NEW YORK (Reuters Health) - Elderly people who report being lonely are more than twice to develop Alzheimer's disease as likely as people who are not lonely, new research indicates.

Social isolation in old age -- being single, having few friends, and participating in few activities with others -- has been associated with risk of developing dementia, Dr. Robert S. Wilson, from Rush University Medical Center, Chicago, and colleagues explain in the Archives of General Psychiatry. "In contrast, little is known about the association of dementia and emotional isolation, or loneliness, which refers to perceived social isolation and feeling disconnected from others, that is, to dissatisfaction with social interactions rather than their absence," they note.

To investigate, Wilson and colleagues followed a group of 823 elderly individuals for up to 4 years. At the first examination, participants' rated their agreement with statements such as "I often feel abandoned" and "I miss having a really good friend" to assess their perceived loneliness. The average loneliness score was 2.3 on a scale of 1 to 5, with higher scores indicating more loneliness.

The subjects were free of dementia at the outset but during follow up 76 individuals developed dementia that could be classified as Alzheimer's disease.

The investigators found that the risk of developing dementia increased roughly 51 percent for each notch on the loneliness scale, so that a person with a high degree of loneliness (a score of 3.2) had about 2.1 times the risk of developing Alzheimer's disease during the study compared with someone with a low degree of loneliness (a score of 1.4).

Loneliness was associated with lower cognitive function at the start of the study and with more rapid cognitive decline during the study, the authors report.

In autopsies performed on 90 individuals who died during the study, loneliness during life was not related to any of the hallmark brain changes associated with Alzheimer's disease. This finding suggests that loneliness may contribute to the development of Alzheimer's-like dementia in some other way, Wilson and colleagues surmise.

The mechanism that does link dementia and loneliness is unclear but it seems unlikely that loneliness is caused by dementia, the authors say, because loneliness levels remained relatively stable even in individuals who developed dementia.