Lonely people are more likely to get sick and die young because it affects their immune systems, according to new research.
American scientists used a "gene chip" to look at the DNA of isolated people and found that those who described themselves as chronically lonely had weaker immune systems.
Study author Steve Cole, a molecular biologist at the University of California said: "This study shows that the biological impact of social isolation reaches down into some of our most basic internal processes - the activity of our genes."
Many studies of large populations have shown that people who describe themselves as lonely or as having little social support are more likely to die prematurely and to have infections, high blood pressure, insomnia and cancer.
"There are two theories," Cole said.
"The social provision theory, which basically is about what other people do for you in a tangible, material sense. Like, if I am sick and I have got people around me, they will take me to the doctors, they will see I take my pills," Cole said.
"The other is that there is something about being isolated and lonely that changes your body."