A loss of smell is a surprisingly 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 people without the gene.
“Testing a person’s ability to detect odors may be a useful way to predict future problems with cognition,” said study co-author Dr. Matthew S. GoodSmith of the University of Chicago.
Previous research has shown that loss of smell can cause inflammation in the brain – which is linked to cognitive decline.
The gene that most strongly increases the risk of dementia is apolipoprotein E (APOE), which transports cholesterol and other fats in the bloodstream.
A new study in the journal Neurology found that people with the APOE e4 gene, which has been linked to Alzheimer’s disease, were 37 percent more likely to have poorer odor detection than those without the gene.
APOE comes in several forms or alleles: APOE e2, APOE e3, and APOE e4.
Each person inherits two APOE genes, one from each biological parent.
While some variants are not associated with dementia, APOE e4 is most commonly associated with an increased risk of the dementia subset of Alzheimer’s disease.
The risk is even greater for those who have two copies of the gene. One in four people carries one copy of APOE e4, but only two to three percent carry two copies.
In the study, published Wednesday in the journal Neurology, 865 participants took a survey at home to evaluate their sense of smell, including their ability to detect an odor at all and their ability to identify which odor they smelled.
These tests were administered 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 risk of Alzheimer’s.
In the odor detection test, participants were asked to score themselves on a scale of zero to six based on the number of odors they could connect.
Participants with at least one copy of APOE e4 were 37 percent more likely to have poorer odor detection than those who lacked the gene. Gene carriers who began to experience this decreased sense of smell were between the ages of 65 and 69.
At that age, they detected an average of 3.2 odors, compared to 3.9 odors in people without the gene.
Participants between the ages of 75 and 79 with the gene began to show an inability to figure out what they were smoking. Once they lost that ability, it declined faster in gene carriers than in non-gene carriers.
Those with the gene suffered greater cognitive losses than those without the gene.
This may be due to loss of smell causing inflammation in the brain, which has long been linked to cognitive decline.
“While more research is needed to confirm these findings and determine what level of smell loss would predict future risks, these results could hold promise, especially in studies aimed at identifying people at risk for dementia in early childhood. stage of the disease,” said Dr. GoodSmith.
This builds on previous research that has linked smell to cognitive impairment.
A study published last year in the Journal of Alzheimer’s diseasefound, for example, that on similar tests, 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 disease.
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 conditions (affecting the brain) that affect 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, repetitive questions, difficulty communicating, taking longer to perform normal daily tasks, acting impulsively, and mobility problems.
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.