An integral part of many people's lives, religion defines patterns of worship and often socialization, but its impact, if any, on health is unclear. Some studies seem to show a benefit from religious practice, while others — including much of the research into prayer — fail to prove its health value.

It's not hard to understand why being religious might be good for the body, some experts say. Religious people often attend regular services — this puts them in a socially supportive environment, which has widely acknowledged health advantages. And some religions promote healthful diets and discourage unhealthy behaviors such as smoking.

"Religions package many of the ingredients of well-being to make them accessible to people," said Richard Eckersley, a visiting fellow at the National Centre for Epidemiology and Population Health at the Australian National University in Canberra. And the "psychological well-being" that religion can promote is "linked to physical health through direct physiological effects, such as on neuroendocrine and immune function, and indirect effects on health behaviors, such as diet, smoking, exercise and sexual activity."

Interest in researching the impacts of religion and spirituality on how we live seems to be rising. David Myers, author of "A Friendly Letter to Skeptics and Atheists" (to be published in August) and a psychology professor at Hope College in Holland, Mich., did a database search to compare recent and past interest in the topic. Between 1965 and 1999, 1,950 study abstracts mentioned religion or spirituality, he found. Myers' search for the same terms in abstracts published between 2000 and 2007 came up with 8,719 hits, he said.

Among that research is some evidence that religion and spirituality can offer health benefits and even longer life spans. A national survey conducted by the Centers for Disease Control and Prevention and mentioned in Myers' book found that people who did not attend religious services were 1.87 times more likely to have died during an eight-year period than those who attended services more than weekly. The life expectancy for infrequent attendees was age 75, and it was 83 for those who attended frequently.

A 1996 study looked at the association of Jewish religious observance with mortality by comparing secular and religious kibbutzim in Israel. Belonging to a religious group appeared to be linked to living longer, and the lower mortality rates seen in the religious group were consistent for all causes of death, the authors wrote.

A 2003 study published in Psychosomatic Medicine found that meditation might alter brain and immune function in positive ways, an effect similarly seen in research involving Buddhist monks.

But researchers have had trouble replicating such statistics in the randomized studies that are the gold standard for medical research. It's hard to show conclusively whether or how a belief system affects one's health; other life experiences may provide health benefits so similar to religion and spirituality that it's hard to differentiate.

It's not unusual for people to pray for their own health and for that of others. In a 2004 survey of more than 31,000 people, 45 percent said they'd prayed for health reasons, 43 percent prayed for their own health, and 25 percent reported that others had prayed for them. About 10 percent said they'd participated in a prayer group for their health, according to the survey released by the National Center for Health Statistics and the National Center for Complementary and Alternative Medicine.

But a 2003 update to an earlier systemic review of clinical trials on distant healing found that intercessory prayer, which involves someone praying for the healing of a person located elsewhere, with or without that person's knowledge, doesn't appear to have specific healing effects.

Any benefit seen from prayer might come from the fact that "knowing that your friends and family are praying for you is part of social support ... and (that is) probably really helpful to people, independent of if there is a higher being that answers those prayers," said David G. Schlundt, an associate professor of psychology at Vanderbilt University, who has researched the connection between faith and health.

Still, there is no question that many people believe that having a religious or spiritual foundation is ultimately beneficial to their health. "Actively religious people are considerably more likely to report that they're more happy than not-religious people," Myers said. "Religion is a communal experience that helps provide hope and motivates healthy living."

Although religion might provide social support, purpose, a belief system, a moral code — and even happiness — these benefits can also come from other sources, including cultural influences, notes a 2007 study by Eckersley published in the Medical Journal of Australia.

Cultural context could be key to understanding how people's beliefs factor into their health outcomes, experts note, because religion and spirituality don't seem to produce a uniform effect on everyone — differences are apparent and vary by socioeconomic and racial and ethnic backgrounds.

"You have to think of a religious group not just in terms of beliefs but also the socioeconomic context of that group," Schlundt said. "For instance, southern Baptists in rural communities will look different than big-city Baptists."

Experts also note that a negative aspect of the purported faith-health connection is fatalism, the belief that health is predetermined and is not within a person's control. Research shows that people who hold such beliefs are less likely than others to participate in health promotion programs and to seek health care.

Studies have found that African Americans are more likely to endorse fatalism than whites. A 2007 study published in the American Journal of Health Behavior reported that such beliefs could be a reaction to chronic illness or poor health rather than something that inhibits beneficial health behavior from the outset.

"Religion can have its dark side, too," Schlundt notes. "If you belong to a community that induces guilt or creates discomfort, that can have the same effect as any source of stress. If your religious belief gives you a feeling of fatalism where you don't step forward to protect your health because you believe God will do that for you, that's another potential downside."