Among older couples, physical illnesses can strain a marriage, but maintaining a healthy sex life could make a difference in how happily both partners cope, a new study suggests.

Researchers have long known the illnesses that come with age are linked to poorer marriage quality, but exactly why has not been clear. According to the new analysis, sexual intimacy is the link that keeps partners positive about their marriages in the face of difficult times, and a lack of sex makes matters worse.

The results "suggest that it may be important to stay sexually connected to protect" the quality of a marriage, lead author Adena Galinsky told Reuters Health.

Galinsky is a research affiliate of the University of Chicago's Center on Demography and Economics of Aging in Illinois.

After analyzing data from 732 "couple units," encompassing 1,464 individuals, most of whom were between 65 and 74 years old, Galinsky and a colleague found that sexual frequency was tied to relationship quality.

"This paper helps contribute to our understanding of why poor health is associated with poor marital quality," said Amelia Karraker, a postdoctoral fellow at the Population Studies Center at the University of Michigan in Ann Arbor. She was not involved in the study.

"Wellbeing in older age incorporates both psychological and physical wellbeing, as well as sexual wellbeing, which can occur at the intersection of those two," Karraker said.

In the current study, most men and women were white and had been couples for 40 years or more. Nearly all of the men and women were married, but about 50 people were just living together.

Couples reported an average of one partnered sexual activity per month. As sexual frequency increased, so did marriage quality.

The research does not prove that a decrease in sexual activity causes a decrease in marital quality. It is also entirely possible that it's the other way around, Galinsky and her coauthor acknowledge in their report.

"The authors did a nice job of highlighting the idea that being sexually active isn't restricted to vaginal intercourse, but people can expand their idea of what sex is," Karraker said.

The researchers attempted to capture the nuances of marriage by asking partners to describe certain aspects of it as positive or negative.

For example, "emotional satisfaction" was a quality participants tended to associate with a positive marriage. But most men and women listed "partner criticizes" as a negative marriage quality.

Generally, husbands reported higher positive qualities in their own marriages than wives. The nearly 66 black couples included in the study reported higher overall levels of negative qualities in marriage than white couples.

The team's findings also confirmed what previous research has found - better physical health is associated with better marital quality.

"Poor physical health affects the psychological well-being of that person and his or her spouse at the same time that it affects the sexual behavior of the couple," the researchers write in The Journals of Gerontology: Series B.

Without doubt, the study "points to the important role that counselors can play in addressing the challenges of poor physical illness for individuals and couples," Karraker said.

"I hope our study inspires research investigating what factors enable some couples struggling with poor health to find the energy and creativity to stay sexually engaged," Galinsky said.