In a highly publicized 2008 study, researchers at Brigham Young University found that people in happy marriages tended to have lower blood pressure than their single counterparts.
But Troiani has seen the opposite happen quite often, as well: "A happy couple can motivate each other to stay healthy—they'll go to the gym together, set goals, and feel responsible for each other." When couples do pack on the pounds, she adds, it may be a symptom of conflict, not slacking off.
"Dissatisfaction in the relationship can lead to passive-aggressive eating behaviors and sleep problems, which will lead to weight gain," she says.
Surprise, surprise: Regular physical intimacy appears to reduce stress and boost well-being.
One study, published in 2009 in the Journal of Sexual Medicine, found that people who frequently had sex were healthier mentally and more likely to report greater satisfaction with their relationship and life overall. And your partner's behavior outside the bedroom can just as easily send stress levels soaring in the opposite direction.
These links can be difficult to untangle, however, since anxiety has been shown to breed relationship problems (and not just vice versa).