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DURHAM, N.C. -- Researchers at Duke University Medical Center have shown that people who regularly attend religious services appear to have a healthier immune system than those who don't. They say the mechanism behind this effect remains unclear, and the results may not apply to people who live in regions where religious participation is not a major cultural force.

In a study of 1,718 older adults in North Carolina, Dr. Harold Koenig and Dr. Harvey Cohen found that those who attended services at least once a week were about half as likely as non-attenders to have elevated levels of interleukin-6 (IL-6), an immune system protein involved in a wide array of age-related diseases.

This effect persisted, albeit to a lesser degree, even when researchers accounted for factors like depression, chronic illness and negative life events that were likely to affect immune status. Results of the study, funded by the National Institute on Aging, were reported in the October issue of the International Journal of Psychiatry in Medicine.

Koenig speculates that religion may affect immune function through better coping skills, psychosocial factors and the mechanisms by which organized religion promote positive thoughts and behaviors.

"Perhaps religious participation enhances immune functioning by yet unknown mechanisms, such as through feelings of belonging, togetherness, even perhaps the experience of worship and adoration," said Koenig, lead author of the study. "Such positive feelings may counteract stress and convey health effects that go far beyond simply the prevention of depression or other negative emotions."

While Koenig found a similar effect between religion and good health last year, that study relied on personal interviews to gauge the subjects' health status and feelings of well being. In the current study, researchers measured blood samples for levels of IL-6 and other substances -- alpha, beta, and gamma globulins, fibrin d-dimers, lymphocytes, and neutrophils -- that regulate immune and inflammatory responses in the body.

Researchers say biological measures are much more accurate than interviews in gauging how healthy a person really is.

"There is so much subjectivity when people say they feel better that you can't rely on self reports alone to truly reflect health status," said Cohen, professor of medicine and director of the Center for the Study of Aging and Human Development at Duke. "By measuring blood levels of IL- 6, we were trying to put rigorous scientific parameters on the positive health effects of religion," he said.

The research team selected IL-6 as a marker of immune status because it has been shown to contribute to a broad spectrum of age-related diseases, including B-cell lymphomas, multiple myeloma, autoimmune diseases and some viral diseases, Cohen said. His own research, published in the July issue of the Journal of Gerontology, showed an association between high levels of IL-6 and poor functional ability -- a phrase used to describe routine tasks of daily living, from walking to a chair to bathing, dressing and cooking.

Still other studies have shown IL-6 levels are elevated with diseases such as cancer, heart disease and high blood pressure in older adults.

Cohen said two prevailing theories could explain the relationship between aging, disease, and high levels of IL-6. The first hypothesis holds that as people age, they acquire more diseases, and their bodies react to those diseases by producing more IL-6.

In the second theory, favored by Cohen and his colleagues, the aging process itself causes the body to lose its ability to regulate IL-6, and "proteins begin to break down and appear where they shouldn't be," thereby making the body vulnerable to disease, Cohen said.

Researchers discounted the possibility that people with high IL-6 levels did not attend church because of fatigue or lethargy, which would have skewed the results, Koenig said. In this study, 60 percent of elderly persons surveyed were attending religious services at least once a week, despite their advanced age and substantial disability.

Researchers cautioned that the results may represent, in part, a regional influence. Participants in the study are from the "Bible Belt" South, where they said religion is "ingrained in the social fabric of the community."

"If nothing else, our study should stimulate further research to replicate these findings, establish a direction of effect, and identify the mechanism by which religious attendance affects immune functioning, or vice versa," said Koenig.