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 that 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 to 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.Individuals with just five people in their social network had 20% 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, in turn, can be lethal in the long term...