Elderly people who really can smell the roses may have a lower risk of dementia, research suggests.
Scientists from the University of California, San Fransisco (UCSF), followed 1,800 people in their seventies for up to 10 years.
They found those who retained good sensory function – including being able to identify the scent of roses, paint-thinner and lemons – were up to half as likely to develop the memory-robbing disease.
As to why this occurs, the scientists wondered if the parts of the brain affected by dementia may also regulate our sense of smell.
Sensory decline in itself may also trigger memory loss, possibly indirectly by hindering socialising or affecting mental health.
Dementia is an umbrella term for conditions that cause a progressive loss in brain function.
In the US, 5.7 million people are said to have dementia.
The UCSF scientists set out to uncover if sensory function is linked to the disease, with past studies suggesting an association.
At the start of the study, all the participants were dementia-free, with 328 (18%) developing the condition over the next decade.
After ranking the participants’ sensory levels, the scientists found 27% of those with a poor score were later diagnosed with dementia.
This is compared to 19% in the middle range and 12% who scored “good”.
As well as smell, the scientists also took hearing, vision and touch into account.
Smell – tested by identifying distinctive odours like onions and turpentine – was found to have the biggest impact.
The participants whose sense of smell declined by 10% had a 19% higher risk of developing dementia, compared to 1% to 3% increased odds for those who had the same decline in vision, hearing or touch.
“The olfactory bulb, which is critical for smell, is affected fairly early on in the course of the disease,” said lead author Dr Willa Brenowitz.
“It's thought that smell may be a preclinical indicator of dementia, while hearing and vision may have more of a role in promoting dementia.”
The scientists have suggested sensory impairment may be due to “underlying neurodegeneration or the same disease processes as those affecting cognition, such as stroke”.
“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,” said Dr Brenowitz.
Even a seemingly minor loss of sensory function was found to increase the risk somewhat.
“We found that with deteriorating multi-sensory functioning, the risk of cognitive decline increased in a dose-response manner,” said study author Dr Kristine Yaffe.
“Even mild or moderate sensory impairments across multiple domains were associated with an increased risk of dementia, indicating that people with poor multi-sensory function are a high-risk population that could be targeted prior to dementia onset for intervention.”
The results – published in Alzheimer's and Dementia: The Journal of the Alzheimer's Association – further revealed the participants with good multi-sensory function were generally healthier, with a lower rate of diabetes, heart disease, high-blood pressure or stroke.
They were also more likely to have completed secondary school.