They used a "gene chip" to look at the DNA of isolated people and found that people who described themselves as chronically lonely have distinct patterns of genetic activity, almost all of it involving the immune system.
The study does not show which came first -- the loneliness or the physical traits. But it does suggest there may be a way to help prevent the deadly effects of loneliness, said Steve Cole, a molecular biologist at the University of California Los Angeles who worked on the study.
"What this study shows is that the biological impact of social isolation reaches down into some of our most basic internal processes -- the activity of our genes," Cole said.
"We have known for years that there is this epidemiological relationship between social support -- how many friends and family members you have around you -- and a whole bunch of physical outcomes," he said in a telephone interview.
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 -- 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 .