people are more likely than others to overestimate how healthy they are.

"We believe marriage is still good for the health of some people, but it is not equally protective for everyone," study lead author Hui Zheng, an assistant professor of sociology, said in a university news release. "For those who are already in poor health, marriage doesn't seem to provide any extra benefits."

The findings were true for both men and women and were similar for those who were separated, divorced, widowed or never married, according to the study, which was published in the March issue of the Journal of Health and Social Behavior.

The researchers analyzed data from nearly 800,000 people who took part in the U.S. National Health Interview Survey from 1986 to 2004. Survey participants rated their own health as either excellent, very good, good, fair or poor. The researchers used follow-up data to identify the more than 24,000 participants who died between 1986 and 2006.

The study confirmed previous findings that, overall, people who are not married have a significantly increased risk of death within three years. Never-married people who rate their health as excellent are twice as likely to die within three years than married people in excellent health.

But this study also found that, as health declines, the lower risk of death for married people declines. Each unit decline in self-rated health (from good to fair, for instance) decreases the death-risk difference between married and unmarried people by 12 percent. Among people who rate their health as poor, there is no significant difference in death risk between married and unmarried people.

"These results suggest that marriage may be important for the prevention of disease, but not as helpful once people become seriously ill," Zheng said. "That's why we see a protective effect of marriage when people are in excellent health, but not when they are in poor health."

Although the study found an association between marital status and health, it didn't show a cause-and-effect relationship.

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