Social networks can shape our health in unexpected ways.

You know your friends are good for your heart, figuratively speaking. How often have you been soothed by the sound of a friend’s voice or felt better after venting to your bestie? But the health benefits of friendship go way beyond emotional support. friends can actually influence our hearts physically—and they impact a staggering number of other health factors, too.

The heartening health benefits of friendship

A fascinating study based out of Montreal, Canada, found that making friends affected heart function in international students who were new to the city. The researchers made sure the students had no friends or family in Montreal and no romantic partners—in other words, that the students were bound to be lonely. And they measured the students’ heart rates.

As months passed and the students made friends, the researchers noticed something: the students who made more friends had greater heart rate variability. Those who cemented fewer social ties had lower heart rate variability. Low heart rate variability has been linked with risk of heart disease and poor health.

In short: grabbing coffee with a new acquaintance could be a lot more important than we once thought.

Friendship doesn’t just have a protective effect on your heart health; it could also help after you’ve had heart trouble. A recent study found that people who thought they had little social support were in a poorer state of health a year after having a heart attack. They also showed lower mental functioning and more symptoms of depression. It gets more serious: people with fewer friends have been shown to die sooner after a heart attack than those with a more robust social circle.

Death from heart troubles isn’t the only bullet that friends could help you dodge. A study of more than 300,000 people found that people with stronger social relationships had a 50 percent increased chance of survival overall—no matter their age, sex or initial state of health. To put it bluntly, a lack of a strong social network could have an effect on your health roughly the same as smoking 15 cigarettes a day.

It’s clear that friends affect our big-picture health. They also affect smaller health measures. A larger social network may help us tolerate more physical pain and boost our immunity. But friends affect our health in more precise, subtle ways, too. If you’ve set a goal to lose weight or quit smoking, for instance, take a good look at your social circle. It could mean the difference between success and struggle.

The science of “rubbing off”

“Social networks have value precisely because they can help us to achieve what we could not achieve on our own,” explain Harvard Medical School internist and social scientist Nicholas Christakis and University of California at San Diego political science professor James Fowler in the book Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives.

“Students with studious roommates become more studious. Diners sitting next to heavy eaters eat more food. Homeowners with neighbors who garden wind up with manicured lawns. And this simple tendency for one person to influence another has tremendous consequences.”

The notion that personal behaviors rub off on people around us is so promising that it’s being investigated to help with some of today’s most pressing health concerns.

Helps you quit smoking

For those who want to stop smoking, chances of success are far greater when smokers connect with others who’ve already kicked the habit or are in the process of doing so.

Research shows that stop-smoking efforts spread via social connections. Based on a study of more than 12,000 people who were assessed for more than 30 years, researchers found that a spouse quitting smoking decreased a person’s chances of smoking by 67 percent. Meanwhile, a friend or co-worker quitting smoking lowered one’s chances of smoking by about 35 percent, and a sibling quitting reduced the chances by 25 percent.

Changes your weight

Social connections seem to affect physical fitness, according to economist Scott Carrell. Carrell headed a study into the spread of fitness habits among more than 3,000 students at the United States Air Force Academy.

Researchers concluded that if half of one person’s friends became out of shape, his or her chances of failing the course’s basic fitness requirements tripled.

“Our estimates imply that each out-of-shape individual creates two additional out-of-shape individuals through their social interactions, thus supporting the provocative notion that poor physical fitness spreads on a person-to-person basis,” the researchers wrote.

Another study found that the impact of obese friends on our own weight is greater than the effect of obese family members and partners. A person’s chances of becoming obese increased by 57 percent if he had a friend who became obese. Among adult siblings, if one became obese, the chance that another would too increased by 40 percent. And if a spouse became obese, the likelihood that the other partner would also gain excess weight increased by 37 percent.

On the flip side, being around those committed to managing their weight may help you achieve your own weight goals. Take Deanna Johnston. She joined a local weight-loss support group and went to regular meetings. Strangers became friends. Surrounding herself with others who had a similar goal made all the difference. Other people’s desire to lose weight rubbed off on her: she lost more than 30 pounds.

“I had lots of support, and I felt like every accomplishment was applauded, small or large,” Johnston says. “There was never any judgment. People who’ve gone through the same thing know what the emotional and physical challenges are like.”

Molds your mental health

Depression among our friends appears to affect us as well. A recent study examined whether depressive symptoms in one person were associated with similar signs in friends, co-workers, siblings, spouses and neighbors.

It found that people were 93 percent more likely to be depressed if a person they were directly connected to (at one degree of separation) was depressed. People at two degrees of separation (the friend of a friend) were 43 percent more likely to be depressed, while people at three degrees of separation (the friend of a friend of a friend) were 37 percent more likely.

“At four degrees of separation the effect disappears,” the authors noted, “a result that is in line with other results that have shown similar drop-offs after three degrees of separation, including obesity, smoking, happiness and loneliness.”

So should you avoid depressed friends—or friends who are struggling with weight or smoking, for that matter? No. Just be aware of the power of influence and learn how to be supportive and compassionate while still looking after your own health needs.

In fact, using your own difficult experiences to help your social circle can have tremendous results. When it comes to depression, for example, being surrounded by peers who can relate to specific mental-health conditions is proving to be an effective component of treatment.

Support from trained peers has helped people with mental illnesses address certain attitudes, values and behaviors more effectively in some cases than advice from health professionals.

What about online social networks?

Social media sites like Facebook and Instagram can have a powerful effect on your well-being—for better or worse.

First, the good. When a person shares a specific goal—say, to quit smoking or lose weight—on social media, they become accountable, explains Christopher Schneider, a professor of sociology.

“When you put yourself out there, and your personal goal goes global for everyone to see, there’s a lot more pressure to succeed,” he says.

Joining online communities also allows people to share their stories more openly than they might in person, and web-based support groups reassure people that they’re not alone.

However, research shows that heavy social media use doesn’t translate into closer relationships offline with those people. In addition, heavy social media use may make us feel that others are happier than us, and could lead us to feel more dissatisfied with our lives. Using social media in moderation is likely the best approach—and making a conscious effort to take advantage of its positives.

The way forward

So how can you enjoy the health benefits of friendship? There’s the obvious: make time to see friends. There’s the nitty-gritty: keep engagements and show up on time. Finally, there are the logical steps: choose healthy activities, like working out together, more often than sedentary ones.

Regularly take stock of the impact of your friends on your health—and your impact on theirs. Are you cheering on your friends’ healthy living goals? Are you supporting those who are going through a tough time? Are you the sort of friend who could save a life?