According to a large study carried out in the United States older people who are lonely are far more likely to develop Alzheimer's disease.
This is not the first study to suggest that social isolation is linked to diseases such as dementia and Alzheimer's disease, but this latest study shows loneliness doubles the risk.
The findings are the result of a study of more than 823 elderly patients with an average age of 80.7, over a four-year period.
Social isolation means people have a small social network, are often unmarried and participate in few activities with others.
The study authors say little is known about the link between dementia and emotional or social isolation where individuals feel disconnected from others and dissatisfied with social interactions.
Although the link between social isolation and dementia has already been established the researchers say this is the first time how alone people actually feel has been examined.
The researchers say the reason for the link is unclear but loneliness appears to have a physical impact as well as an emotional one.
Professor Robert Wilson and colleagues at Rush University Medical Center, Chicago, assessed the participants loneliness each year between November 2000 to May 2006, for up to four years.
Evaluations that included questionnaires to assess loneliness, classifications of dementia and Alzheimer’s disease were used along with tests of thinking, learning and memory.
Their loneliness was measured on a scale of one to five, with higher scores indicating more loneliness.
The group were also assessed for signs of dementia and Alzheimer's disease.
When autopsies were carried out on 90 patients who died during the study, to look for certain physical signs associated with Alzheimer's disease such as deposits of protein outside and around nerve cells, no association between loneliness and the brain pathology associated with Alzheimer's disease was seen.
However the team found that the risk of developing Alzheimer's disease increased by 51% for each point of the loneliness score and those individuals with the highest loneliness score of 3.2 had about 2.1 times the risk of developing Alzheimer's disease compared to those with a low score of 1.4.
When the researchers factored in social isolation, such as if people had a small social network, the results did not change significantly.
Professor Wilson, says the results suggest that loneliness is a risk factor for Alzheimer's disease and may affect systems in the brain dealing with cognition and memory, making lonely people more vulnerable to effects of age-related decline in neural pathways.
Experts say the study is interesting as it shows a clear link between poor social activity and a higher risk of dementia symptoms but suggest more research is needed to understand the exact link between loneliness and dementia symptoms.
The study is published in Archives of General Psychiatry.