Older people with a poor sense of smell have a greater chance of developing Alzheimer’s and other forms of dementia, US research has shown.
The discovery raises the prospect of using “sniffing sticks” to flag individuals needing closer monitoring in the next five years.
Though humans lack the acute sense of some animals such as dogs, they can distinguish up to a trillion different odours.
The brain’s ability to sort and recognise smells may provide a way of spotting early damage caused by neurodegenerative disease, the research suggests.
But experts warned that impaired smell detection was not always a symptom of early dementia.
Almost 3000 adults aged 57 to 85 took part in the long-term US study, which involved waving “sniffing sticks” with various smell flavours in front of their noses.
Those with a “normal” sense of smell could identify at least four out of five common odours.
Compared with this group, people who failed the test were more than twice as likely to develop dementia five years later.
The vast majority of those tested, 78 per cent, had a “normal” sense of smell and could accurately identify the scents.
But 14 per cent could name just three out of five, 5 per cent could identify only two, and 2 per cent could recognise only one.
Just 1 per cent of participants were not able to name a single smell.
Five years after the initial test, almost every participant who was unable to name any of the smells had been diagnosed with dementia.
Nearly 80 per cent of those who provided only one or two correct answers had developed the condition.
There was a dose-dependent effect, the study found, with dementia rates rising in step with increasingly poor smell sense.
Lead scientist Professor Jayant Pinto, from the University of Chicago, said: “These results show that the sense of smell is closely connected with brain function and health.
“We think smell ability specifically, but also sensory function more broadly, may be an important early sign, marking people at greater risk for dementia.
“Loss of the sense of smell is a strong signal that something has gone wrong and significant damage has been done. This simple smell test could provide a quick and inexpensive way to identify those who are already at high risk.”
The findings are reported in the Journal of the American Geriatrics Society.