Feeling lonely, as distinct from being/living alone, is linked to an increased risk of developing dementia in later life, indicates research published online in the Journal of Neurology Neurosurgery and Psychiatry.
Various factors are known to be linked to the development of Alzheimer's disease, including older age, underlying medical conditions, genes, impaired cognition, and depression, say the authors.
But the potential impacts of loneliness and social isolation--defined as living alone, not having a partner/spouse, and having few friends and social interactions -- have not been studied to any great extent, they say.
This is potentially important, given the ageing population and the increasing number of single households, they suggest.
They therefore tracked the long term health and wellbeing of more than 2000 people with no signs of dementia and living independently for three years.
All the participants were taking part in the Amsterdam Study of the Elderly (AMSTEL), which is looking at the risk factors for depression, dementia, and higher than expected death rates among the elderly.
At the end of this period, the mental health and wellbeing of all participants was assessed using a series of validated tests. They were also quizzed about their physical health, their ability to carry out routine daily tasks, and specifically asked if they felt lonely. Finally, they were formally tested for signs of dementia.
At the start of the monitoring period, around half (46%; 1002) the participants were living alone and half were single or no longer married. Around three out of four said they had no social support. Around one in five (just under 20%; 433) said they felt lonely.
Among those who lived alone, around one in 10 (9.3%) had developed dementia after three years compared with one in 20 (5.6%) of those who lived with others.
Among those who had never married or were no longer married, similar proportions developed dementia and remained free of the condition.
But among those without social support, one in 20 had developed dementia compared with around one in 10 (11.4%) of those who did have this to fall back on.
And when it came to those who said they felt lonely, more than twice as many of them had developed dementia after three years compared with those who did not feel this way (13.4% compared with 5.7%).
Further analysis showed that those who lived alone or who were no longer married were between 70% and 80% more likely to develop dementia than those who lived with others or who were married.
And those who said they felt lonely were more than 2.5 times as likely to develop the disease. And this applied equally to both sexes.
When other influential factors were taken into account, those who said they were lonely were still 64% more likely to develop the disease, while other aspects of social isolation had no impact.
"These results suggest that feelings of loneliness independently contribute to the risk of dementia in later life," write the authors.
"Interestingly, the fact that 'feeling lonely' rather than 'being alone' was associated with dementia onset suggests that it is not the objective situation, but, rather, the perceived absence of social attachments that increases the risk of cognitive decline," they add.
They suggest that loneliness may affect cognition and memory as a result of loss of regular use, or that loneliness could itself be a sign of emerging dementia, and either be a behavioural reaction to impaired cognition or a marker of undetected cellular changes in the brain.
[Feelings of loneliness, but not social isolation, predict dementia onset: results from the Amsterdam Study of the Elderly (AMSTEL) Online First doi 10.1136/jnnp-2012-302755]