A decreased sense of smell can increase the risk of developing depression in the elderly, a recent study has found.

Hyposomia is a condition in which a person has decreased or complete loss of smell. It can occur due to allergies, infections, smoking, hormonal imbalance, use of recreational drugs and head injury. Hyposomia is also considered a sign of neurological conditions such as Parkinson's disease, multiple sclerosis (MS) and Alzheimer's disease.

In the latest study, researchers evaluated the association between the loss of smell in the elderly and the development of depression later in life.

"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," said Vidya Kamath, the lead author of the study from Johns Hopkins University School of Medicine in Baltimore.

"Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation," she added.

The team used data from another study – the Health, Aging and Body Composition Study (Health ABC) – and evaluated more than 2,100 community-dwelling older adults over eight years. At the beginning of the study in 1997, the participants who were aged between 70 to 73 could perform normal physical activities such as walking and climbing steps.

Researchers followed them up every year and their health conditions were screened. After two years, 28% of the participants showed a decreased sense of smell and 24% had a profound loss of smell. During the follow-up period, around 25% of participants developed significant depressive symptoms.

The researchers observed that those with decreased or significant loss of smell had an increased risk of developing significant depressive symptoms.

Olfaction and depression may be linked through both biological and behavioral routes, the researchers said. The olfactory bulb responsible for processing the sense of smell in the brain interacts closely with the brain structures responsible for regulating memory, decision-making and emotional responses.

"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. The 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," Kamath said.

The researchers plan to investigate if smell can be used to help treat depression later in life.