Researchers combined data from four large studies that have been following, for decades, the physical and mental well-being of thousands of Americans between the ages of 12 and 91. They focused on social ties that participants reported, such as number of friends and amount of family support, and markers of physical health, including obesity, blood pressure and inflammation, over subsequent years.
The researchers found that the more socially connected a person was, the lower their blood pressure down the road. For adolescents, being popular also seemed to protect against becoming overweight and, specifically, from gaining weight in the mid-section.
"These findings add support for the theory that social integration buffers the daily stresses that we all experience [by] having people to talk to, share experiences and the hassles of everyday life with," said Kathleen Mullan Harris, professor of sociology at the University of North Carolina. Harris led the new research, which was published
on January 4 in the journal Proceedings of the National Academy of Sciences.
When we are socially isolated and don't have this buffer, we may have higher levels of stress hormones in the body such as adrenaline that "breaks down the body and biological systems," Harris said.
"We hope our research will bring attention within the biomedical field to the importance of social factors and that doctors seeing their patients even in an annual visit will not just see what risks they have like diabetes but ask them about their social activities," Harris said. Doctors could encourage patients to develop their social connections and engage in more activities, she said.
Why social connections boost health
Research has piled up over the years suggesting that loneliness can kill
. Social isolation has been linked with 30% higher risk of early death. It has also been associated with higher risk of diseases "across the board," such as heart disease, stroke and cancer, Harris said.
The current study is a big stride forward because it supports the idea that social connections could be directly influencing health rather than the other way around, said Julianne Holt-Lunstad, associate professor of psychology and neuroscience at Brigham Young University.
There have been questions over whether health outcomes such as obesity could actually be causing people to become more socially isolated, said Holt-Lunstad, who was not involved in the current study but has conducted research on the social relationships and risk of death. However, because the current study followed participants over time, it could tell that people were already socially isolated before they became overweight or developed high-blood pressure.
Friends, family and spouses could be having beneficial effects on stress levels and other physiological markers, Holt-Lunstad said. But there could also be a slew of additional ways that these relationships boost our health.
"It can be everything from the time we're little we have our parents encouraging us to eat our vegetables ... to a spouse or romantic partner encouraging us to get more sleep," Holt-Lunstad said. Friends and family can make us more likely to adhere with medical treatments and make doctor's appointments, she added.
However, friends can also have negative effects on your health. If you have close relationships with smokers, you are more likely to smoke, for example, Holt-Lunstad said.
It depends on how old you are
The kinds of health benefits that we stand to gain through better social relationships probably depends on what age we are, Harris said.
For adolescents, social connections have a similar effect on weight as exercise. The young people in this study could be especially at risk of becoming overweight because they belong to a cohort from the late 1990s when the obesity epidemic really took off, said Harris, who is director of the adolescent cohort Add Health.
Among the older adults in this study, social isolation was about as big a risk factor for developing high-blood pressure as having diabetes. These connections could be especially important later in life because that is when people are really at risk of high-blood pressure later, Harris said.
Unlike with the young and older age groups, social integration did not seem to influence measures such as weight and blood pressure for middle-aged adults. However, while the quantity of relationships did not seem to matter, the quality did. Participants in this age group who reported having the most relationship strain had higher levels of BMI and C-reactive protein than those reporting the least strain.
"It makes perfect sense from a life course perspective -- in middle age you are naturally embedded in so many networks [with children, parents, your community], it's almost involuntary that you're in all these networks," Harris said. Instead, "it was more what those connections give you."
You have to have the right friends
One of the strengths of this study is that it found a dose effect of social relationships, meaning the more relationships you have, the greater the health benefits, Holt-Lunstad said.
"Many people assume there's a threshold effect -- if you're lonely or isolated you're at risk, but as long as you've got someone in your life you're OK," Holt-Lunstad said.
Having a mix of different relationships also could be beneficial.
"Different people in your life potentially influence you in different ways ... by having these different relationships we may be tapping into additional (biological) pathways that combine to a stronger effect," Holt-Lunstad said.
"We can all benefit from taking our relationships just as seriously for our health as we do other lifestyle factors," she said.
Follow CNN Health on Facebook and Twitter