A long-term study of nearly 3,000 adults, aged 57 to 85, found that those who could not identify at least four out of five common odors were more than twice as likely to develop dementia within five years.
Although 78 percent of those tested were normal — correctly identifying at least four out of five scents — about 14 percent could name just three out of five, five percent could identify only two scents, two percent could name just one, and one percent of the study subjects were not able to identify a single smell, according to researchers.
Five years after the initial test, almost all of the study subjects who were unable to name a single scent had been diagnosed with dementia. Nearly 80 percent of those who provided only one or two correct answers also had dementia, the study found.
“These results show that the sense of smell is closely connected with brain function and health,” said the study’s lead author, Jayant M. Pinto, M.D., a professor of surgery at the University of Chicago and ENT specialist who studies the genetics and treatment of olfactory and sinus disease.
“We think smell ability specifically, but also sensory function more broadly, may be an important early sign, marking people at greater risk for dementia.”
“We need to understand the underlying mechanisms, so we can understand neurodegenerative disease and hopefully develop new treatments and preventative interventions,” he continued.
“Loss of the sense of smell is a strong signal that something has gone wrong and significant damage has been done,” Pinto added. “This simple smell test could provide a quick and inexpensive way to identify those who are already at high risk.”
The new study, published in the Journal of the American Geriatrics Society, follows a related 2014 study, in which olfactory dysfunction was associated with increased risk of death within five years. In that study, loss of the sense of smell was a better predictor of death than a diagnosis of heart failure, cancer, or lung disease.
For both studies, the researchers used “Sniffin’Sticks,” which look like a felt-tip pen, but instead of ink, they are infused with distinct scents.
Study subjects smell each item and are asked to identify that odor, one at a time, from a set of four choices. The five odors, in order of increasing difficulty, were peppermint, fish, orange, rose, and leather.
Test results showed that:
- 78.1 percent of those examined had a normal sense of smell;
- 48.7 percent correctly identified five out of five odors and 29.4 percent identified four out of five;
- 18.7 percent, considered “hyposmic,” got two or three out of five correct;
- The remaining 3.2 percent, labelled “anosmic,” could identify just one of the five scents (2.2 percent), or none (one percent).
The olfactory nerve is the only cranial nerve directly exposed to the environment. The cells that detect smells connect directly with the olfactory bulb at the base of the brain, potentially exposing the central nervous system to environmental hazards such as pollution or pathogens, researchers explain.
Olfactory deficits are often an early sign of Parkinson’s or Alzheimer’s disease. They get worse with disease progression, the researchers noted.
Losing the ability to smell can have a substantial impact on lifestyle and wellbeing, said Pinto.
“Smells influence nutrition and mental health,” Pinto said. “People who can’t smell face everyday problems, such as knowing whether food is spoiled, detecting smoke during a fire, or assessing the need for a shower after a workout. Being unable to smell is closely associated with depression as people don’t get as much pleasure in life.”
“This evolutionarily ancient special sense may signal a key mechanism that also underlies human cognition,” noted study co-author Martha K. McClintock, Ph.D., a professor of psychology at the University of Chicago.
She noted that the olfactory system also has stem cells which self-regenerate, so “a decrease in the ability to smell may signal a decrease in the brain’s ability to rebuild key components that are declining with age, leading to the pathological changes of many different dementias.”
“Our test simply marks someone for closer attention,” Pinto added. “Much more work would need to be done to make it a clinical test. But it could help find people who are at risk. Then we could enroll them in early-stage prevention trials.”
“Of all human senses,” Pinto added, “smell is the most undervalued and underappreciated — until it’s gone.”
Source: University of Chicago Medical Center
Photo: For both studies, the researchers used a well-validated tool, known as ‘Sniffin’Sticks.’ Credit: Rob Kozloff, for the University of Chicago Medicine.