THURSDAY, July 2 (HealthDay News) -- Middle-aged adults who live alone are twice as likely to develop dementia or Alzheimer's disease later in life compared to those who are married or live with a partner. And the risk is three times higher among those who are divorced or widowed, according to a new study by Swedish and Finnish researchers.

The study included 2,000 men and women in Finland who were initially surveyed when they were about 50 years old and again 21 years later.

In addition to looking at the association between marital status and dementia, the researchers also examined the link between living alone and being a carrier of the apolipoprotein E e4 gene variant, a known risk factor for Alzheimer's disease.

The study found the highest risk of developing Alzheimer's disease among people with the gene variant who live alone after losing their partner.

The findings add to a growing body of evidence that shows that social factors play an important role in brain health, and indicate that "supportive intervention for individuals who have lost a partner might be a promising strategy in preventive health care," according to Dr. Miia Kivipelto, of the Karolinska Institute in Sweden, and colleagues.

The study, which appears online July 2 in the BMJ, is one of the first to examine mid-life marital status and dementia risk.

In an accompanying editorial, epidemiologist Dr. Catherine Helmer of Victor Segalen University in Bordeaux, France, noted that the study strengthens the theory that cognitive impairment and dementia are affected by various factors throughout life and develop over a long period of time.

The findings could lead to preventive strategies that encourage unmarried people to boost their social involvement by taking part in community, cultural and sporting activities, Helmer said.

More information

The U.S. National Institute on Aging has more on Alzheimer's disease.