Study Shows Having No Friends 'As Deadly As Smoking'

A man smokes a cigarette, London, December 16, 2015. A report has shown that not having many friends has the same impact on levels as taking up smoking. Leon Neal/Getty

Having no friends could be as deadly as smoking, according to reports.

Researchers at Harvard University have discovered a link between loneliness and the levels of a blood-clotting protein that can cause heart attacks and stroke.

Social isolation is known to activate the "fight or flight" stress signal which increases levels of the protein fibrinogen in anticipation of injury and blood loss.

But too much fibrinogen is bad for health, raising blood pressure and causing the build-up of fatty deposits in the arteries, The Daily Telegraph reports.

Harvard researchers compared levels of the blood-clotting protein with the numbers of friends and family in a person's social network and found a correlation. As the number of social connections fell, the level of fibrinogen rose.

People with just five people in their social network had 20 percent higher levels of fibrinogen than those with 25. Having 10-12 fewer friends than that had the same impact on levels as taking up smoking.

"Measurement of the whole social network can provide information about an individual's cardiac risk that is not necessarily apparent to the individual herself," said lead author Dr David Kim of Harvard Medical School.

"Social connectedness displays a significant association with fibrinogen," he continued. "If there is an independent causal relationship between social isolation and fibrinogen and, subsequently, heart disease and stroke, then policies and interventions that improve social connectedness may have health effects even beyond the well-known benefits of improved economic conditions."