A medical study published in JAMA Otolaryngology-Head & Neck Surgery revealed that a diminished sense of smell (i.e., a person feeling that they cannot smell well) is strongly and consistently associated with depression.
As for objective olfactory impairment (measured by testing devices), its association with depression is less clear, but it remains strong in older adults over 65 and those with complete loss of smell.
Dr. Henry Bode of the Karolinska Institute in Stockholm and his colleagues conducted a comprehensive review of previous scientific studies to assess the relationship between two types of olfactory impairment and depression. The first type is subjective olfactory impairment, which is a condition a person experiences spontaneously without medical measurement. The second type is objective olfactory impairment, which is detected through specialized medical tests and equipment.
The researchers relied on an analysis of data from 7 independent studies to examine the relationship between impaired subjective smell and depression, and 11 studies to examine the relationship between impaired objective smell and depression.
These studies included diverse age groups and used different measures to diagnose depression (whether the depression was self-reported by the participants or medically diagnosed in objective ways).
The study's findings were interesting. Regarding autosomal impairment, meaning a person's feeling of having a diminished sense of smell, the researchers found that these individuals were 1.19 to 2.05 times more likely to suffer from depression compared to those without this complaint, with an average odds ratio of 1.56. Even more interestingly, this association did not weaken when the analysis was limited to the most accurate and least biased studies; rather, the ratio increased to 1.71, reinforcing confidence that this relationship is real and not just a statistical coincidence.
Most importantly, this association remained consistent across all age groups, regardless of how depression was diagnosed, whether based on the patient's own feelings or through objective medical evaluation.
As for objective olfactory impairment, measured by specialized testing devices, the 11 studies showed a correlation of 1.26 with depression, meaning that individuals with objective olfactory impairment have a 26% higher likelihood of developing depression compared to those without. However, surprisingly, this correlation lost its statistical significance after adjusting the data for publication bias, the tendency of published studies to show more positive than negative results. After adjustment, the correlation dropped to 1.12 with a confidence interval of 1, meaning the result is no longer statistically significant and suggests that the relationship between objective olfactory impairment and depression may be less strong than initially appeared.
Despite the overall decline in the strength of the correlation, the relationship between impaired subjective smell and depression remained strong and clear in two specific categories:
• Elderly people over 65 years of age.
• People with a complete loss of the sense of smell (anosmia).
The findings suggest that a loss of smell, especially when experienced by the individual themselves, may be a warning sign of depression, as the sense of smell is intertwined with emotions and psychological processes in the brain. However, the researchers emphasize that current evidence is still insufficient to provide clear medical recommendations, such as routine smell tests for depression or considering a diminished sense of smell as a reliable diagnostic indicator.
The study concludes that the link between the sense of smell and mental health is real and significant, especially when a person experiences a decline in their ability to smell. However, more research is still needed before this link can form the basis of formal medical recommendations. Nevertheless, these findings open an important door to a better understanding of how our senses are connected to our mental state and may help clinicians in the future improve the diagnosis and treatment of depression by paying attention to patients' complaints related to their sense of smell.
