Is sense of smell linked to increased risk of depression?

A study conducted by researchers at the highly-prolific Johns Hopkins Medicine in Baltimore, which involved following the cases of more than 2,100 community-dwelling adults over an eight-year period, has unearthed new evidence of a significant link between decreased sense of smell, and the risk of developing depression in later life.

Professor Vidya Kamath, PhD, associate professor of psychiatry and behavioural sciences at Johns Hopkins University School of Medicine, commented on the nature of the study, saying: "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."

"Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation," Professor Kamath continued, respectively.

The study involved gathering data from 2,125 participants, all of which were healthy older adults aged between 70 and 73, in a federal government study known as the Health, Ageing and Body Composition (Health ABC). By using this data, researchers examined the activities of each of the participants which included walking, climbing and the performance of basic activities. Other tests also involved being able to detect certain odours, depression and mobility assessments.

When smell was first measured, 48 per cent of participants displayed a normal sense of smell, 28 per cent showed a decreased sense of smell, known as hyposmia, whilst 24 per cent experienced a profound loss of smell, known as anosmia. By adjusting for factors such as age, income, lifestyle, health factors and use of antidepressants, the researchers noticed that the participants with a stronger sense of smell tended to be younger than those who reported significant losses of smell.

Further analysis showed that approximately a quarter of participants developed symptoms of depression. When the researchers investigated this further, they found that individuals with a decreased or significant loss of smell had a higher risk of developing severe depressive symptoms, than those with a better sense of smell.

The researchers associated a poorer sense of smell with an increased chance of a participant falling into moderate to severe depressive states, signifying that the worse a person's sense of smell is, the stronger their depressive symptoms would be.

Professor Kamath said: "Losing your sense of smell influences many aspects of our health and behaviour, 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."

He continued: "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."

While the findings, which were published on June 26th in the Journal of Gerontology: Medical Sciences, do not directly demonstrate that a loss of smell causes depression, they do suggest that it may serve as a potential indicator of well-being and overall health.

The Johns Hopkins researchers commented that their study highly suggests olfaction and depression may be linked through both biological and behavioural mechanisms, before stating their plan to replicate their findings in more groups of older adults in order to determine if smell can be implemented in intervention strategies to mitigate the risks of depression in later life.