The olfactory system is involved in a wide variety of physiological processes, ranging from detecting environmental hazards to triggering memories.
"Olfaction is a critical, if underappreciated, component of human physiology," write the authors, led by Dr. Jayant Pinto and based out of the University of Chicago, IL. "Although potentially less dependent on olfaction than many other mammals, humans still rely on this ancestral system which plays an essential role in health and behavior."
They point out that our ability to distinguish scents performs a wide variety of tasks. It influences nutrition through appetite and food preferences, enables us to identify environmental hazards and is inextricably linked with memory and social relationships.
Indeed, a failing olfactory system has already been observed to signal major degenerative diseases such as Alzheimer's disease and Parkinson's disease. Due to its links to diverse physiological processes, the authors hypothesized that it could also potentially be an indicator of mortality.
The study, published in the journal PLoS ONE, was part of the National Social Life, Health and Aging Project (NSHAP). This project is the first in-home study of social relationships and health within a nationally representative sample of participants aged between 57-85.
From 2005-2006, the researchers interviewed 3,005 of the participants (1,454 men and 1,551 women) and assessed their ability to correctly identify five common odors. They were presented with the five scents - peppermint, fish, orange, rose and leather - one at a time and had to choose from four possible answers.
Participants were considered to have failed the smelling test (anosmic) with four to five errors, to have moderate smell loss (hyposmic) with two to three errors and to have a healthy sense of smell (normosmic) with one or no errors.
A second round of interviews was conducted from 2010-2011, and the researchers recorded which of the subjects were still alive. In the 5-year gap between surveys, 430 (12.5%) of the original participants had died, leaving 2,565 still living.
Of the participants who had failed the first smelling test, 39% had died before the follow-up survey 5 years later. In contrast, 19% of participants with moderate smell loss and 10% of those with a good sense of smell died during the same period.
Subjects who were identified as being at a high risk of mortality had their probability of death seemingly doubled by lacking a sense of smell.
After adjusting for variables such as race, age and socioeconomic status, the researchers found that the subjects with the greatest smell loss when first tested were significantly more likely to have died 5 years later.
'The canary in the coal mine'
The researchers are not certain in what way, if any, the loss of smell contributes to the risk of mortality, but they found that during the study it was a better predictor of death than cancer, heart failure and lung disease. Only severe liver damage was a more effective predictor.
The authors acknowledge that their study has its limitations. The five-item smell test could have been more extensive. The home setting of the interviews meant that additional clinical assessment was unable to be conducted, and the causes of death were not recorded, which could have given further insight to these findings.
The authors believe that what makes the olfactory system so important is that it is reliant on stem cell turnover, meaning that it could be an indicator of deterioration in the body's capacity to regenerate throughout the aging process.
They also speculate that because the olfactory nerve is the only cranial nerve with direct exposure to the environment, it could be the gateway to the central nervous system for harmful toxins and pathogens. Further research will be necessary to find out whether either of these two explanations is behind their findings.
Dr. Pinto adds:
"We think loss of the sense of smell is like the canary in the coal mine. It doesn't directly cause death, but it's a harbinger, an early warning system, that something has already gone badly wrong, that damage has been done. Our findings could provide a useful clinical test, a quick and inexpensive way to identify patients most at risk."
Recently, Medical News Today reported on a study that suggested memory complaints could be an early indicator of future dementia risk.