Having few friends is 'as bad for you as smoking'
Having no friends could be as deadly as smoking, researchers at Harvard University have suggested, after discovering a link between loneliness and the levels of a blood-clotting protein which can cause heart attacks and stroke.
Social isolation is known to activate the 'fight or flight' stress signal which increases levels of 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.
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 striking correlation. As the number of social connections fell, the level of fibrinogen rose.
People with just five people in their social network had 20pc higher levels fibrinogen than those with 25. Having 10-12 fewer friends had the same impact on levels as taking up smoking.
It is thought that social isolation leaves people feeling threatened and vulnerable which triggers an ongoing 'fight or flight' response which can be lethal in the long term.
"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. If there is indeed 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."
Previously loneliness has been linked to a compromised immune system, high blood pressure, and ultimately, premature death.
A recent study by the University of York found that lonely people are around 30pc more likely to suffer a stroke or heart disease. But the reasons have remained unclear. Some researchers thought it was simply that there were fewer people to notice when a person was ill or encourage them to take care of their health.
Dr Nicole Valtorta who led the University of York research said: "These findings are consistent with a growing body of research indicating that social relationships are important for health.
"Our recent review, based on self-reports of social relationships, found that individuals who felt lonely or who were socially isolated had on average a 30pc increased risk of developing coronary heart disease or having a stroke.
"It may be that some of the effect observed by the authors of the article is a result of people's social relationships being affected by poor health. To build on this study, future studies are needed to investigate whether interventions that tackle social isolation have an effect on health."
In January, the Local Government Association said loneliness should be treated as a 'major health issue', while charity Age UK claim the issue "blights the lives" of over a million older people.