People with strong social relationships were 50 percent less likely to die early than people without such support, the team at Brigham Young University in Utah found.
They suggest that policymakers look at ways to help people maintain social relationships as a way of keeping the population healthy.
"A lack of social relationships was equivalent to smoking up to 15 cigarettes a day," psychologist Julianne Holt-Lunstad, who led the study, said in a telephone interview.
Her team conducted a meta-analysis of studies that examine social relationships and their effects on health. They looked at 148 studies that covered more than 308,000 people for their analysis, published in the Public Library of Science journal PLoS Medicine
Having low levels of social interaction was equivalent to being an alcoholic, was more harmful than not exercising and was twice as harmful as obesity.
Social relationships had a bigger impact on premature death than getting an adult vaccine to prevent pneumonia
"I certainly don't want to downplay these other risk factors because of course they are very important," Holt-Lunstad said. "We need to start taking social relationships just as seriously."
PEOPLE INCREASINGLY ISOLATED
Government policies to encourage social relationships will not necessarily be easy, Holt-Lundstad said. "Air pollution and the clean air act -- that is simple policy," she said.
But she has some ideas -- such as making it easier for friends or relatives to take part in medical care, and city planning that encourages interaction.
The different studies measured social interaction in different ways, so the researchers said it was impossible to precisely define positive social interaction.
It equally difficult to study systematically, as it is impossible to randomly assign people to have friends or not have friends. But Holt-Lundstad said there is some evidence that assigning caretakers does not help improve people's health.
"Naturally occurring relationships may be different than support received from someone who is hired for that purpose," she said.
Her team found some troubling evidence that Americans are becoming more isolated, and thus losing the support and care that love and friendship provide.
"For instance, trends reveal reduced intergenerational living, greater social mobility, delayed marriage, dual-career families, increased single-residence households, and increased age-related disabilities," they wrote.
"More specifically, over the last two decades there has been a three-fold increase in the number of Americans who report having no confidant," they added.
"Such findings suggest that despite increases in technology and globalization that would presumably foster social connections, people are becoming increasingly more socially isolated."
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