Does a Diminished Sense of Smell Indicate Risk of Late-Life Depression?

Researchers at Johns Hopkins Medicine say they have significant new evidence of a relationship between a diminished sense of smell and the risk of developing late-life depression in a study that followed over 2,000 community-dwelling older persons for eight years. Their findings, published in the Journal of Gerontology: Medical Sciences, do not prove that loss of smell causes depression, but they do show that it may be a powerful indication of overall health and well-being (1^✔ ✔Trusted Source
Olfactory Dysfunction Predicts the Development of Depression in Older US Adults

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Link Between Impaired Sense of Smell and Neurodegenerative Diseases

“We’ve seen repeatedly that a poor sense of smell can be an early warning sign of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease, as well as a mortality risk. This study underscores its association with depressive symptoms,” says Vidya Kamath, Ph.D., associate professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine. “Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation.”

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Impaired Sense of Smell Linked to Higher Risk of Depressive Symptoms

When the scent was originally assessed in 1999, 48% of participants had a normal sense of smell, 28% had a diminished sense of smell (hyposmia), and 24% had a profound loss of sense (anosmia). Participants who reported considerable loss or hyposmia tended to be younger than those who reported major loss or hyposmia. Throughout the study, 25% of patients developed major depression symptoms.

Researchers discovered that persons with diminished or major loss of smell had a higher probability of acquiring significant depressive symptoms at the end of the study than those with normal olfaction. Participants who reported considerable loss or hyposomia tended to be younger than those who reported major loss or hyposomia.

In the study group, researchers also found three depressed symptoms "trajectories": stable low, stable moderate, and stable high depressive symptoms. A poor sense of smell was linked to an increased likelihood of a participant having moderate or severe depressive symptoms, implying that the worse a person's sense of smell, the greater their depression symptoms. These findings remained after controlling for age, income, lifestyle, health conditions, and antidepressant medication use.

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Connection Between Sense of Smell and Overall Health

“Losing your sense of smell influences many aspects of our health and behavior, such as sensing spoiled food or noxious gas, and eating enjoyment. Now we can see that it may also be an important vulnerability indicator of something in your health gone awry,” says Kamath. “Smell is an important way to engage with the world around us, and this study shows it may be a warning sign for late-life depression.”

The smell is one of two chemical senses in humans. It operates via olfactory neurons, which are specialized sensory cells located in the nose. These neurons only have one odor receptor; it detects chemicals generated by substances around us and relays them to the brain for interpretation. The stronger the smell, the higher the concentration of these smell molecules and different combinations of molecules results in diverse experiences.

The olfactory bulb in the brain processes smell, and it is thought to interact closely with the amygdala, hippocampus, and other brain regions that govern and permit memory, decision-making, and emotional reactions.

According to Johns Hopkins researchers, olfaction and depression may be connected through both biological (e.g., changed serotonin levels, changes in brain volume) and behavioral (e.g., impaired social function and appetite) factors.

The researchers intend to reproduce their findings in larger groups of older folks, as well as evaluate alterations in individuals' olfactory bulbs to see if this system is altered in those diagnosed with depression. They also intend to investigate if smell may be employed in intervention efforts to reduce the incidence of late-life depression.

Reference:

1. Olfactory Dysfunction Predicts the Development of Depression in Older US Adults - (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7909297/)

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