In a study examining factors that influence successful aging, researchers found that among a group of men in their 70s, social isolation was linked to increased levels of C-reactive protein (CRP), interleukin-6 (IL-6) and fibrinogen in the blood. These blood components are elevated during inflammation.
Recent research has suggested that inflammation in the body is a risk marker for cardiovascular disease. People with elevated CRP and fibrinogen have higher risks for heart disease and stroke.
"Social isolation may influence these different inflammatory markers and may be one way social relationships influence our health," said lead author Eric B. Loucks, Ph.D., research fellow at Harvard School of Public Health in Boston. He is a co-investigator for this endeavor in the ongoing MacArthur Successful Aging Study, a research project which follows 1,189 men and women from Durham, N.C., Boston, and New Haven, Conn.
Social relationships have been linked to better health and protection against heart disease in many studies. However, the unanswered question is how social relationships translate into biological processes that affect a person's health.
Loucks and his colleagues at Harvard and the University of California, Los Angeles, investigated CRP, IL-6 and fibrinogen as potential biological links between friends, family and health.
As part of that study, researchers drew and froze blood samples in 1988. They gave a questionnaire to participants to gauge their social relationships. The questions included marital status, the number of close friends and family members, and the extent of religious and social club participation.
In 1988, the potential importance of inflammatory markers in heart disease had not been fully recognized, nor did today's highly sensitive techniques exist to measure CRP and IL-6.
Several years ago, however, the research team began analyzing blood samples drawn from 388 men and 438 women when they entered the MacArthur study. Levels of the participants' biomarkers were correlated with their degree of social relationships.
Researchers failed to find any correlation between the degree of social isolation in women and their levels of the inflammatory biomarkers.
"Men may respond differently than women to social relationships," Loucks said.
"Women also live longer than men," he added. "So another possibility is that in this particular age group, 70 to 79, men's inflammatory biomarkers may be more influenced by social relationships than women's at that age."
Among the 388 men, CRP levels were 3.69 for those in the lowest fourth of social network index (i.e. those most socially isolated) compared to 2.33 for those in the highest fourth. Levels of IL-6 were 5.54 for those in the lowest fourth and 4.10 in the highest fourth. Fibrinogen levels were only slightly different: 2.98 compared to 2.73.
When the researchers statistically controlled for age, education, race, physical functioning, and the presence of other diseases, they still found a significant inverse correlation between people's social network and their levels of the three biomarkers.
However, when the team further controlled for behavioral factors that can affect health - such as smoking, alcohol consumption, physical exercise and obesity - the association was no longer statistically significant.
This last finding shines further light on how social relationships may influence a person's level of biomarkers because social relations may influence behavior, Loucks noted.
"If your spouse eats a high-fat diet, chances are you will eat a high-fat diet, or if your spouse exercises, chances are you will too," he said. "People who have a low variety of social relationships may not have people to support them in behaviors such as exercise, or in stopping smoking.
"Stress can raise levels of IL-6 and fibrinogen and may be another pathway by which social isolation can influence health," Loucks said.
Future long-term studies are planned to examine causes.
Co-authors are Lisa F. Berkman, Ph.D.; Tara L. Gruenewald, Ph.D. and Teresa E. Seeman, Ph.D.