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The metaphorical heart, in the form of stable, loving relationships, can have a positive effect on the physical heart, reducing the risk of heart attack and mortality.

Considerable research confirms the health benefits of supportive relationships in general and marriage in particular.

A 2017 study in the  found unmarried people with heart disease were 52% more likely to have a heart attack or die from a cardiovascular problem after nearly four years compared with married heart patients. Overall, the death rate for married people is lower than for those who were never married, divorced or widowed, according to the .

Imaging studies show that viewing pictures of one's romantic partner activates brain regions related to mood and pain regulation. Thinking about a partner also may boost energy by positively affecting blood glucose levels.

Dr. Robert Waldinger is director of the , which has been tracking two groups of men since 1938. His TED talk on  has been viewed tens of millions of times. Among other things, the study has found stable relationships at midlife are a better predictor of being healthy and happy 30 years later than cholesterol levels.

The clearest message from the study, he tells the audience, is this: "Good relationships keep us happier and healthier. Period."

But why?

One theory is that good relationships calm people down from the fight-or-flight response that kicks in when they're scared or angry, said Waldinger, a psychiatrist and professor at Harvard Medical School in Boston.

Stress hormones released at such moments can be harmful. But, he said, "If you have a really lousy day and something bad happens, and you go home and there's somebody you can talk to about it, you can almost literally feel your body decompress as you talk about what was wrong."

Studies have shown physical intimacy, such as holding hands or hugging, can lower levels of stress hormones.

But relationships play a bigger role beyond regulating stress, said Julianne Holt-Lunstad, a professor of psychology and neuroscience at Brigham Young University in Utah. A supportive partner might encourage you in healthy ways – to exercise, eat better or see a doctor when you need one.

Her  published in the journal PLOS Medicine examined data from 148 studies involving more than 300,000 people. It found the odds of being alive at the end of a study's given time period was 50% higher for those with the strongest social relationships compared with people without such ties. As a predictor of survival, this is on par with the effect of quitting smoking.

Other studies led by Holt-Lunstad focused on the health effect of marriage itself. The lesson there: Quality matters.

The work found people in happy marriages had lower blood pressure than people who weren't married. But people in strained marriages fared worse than single people.

Elements of a positive relationship, whether it's a marriage or something else, include trust and security, she said. As does how well you respond to your partner's needs – "the extent to which you are both giving and receiving, so it's not a one-way kind of relationship."

Waldinger said a relationship doesn't have to be perfect to provide a health benefit. The research isn't conclusive, he said, but experts think the key may be simply knowing that somebody has your back.

Holt-Lunstad said more people need to understand that relationships have a huge influence on physical well-being, particularly when it comes to heart health.

People are used to hearing messages about the importance of exercising and not smoking, she said. "We need to start taking our relationships just as seriously as we take those things."