Pensioners who can identify distinctive smells like roses, lemons, turpentine and paint-thinner are half as likely to develop dementia, a study has revealed.
In a study of nearly 1,800 participants tracked for a decade, US scientists found that a decline in the ability to smell was an early indicator of the syndrome.
A keen sense of smell has a stronger association against the degenerative brain condition than touch, hearing or vision, they claim.
Participants with higher brain capacity, meanwhile tended to have no sensory impairments.
Worsening multisensory function, even at mild levels, was associated with accelerated cognitive aging in study participants
‘Sensory impairments could be due to underlying neurodegeneration or the same disease processes as those affecting cognition, such as stroke,’ said first author Willa Brenowitz at the University of California, San Francisco (UCSF).
‘Alternatively, sensory impairments, particularly hearing and vision, may accelerate cognitive decline, either directly impacting cognition or indirectly by increasing social isolation, poor mobility and adverse mental health.’
The 1,794 participants were recruited from a random sample of Medicare-eligible adults in the Health, Aging and Body Composition (Health ABC) study.
Health ABC is a government-backed interdisciplinary study focused on risk factors for the decline of function in healthier older persons – particularly change in body composition with age.
The researchers tracked the participants in their seventies for a period of up to 10 years to see if their sensory functioning correlated with the development of dementia.
At the time of enrollment, all participants were dementia-free.
Cognitive testing was done at the beginning of the study and repeated every other year.
Dementia was defined by testing that showed a significant drop from baseline scores, documented use of a dementia medication or hospitalisation for dementia as a primary or secondary diagnosis.
By the end, 328 participants (18 percent of the total) went on to develop the condition over the course of the study period.
The researchers conducted tests to determine their multi-sensory ability – not just smell, but hearing, vision and touch.
The tests included contrast-sensitivity tests for vision (glasses were permitted), touch testing in which vibrations were measured in the big toe, and smell tests.
The latter involved identifying distinctive odours like paint-thinner, roses, lemons, onions and turpentine.
The 328 participants were divided into three ranges, based on their sensory ability – ‘poor’, ‘middle’ and ‘good’.
Among those whose sensory levels ranked in the middle range, 141 of the 328 (19 per cent) developed dementia.
This compared with 83 in the good range (12 percent) and 104 (27 percent) in the poor range, according to the study.
While multiple impairments were key to the researchers work, a keen sense of smell, known as olfaction, has a stronger association than any other sensory ability monitored in the study — touch, hearing or vision.
Participants whose smell declined by 10 percent had a 19 percent higher chance of dementia, compared with a 1 to 3 per cent increased risk for corresponding declines in vision, hearing and touch.
‘The olfactory bulb, which is critical for smell, is affected fairly early on in the course of the disease,’ said Brenowitz.
‘It’s thought that smell may be a pre-clinical indicator of dementia, while hearing and vision may have more of a role in promoting dementia.’
Meanwhile, the remaining study participants who remained dementia-free generally had higher cognition at enrollment and tended to have no sensory impairments.
‘We found that with deteriorating multisensory functioning, the risk of cognitive decline increased in a dose-response manner,’ said senior author Kristine Yaffe, a cognitive decline and dementia researcher at UCSF.
‘Even mild or moderate sensory impairments across multiple domains were associated with an increased risk of dementia, indicating that people with poor multisensory function are a high-risk population that could be targeted prior to dementia onset for intervention.’
The 780 participants with good multi-sensory function were more likely to be healthier than the 499 participants with poor multisensory function, suggesting that some lifestyle habits may play a role in reducing risks for dementia.
The former was more likely to have completed high school (85 percent versus 72.1 percent), had less diabetes (16.9 percent versus 27.9 percent) and were marginally less likely to have cardiovascular disease, high-blood pressure and stroke.
The study has been published in Alzheimer’s and Dementia: The Journal of the Alzheimer’s Association.