Alzheimer and other dementias are linked to risk factors such as older age, low education, genetics, earlier head injury, mild cognitive impairment, stroke, hypertension, lipid abnormalities, diabetes, obesity and other cardiac arrhythmias. We also know that late onset depression can be linked to cognitive decline as well. Depression may impact on brain activity.

We also know that social isolation, not being married, living alone, having a small social network, little participation in activities and lack of social engagement are also associated with cognitive declines. The question these researches posed is whether loneliness and feeling lonely is a risk as well.

And in fact, the researchers found that feeling lonely, as distinct from being or living alone, is indeed linked to an increased risk of developing dementia in later life. This is important, given the ageing population and the increasing number of single households, they suggest.

The researchers tracked the long term health and wellbeing of more than 2,000 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 tested for signs of dementia.

At the start of the monitoring period, around half (46 per cent; 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 per cent; 433) said they felt lonely.

Among those who lived alone, around one in 10 (9.3 per cent) had developed dementia after three years compared with one in 20 (5.6 per cent) 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.

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 per cent compared with 5.7 per cent).

Also those who lived alone or who were no longer married were between 70 per cent and 80 per cent more likely to develop dementia than those who lived with others or who were married. 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  per cent 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.