People can temporarily and partially lose their sense of smell (hyposmia) or completely lose it (anosmia) due to a cold or flu, COVID-19 or sinusitis infection, an allergy like hay fever or nasal polyps. They can also start smelling things that aren’t there (phantosmia) like smoke or burnt toast.
But as they get older, many people permanently lose their ability to smell, and this reduces their pleasure from food and enjoyable surroundings.
In an eight-year study that followed more than 2,000 older adults living in geriatric facilities, researchers at Johns Hopkins Medicine in Baltimore say they have significant new evidence of a link between decreased sense of smell and risk of developing late-life depression.
Their findings, published June 26 in the Journal of Gerontology: Medical Sciences under the title “Olfactory Dysfunction and Depression Trajectories in Community-Dwelling Older Adults,” do not claim that loss of smell causes depression – but that it may serve as a potent indicator of overall health and 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,” said psychiatry and behavioral sciences Prof. Vidya Kamath at his university’s School of Medicine. “Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation.”
The study's sample and methods
The study used data gathered from 2,125 healthy participants aged 70 to 73 in a government study known as the Health, Aging and Body Composition Study. Participants showed no difficulties in walking half a kilometer, climbing 10 steps or performing normal activities at the start of the study. They were assessed in person every year and by phone every six months; tests included those for the ability to detect certain odors, depression and mobility assessments.
In 1999, when smell was first measured, 48% of participants displayed a normal sense of smell, 28% showed a decreased sense of smell and 24% had anosmia. Participants with a better sense of smell tended to be younger than those reporting significant loss or hyposmia.
The follow-up showed that a quarter of participants developed significant depressive symptoms. When analyzed further, researchers found that individuals with decreased or significant loss of smell had an increased risk of developing significant depressive symptoms during the long term than those in the normal olfaction group.
Researchers also identified three depressive symptom trajectories in the study group – stable-low, stable-moderate and stable-high depressive symptoms. A poorer sense of smell was associated with an increased chance of a participant falling into the moderate or high depressive symptoms groups, meaning that the worse a person’s sense of smell, the higher their depressive symptoms.
“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,” commented 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.”
Humans’ sense of smell works through specialized sensory cells called olfactory neurons, which are found in the nose. These neurons have one odor receptor; it picks up molecules released by substances around us, which are then relayed to the brain for interpretation. The higher the concentration of these smell molecules the stronger the smell, and different combinations of molecules result in different sensations.
Smell is processed in the brain’s olfactory bulb, which is believed to interact closely with the amygdala, hippocampus and other brain structures that regulate and enable memory, decision-making and emotional responses.
The researchers concluded that olfaction and depression may be linked through both biological (altered serotonin levels, brain volume changes) and behavioral (reduced social function and appetite) mechanisms. They plan to replicate their findings from this study in more groups of older adults and study changes to individuals’ olfactory bulbs to find out if this system is in fact changed in those diagnosed with depression. They also want to know if smell can be used in intervention strategies to mitigate risk of late-life depression.