Andrew Steptoe, Aparna Shankar, Panayotes Demakakos and Jane Wardle of the University College London said they assessed social isolation in terms of contact with family and friends and participation in civic organizations in 6,500 men and women age 52 and older who took part in the English Longitudinal Study of Aging in 2004-05. A standard questionnaire measure of loneliness was also administered.
The researchers monitored all-cause mortality up to March 2012 -- mean follow-up of seven years -- and analyzed results using Cox proportional hazards regression.
After adjusting statistically for demographic factors and baseline health, social isolation remained significantly associated with mortality for the top 20 percent of the most isolated but loneliness did not.
Both social isolation and loneliness were associated with increased mortality but the effect of loneliness wasn't independent of demographic characteristics or health problems and didn't contribute to the risk associated with social isolation.
"Although both isolation and loneliness impair quality of life and well-being, efforts to reduce isolation are likely to be more relevant to mortality," the researchers wrote in the study.
The findings were published in the Proceedings of the National Academy of Sciences.