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.