(Reuters/Michael Buholzer)

Yes, love is grand and can heal all wounds, as they say. But bad marriages can do just the opposite. People in unhappy marriages are at a much higher risk for heart disease than people in more joyous unions, according to a first-of-its kind study released this week.

"Married people seem healthier because marriage may promote health," said lead study investigator Hui Liu, a Michigan State University sociologist. "But it's not that every marriage is better than none. The quality of marriage is really important."

That negative effect on cardiovascular health was even more pronounced for women and older adults, as found in the National Institutes of Health-funded study published this week in the Journal of Health and Social Behavior. It's one of the first to take a nationally-representative sample of adults and examine the impact of marriage quality on heart health over time.

Liu and researchers combed through five years of data taken from 1,200 adults in their late 50s to 80s, and determined respondents' heart health, measured by factors such as heart attacks, strokes and cholesterol levels. They then compared heart health to how these adults said they felt about their marriages. Wives and husbands (who were not married to each other) answered questions about how close they felt to their spouses, how happy their marriages were, and how demanding and critical they felt their spouses were.

Both negative and positive marital qualities were taken into account as "some people really love each other and have a lot of happiness, but at other times they may have a lot of arguments," Liu said. But the bad was more powerful than the good; Liu found that negative martial qualities hurt a spouse's heart health more than positive qualities helped.

Being in an unhappy marriage can cause stress, which has a direct link with cardiovascular health. And those effects accumulate. "It's not like you have contact with your spouse and the next day you have heart disease," Liu said. "It really takes time. That may explain why it's stronger for older people. Your body will remember the effect."

Such marriages can also push you toward unhealthy and harmful habits, like drinking a lot or smoking.

But why is it that women were hurt even more by unhappiness in a marriage? Liu said it's possible that women are more likely to internalize their feelings, feel depressed and be more sensitive than the men in their relationships. They also found that when women were sick with heart disease, it lowered the quality of a marriage, but not when men were sick. Liu said women are more likely to serve in a caregiver role for their sick husbands and be more sensitive to not exacerbating stress, but husbands may not be as sensitive about the relationship when their wives are sick.

Heart health isn't only thing Liu and her team are interested in exploring; next they'll look at the impact of marriage quality as it relates to diabetes, and the health dynamics within couples themselves.

So while a lot of marriage counseling may focus on younger couples, the study authors emphasize that older couples would be wise to pay attention to the qualities of their marriages, too. Their hearts may very well depend upon doing so.