In a strange piece of science, researchers have discovered a connection between having a diminished sense of smell and an increased risk of developing late-life depression.
While the study does not confirm that the loss of smell directly causes depression, it suggests that it could serve as a powerful indicator of overall health and wellbeing.
A poor sense of smell has long been associated with neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease.
The study, published in the Journal of Gerontology: Medical Sciences, used data from the Health, Aging and Body Composition Study (Health ABC), a US federal government study that followed 2125 community-dwelling older adults over an eight-year period.
The participants, aged 70-73 at the start of the study in 1997-98, underwent regular assessments for their odour detection ability, depression symptoms, and mobility.
The results of the assessments showed 48 per cent of participants had a normal sense of smell, 28 per cent experienced a decreased sense of smell (hyposmia), and 24 per cent could be classed as having a profound loss of smell (anosmia).
Over the eight-year follow-up period, 25 per cent of participants developed significant depressive symptoms and further analysis showed that it was the individuals with decreased or significant loss of smell who were at a higher risk of developing depressive symptoms compared to those with normal smelling ability.
The association remained even after adjusting for various factors such as age, income, lifestyle, health, and antidepressant medication use.
“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 Professor Vidya Kamath, lead author of the study.
“Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation.”
The research 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 linked to an increased likelihood of falling into the moderate or high depressive symptom groups. In other words, 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 behaviour, such as sensing spoiled food or noxious gas, and eating enjoyment,” Prof. Kamath says.
“Now we can see that it may also be an important vulnerability indicator of something in your health gone awry. 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.”
As to why a lack of smelling ability is connected to depression in later life, the research team has some theories.
The human sense of smell is one of the two ‘chemical senses’ and relies on specialised sensory cells called olfactory neurons located in the nose.
These neurons contain odour receptors that detect molecules released by substances in our surroundings. The brain then interprets these signals, with different combinations of molecules resulting in different smells.
From there, the olfactory bulb in the brain processes smell and interacts closely with structures responsible for memory, decision-making, and emotional responses. So, in this case, a lack of smell is denying the brain a whole range external information the brain needs to properly classify its surroundings.
This study sheds light on the significance of the sense of smell in older adults and its potential as an early warning sign for late-life depression.
Further research is needed to fully understand the underlying mechanisms and develop interventions that can help mitigate the risk of depression in this population.
How is your sense of smell these days? Has it changed as you’ve aged? Let us know in the comments section below.