June 27 (HealthDay News) -- The key to a healthy mind in old age may lie in an active social life, a new study suggests.

"If you are socially engaged, you are at lower risk of dementia," said Dr. Valerie C. Crooks, a researcher at the Department of Research and Evaluation, Kaiser Permanente Southern California.

During her study, which followed more than 2,200 women ages 78 and older for four years, those with large social networks reduced their risk of getting dementia by 26 percent, she said.

Previous studies about the association between social engagement with family and friends and cognitive functioning in old age have yielded mixed results, Crooks noted. For example, "there were studies that said being married is helpful, and studies that said being married is not so helpful," she said.

In recent studies, social contact has been generally found to be protective of cognitive functioning, however, she said.

For this new study, published in the July issue of the American Journal of Public Health, Crooks and her colleagues conducted telephone interviews with the women, all of who were members of the Kaiser Permanente HMO. The women were free of dementia when the study started in 2001.

The team tested each woman's cognitive status by phone and reviewed her medical records to help assess it, as well. They also asked about social interactions with a spouse and/or other family and friends, including how many people they interacted with and how often.

Crooks' team also asked the women how many people they felt they could rely on if they needed help and whether they had a person or persons they could talk to about personal issues.

At the end of the follow-up, 268 of the women had been diagnosed with dementia.

Those with larger social networks also showed a reduced risk for dementia, whose most common form is Alzheimer's disease.

"In this study, we found marriage didn't make a difference in terms of dementia risk," she said. In other words, it was the social network that was protective, regardless of whether the woman was married or not.

"Those with daily contact or more had a lower risk of dementia," she said. The contact didn't have to be face-to-face -- e-mail and telephone interaction counted, Crooks added.

It's impossible to say how many friends and family makes up a big enough social circle to be protective, the researcher said. "Two or fewer is probably not a sufficient amount. You could have three really close friends [or family] and be fine," Crooks speculated.

"We can't tell you what the magic number is," she said.

And she emphasized that her team found only an association between social networks and reduced risk of dementia, which doesn't point to a cause-and-effect relationship, necessarily.

"There could be a person with one person [in his or her social network] who is doing perfectly fine," she said.

While more study is needed to zero in on exactly which aspects of social support are linked with a decrease in dementia risk, Crooks said the findings make perfect sense, neurologically speaking. "The more interaction, the more you challenge your brain."

Dr. William Thies, vice president of medical and scientific relations at the Alzheimer's Association, said the finding "fits with a large body of evidence that being isolated is bad for you."

But he added, the finding is merely an association, not cause and effect.

"You don't know whether the bigger network prevents Alzheimer's or [whether] people who don't get Alzheimer's maintain bigger networks," he said.

SOURCES: Valerie C. Crooks, D.S.W., researcher, Kaiser Permanente Southern California, Pasadena, Calif.; William Thies, Ph.D., vice president, medical and scientific relations, Alzheimer's Association, Chicago; July 2008, American Journal of Public Health