A long-term study of nearly 3,000 older adults (57-85) has found that those who couldn’t identify at least four out of five common odors were more than twice as likely as those with a normal sense of smell to develop dementia within five years.
Of the participants, some 14% could name just three out of five, 5% could identify only two scents, 2% just one, and 1% couldn’t identify a single smell.
Five years after the smell test, almost all of the study subjects who were unable to name a single scent had been diagnosed with dementia, and nearly 80% of those who provided only one or two correct answers.
The test involved a well-validated tool known as "Sniffin'Sticks." The five odors, in order of increasing difficulty, are peppermint, fish, orange, rose and leather.
A study involving nearly 300 older adults (average age 63) who had a parent with Alzheimer’s has found that those with the most difficulty in identifying odors were those in whom Alzheimer's biomarkers were most evident.
Sense of smell was assessed using multiple choice scratch-and-sniff tests to identify scents as varied as bubble gum, gasoline or the smell of a lemon. A hundred of the participants had regular lumbar punctures to measure the Alzheimer's biomarkers in the cerebrospinal fluid.
A seven-year study involving a multi-ethnic (34% White, 30% African-American, 36% Hispanic) sample of 757 healthy older adults (average age 80.7) found that lower odor identification scores on UPSIT were significantly associated with both the transition to dementia and cognitive decline.
For each point lower that a person scored on the UPSIT, the risk of Alzheimer's increased by about 10%.
The report was reported at the Alzheimer's Association International Conference® 2014 in Copenhagen
Loss of smell sense linked to amyloid-beta protein
An animal study has shown that olfactory dysfunction occurs much earlier than cognitive dysfunction, and that this is related to the amyloid-beta protein. Although it’s been thought that this protein is expressed only in the central nervous system, the study detected direct expression of the protein in the olfactory epithelium, part of the peripheral nervous system. Moreover, the amyloid-beta protein had a fatal effect on olfactory nerve cells in the olfactory epithelium and directly induced the failure of olfactory function.
A less alarming explanation for why our sense of smell tends to decline in old age comes from a mouse study that found that fewer stem cells become olfactory cells in old age as they tend to remain in the stem cell pool and become less active.
Poor sense of smell linked to greater mortality risk
Following on from a previous study in which more than 2,200 older adults (71-82) undertook smell identification tests, investigation 13 years later found that a poor sense of smell was linked to a 46% greater risk of dying within 10 years compared with those ranked as having a good sense of smell. Poor sense of smell was particularly linked to death from dementia and Parkinson’s disease, with some signs that poor smell might also be linked to death from cardiovascular disease. There was no link between poor sense of smell and death from cancer or respiratory diseases. 22% of the overall increased risk of death among those with a poorer sense of smell was down to neurodegenerative diseases.
The link was only present among those who were in very good health at the start of the study.
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(2017). Differential spatial expression of peripheral olfactory neuron-derived BACE1 induces olfactory impairment by region-specific accumulation of β -amyloid oligomer.
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(2019). Relationship Between Poor Olfaction and Mortality Among Community-Dwelling Older Adults: A Cohort Study.
Annals of Internal Medicine. 170(10), 673.
(2018). Increasing Neural Stem Cell Division Asymmetry and Quiescence Are Predicted to Contribute to the Age-Related Decline in Neurogenesis.
Cell Reports. 25(12), 3231 - 3240.e8.