Decreased sense of smell may be linked to an increased risk of depression later in life. The corresponding study was published in The Journals of Gerontology.
"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 study author Vidya Kamath, Ph.D., associate professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine, in a press release.
"Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation,” she added.
For the study, the researchers analyzed healthcare data from 2125 adults who were aged between 70 and 73 years years old at baseline. Data included cognitive assessments, depressive symptoms, and measures of inflammatory markers collected over eight years. Sense of smell was also assessed at the start of the study.
The researchers found that 48% of participants had a normal sense of smell at the beginning of the study. Meanwhile, 28% had a decreased sense of smell, whereas 24% had a profound loss of smell, known as anosmia. They noted that participants with a stronger sense of smell tended to be younger than those reporting olfactory loss.
Over the follow-up period, the researchers noted that 25% of participants developed significant depressive symptoms and that those with decreased or significant loss of smell were at more risk of developing depressive symptoms than those with normal olfaction.
In particular, those with poorer olfaction were 6% more likely to have moderate or 'high' levels of depressive symptoms than fewer symptoms. The findings remained after adjusting for age, income, lifestyle, and health factors, including the use of antidepressants.
Sense of smell is processed by the olfactory bulb, which closely interacts with areas of the brain linked to emotional response, decision-making, and memory, such as the amygdala and hippocampus. The researchers noted that sense of smell and depression may be linked to such biological mechanisms but that behavioral mechanisms may also be involved.
Going forward, the researchers hope to investigate whether their findings can be replicated in more groups of older adults, and to examine whether the olfactory bulb is altered in those with depression. They also aim to see whether smell could be used in preventative strategies for late-life depression.
Sources: Science Daily, The Journals of Gerontology