A loss of smell is a surprising early warning sign of Alzheimer’s disease, a new study suggests.
Researchers found that people with the APOE e4 gene, the same one Chris Hemsworth revealed he had two copies of last year, were more likely to lose their sense of smell and suffer cognitive decline than those without the gene.
‘Testing a person’s ability to detect odors may be a useful way to predict future problems with cognition,’ study author Dr Matthew S. GoodSmith of the University of Chicago said.
Previous research has shown that loss of smell can create inflammation in the brain – which is linked to cognitive decline.
The gene that most significantly raises the risk of dementia is apolipoprotein E (APOE), which carries cholesterol and other fats in the bloodstream.
A new study in the journal Neurology found that people with the gene APOE e4, which is linked to Alzheimer’s, were 37 percent more likely to have worse odor detection than those without it
APOE comes in several different forms, or alleles: APOE e2, APOE e3, and APOE e4.
Each person inherits two APOE genes, one from each biological parent.
While some variants aren’t associated with dementia, APOE e4 is most associated with an increased risk of the dementia subset Alzheimer’s disease.
The risk is even greater for those who have two copies of the gene. One in four people carry one copy of APOE e4, but only two to three percent have two copies.
The study, published Wednesday in the journal Neurology, involved 865 participants taking an at-home survey to evaluate their sense of smell, including their ability to detect an odor at all and their ability to identify what odor they were smelling.
These tests were given every five years. Memory and thinking skills were also evaluated twice, five years apart.
The researchers also took samples from participants to see who had genes associated with a higher Alzheimer’s risk.
In the odor detection test, participants were asked to score themselves on a scale from zero to six based on how many smells they could connect.
Participants with at least one copy of APOE e4 were 37 percent more likely to have worse odor detection than those who didn’t have the gene. Gene carriers who started experiencing this diminished sense of smell were between ages 65 and 69.
At that age, they detected an average of 3.2 smells, compared to 3.9 smells in those without the gene.
Participants between ages 75 and 79 with the gene started showing an inability to figure out what they were smelling. Once they lost that ability, it declined quicker in gene carriers than non-gene carriers.
Those with the gene suffered greater cognitive losses than those without the gene.
This could be due to loss of smell creating inflammation in the brain, which has long been associated with cognitive decline.
‘While more research is needed to confirm these findings and determine what level of smell loss would predict future risk, these results could be promising, especially in studies aiming to identify people at risk for dementia early in the disease,’ Dr GoodSmith said.
This builds on previous research which has linked smell to cognitive impairment.
A study published last year in the Journal of Alzheimer’s disease, for example, found that in similar testing, each point of lower odor identification test performance was associated with a 22 percent higher chance of developing mild cognitive impairment, which could lead to Alzheimer’s.
The researchers in the Neurology study noted that a limitation of the study is that patients with severe dementia were not included.
Dementia is an umbrella term used to describe a range of progressive neurological disorders (those affecting the brain) which impact memory, thinking and behavior.
Alzheimer’s disease is the most common form of dementia, accounting for 60 to 70 percent of all cases, according to the World Health Organization (WHO).
Common symptoms include memory loss, poor judgment, confusion, repeating questions, difficulty communicating, taking longer to complete normal daily tasks, acting impulsively, and mobility issues.
The study was funded in part by the National Institutes of Health, National Institute on Aging, and National Institute of Diabetes and Digestive and Kidney Diseases.
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