The researchers say that their findings link social isolation to alterations in the activity of genes that drive inflammation, the first response of the immune system.
According to them, their study provides a molecular framework for understanding why social factors are linked to an increased risk of heart disease, viral infections and cancer.
Previous studies had shown that lonely people suffered from higher mortality than others. The researchers are now trying to determine whether this risk is an outcome of reduced social resources like physical or economic assistance or of the biological impact of social isolation on the function of the human body.
"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." said Steve Cole, an associate professor of medicine in the division of Hematology-Oncology at the David Geffen School of Medicine, and a member of the UCLA Cousins Center for Psychoneuroimmunology.
"We found that changes in immune cell gene expression were specifically linked to the subjective experience of social distance. The differences we observed were independent of other known risk factors, such as health status, age, weight, and medication use. The changes were even independent of the objective size of a person's social network," said Cole, who is also a member of the Jonsson Comprehensive Cancer Center.
Working with collaborators from the University of Chicago, Cole and his colleagues used DNA micro-arrays to survey the activity of all known human genes in white blood cells from 14 individuals in the Chicago Health, Aging, and Social Relations Study.
The researchers say that six participants scored in the top 15 per cent of the UCLA Loneliness Scale, a widely used measure of loneliness that was developed in the 1970s. Other study subjects scored in the bottom 15 per cent, they add.
It was observed that 209 gene transcripts (the first step in the making of a protein) were differentially expressed between the two groups, with 78 being overexpressed and 131 underexpressed.
"Leukocyte (white blood cell) gene expression appears to be remodelled in chronically lonely individuals," said Cole. He said that genes overexpressed in lonely individuals included many involved in immune system activation and inflammation, but certain genes involved in antiviral responses and antibody production were underexpressed.
"These findings provide molecular targets for our efforts to block the adverse health effects of social isolation," said Cole. "We found that what counts at the level of gene expression is not how many people you know, it's how many you feel really close to over time," he added.
He said that the transcriptional fingerprint identified by his team might become useful as a 'biomarker' to monitor interventions designed to reduce the impact of loneliness on health in future.
The study has been published in the journal Genome Biology.