Suffering bouts of nervousness or struggling to concentrate in mid-life could be an early warning sign of dementia, experts claim.
British scientists who tracked more than 5,000 adults over the age of 55 for more than two decades found that people showing clusters of specific depressive symptoms were significantly more likely to develop the condition later in life.
Those most at risk had five or more out of six warning signs, including worsening anxiety, difficulty concentrating, an inability to face up to problems, a lack of warmth or affection towards others, and a loss of self-confidence.
Another red flag was dissatisfaction with how everyday tasks were carried out.
Researchers said the findings, published in The Lancet Psychiatry, could give doctors a way to spot people at risk of dementia years before memory loss and confusion set in.
Almost one million people in the UK are currently living with dementia - a figure expected to soar as the population ages.
The condition has no cure, but experts say identifying those at risk as early as possible could allow doctors to encourage lifestyle changes that may help slow its progression.
Depression has long been known to be common in people with dementia, but the researchers said this was one of the first studies to track specific mental health symptoms over decades to see how they relate to the disease later in life.
Dr Philipp Frank - a research fellow in the mental health of older people at University College London and the study's lead author - said the findings suggest dementia risk is linked to specific depressive symptoms, rather than depression as a whole.
'Our findings show that dementia risk is linked to a handful of depressive symptoms rather than depression as a whole,' he said.
'This symptom-level approach gives us a much clearer picture of who may be more vulnerable decades before dementia develops.
'Everyday symptoms that many people experience in midlife appear to carry important information about long-term brain health.
'Paying attention to these patterns could open new opportunities for early prevention.'
In the study, researchers analysed health data from 5,811 adults aged between 45 and 69, with an average age of 55.
Participants were asked detailed questions about their mental health, covering 30 common depressive symptoms. Over a follow-up period of 23 years, 586 people went on to develop dementia.
Those who reported five or more of the six key depressive symptoms were found to have a 27 per cent higher risk of developing the condition compared with people who did not report low mood.
In particular, volunteers who reported that they had lost confidence in themselves in midlife appeared to carry a 51 per cent increased risk of dementia in later life.
People who said they were 'not able to face up to their problems' had a 49 per cent increased risk.
Reporting 'not feeling warmth and affection for others' had a 44 per cent raised risk while feeling nervous and 'strung up' held a 34 per cent rise of a third in risk of dementia later in life.
Those who were not satisfied with the way tasks are carried out had a 33 per cent increased risk of dementia more than 20 years later, while those who reported difficulties concentrating had a 29 per cent elevated risk.
Writing in The Lancet Psychiatry, the scientists said: 'A distinct set of midlife depressive symptoms was associated with an increased risk of dementia, suggesting that these symptoms might be early markers of underlying neurodegenerative processes.'
Commenting on the paper, Dr Richard Oakley, associate director of research and innovation at Alzheimer's Society, said: 'The connection between dementia and depression is complicated.
'It's encouraging to see this new observational study begin to unpick how dementia and depression are interlinked.
'However, more research is needed to confirm whether these six symptoms also apply to women and ethnic minorities.
'It's important to note that not everyone who has depression will go on to develop dementia, and people with dementia won't necessarily develop depression.'
Dementia claims 76,000 lives each year and is the UK's biggest killer — usually from problems arising from a weakened immune system, such as pneumonia or difficulty swallowing.
Costs to the UK economy, including that of the round-the-clock care many need as dementia takes hold, is an eye-watering £42 billion a year, according to the charity Alzheimer's Society.
That figure will double by 2040, it warns.
Earlier this month, the Daily Mail launched its Defeating Dementia campaign, in association with Alzheimer's Society, to raise awareness of the disease, increase early diagnosis, boost research and improve care.
Scientists now believe around 45 per cent of all cases of dementia may be preventable or — at the very least — that the symptoms can be delayed, in some cases for many years, allowing everyone to live longer, healthier lives.
