Sense of Smell Fades Earlier in People With Alzheimer's Gene

The results of a new study suggest that people with a gene variant linked to an increased risk of Alzheimer's Disease may lose their ability to detect odors at a younger age than those without the variant.

Determining if a person is more likely to develop Alzheimer's disease or dementia through simple testing could help healthcare providers start interventions earlier and perhaps reduce the impact of the condition.

Though scientists have identified gene variants associated with Alzheimer's, there are still no reliable tests that can predict a person's risk of future cognitive problems.

Now, in new research involving over 865 participants, scientists found that people who carry the Alzheimer's gene variant APOE e4 may lose their ability to detect odors earlier than those who do not have the variant.

In the study, published on July 26 in Neurology, the medical journal of the American Academy of Neurology, researchers investigated the association between APOE e4 and reduced odor sensitivity and identification and whether any loss of smell predicted cognitive decline.

To conduct the research, scientists had the participants complete in-home tests that assessed the ability to detect or identify scents in 2005, 2010, and 2015.

The team also measured the participants' cognitive abilities in 2010 and 2015.

In addition, the researchers obtained DNA samples from the participants to determine who carried the Alzheimer's gene variant.

After analyzing the data and adjusting for factors that could potentially skew the results, the scientists found that, at a single timepoint, individuals carrying the gene variant were 37% less likely to have an adequate scent-detecting ability than individuals without the variant.

Moreover, people with the gene variant began losing their sense of smell around ages 65 to 69, while non-carriers didn't experience odor sensitivity deficits until ages 75 to 79.

Still, odor sensitivity did not decline more rapidly in those with APOE e4 compared to individuals without the gene variant.

However, as they aged, gene carriers experienced a more rapid decline in their ability to identify the odors they smelled than non-carriers.

In addition, both groups' initial thinking and memory skills were similar at the beginning of the study. Nonetheless, as time passed, those carrying the gene variant exhibited more pronounced declines in their cognitive functions.

"Testing a person's ability to detect odors may be a useful way to predict future problems with cognition," says study author Matthew S. GoodSmith, MD, of the University of Chicago, in a news release.

"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," GoodSmith says.

Notably, the study did not include people with severe dementia. Still, the results could help identify mechanisms behind cognitive decline in people with Alzheimer's gene variants and the role smell loss plays in neurodegeneration.