BALTIMORE — Hyposmia, or a reduced sense of smell, has been linked to Alzheimer’s disease and other forms of dementia in the past. Now, new research by a team at Johns Hopkins Medicine finds that a poor sense of smell is also associated with depression in older individuals. The project tracked over 2,000 community-dwelling older adults over the course of eight years. Study authors collected a significant amount of compelling evidence pointing to a link between decreased sense of smell and risk of developing late-life depression.
To be clear, the research team stresses that these findings do not demonstrate that loss of smell causes depression. This study simply suggests that hyposmia may serve as a potent indicator of overall health and mental well-being.
“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., an associate professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine, in a media release. “Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation.”
Researchers made use of data originally gathered from 2,125 people who participated in a federal government study called the Health, Aging and Body Composition Study (Health ABC). That group included healthy older adults (ages 70-73) at the time the eight-year study period began in 1997. Participants generally showed no difficulties in walking 0.25 miles, climbing 10 steps, or performing normal activities at the start of the study. Researchers also assessed these seniors in-person on an annual basis and over the phone every six months. Tests conducted included exams gauging their ability to detect certain odors, depression, and mobility assessments.
The team then measured smell starting in 1999. At that time, 58 percent of the group had a normal sense of smell, 28 percent showed a decreased sense of smell (hyposmia), and another 24 percent displayed a profound loss of the sense (anosmia). Those with a stronger sense of smell were usually younger than others reporting significant smell loss or hyposmia.
During the follow-up period, a quarter (25%) developed significant depressive symptoms. Upon further analysis, study authors discovered that participants with decreased or significant loss of smell also had an increased risk of developing significant depressive symptoms during follow-up visits in comparison to others in the normal smelling group.
The research team also worked to identify three depressive symptom “trajectories” prevalent among the study group: stable low, stable moderate, and stable high depressive symptoms. A poorer sense of smell displayed a link to an increased chance of an older adult falling into either the moderate or high depressive symptoms group. In other words, the worse a person’s sense of smell gets, the more their depressive symptoms grow. These results remained even after researchers adjusted for age, income, lifestyle, health factors, and any use of antidepressant medications.
“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,” Prof. Kamath explains. “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.”
Our sense of smell is one of two human chemical senses. It works via specialized sensory cells (olfactory neurons) that reside in the nose. Those neurons feature just one odor receptor, which picks up molecules released by substances around us, that are then relayed to the brain for interpretation. The higher the concentration of smell molecules, the stronger the smell. Different mixtures of molecules produce different sensations.
The brain’s olfactory bulb processes smells, and that structure is believed to interact closely with the amygdala, hippocampus, and other brain structures involved in the regulation of memory, decision-making, and emotional responses. The research team at Johns Hopkins say their findings indicate that both olfaction and depression may be linked by both biological (altered serotonin levels, brain volume changes) and behavioral (reduced social function and appetite) mechanisms.
Moving forward, study authors plan to replicate their findings using larger cohorts of older adults, and examine changes to individuals’ olfactory bulbs. This approach will hopefully determine if the olfactory system is indeed altered among those diagnosed with depression. Researchers also want to assess if smells can perhaps be used in intervention strategies aimed at mitigating the risk of late-life depression.
The study is published in the Journal of Gerontology: Medical Sciences.
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