They say you should trust your gut, but maybe it’s really the nose that knows. Turns out, the olfactory organ we all take for granted can do more than let you know when it’s time to take out the garbage — it can sniff out death, too. In fact, according to new research released this week, severe loss of one’s sense of smell is a stronger indicator of impending death than a diagnosis of heart failure or cancer.
In the study, researchers at the University of Chicago asked more than 3,000 people perform a smell test and then followed up with them five years later to see who was still living. Subjects were given sets of four scents and asked to sniff out recognizable everyday odors, including peppermint, orange, rose, leather and fish. At a five-year follow-up, one in eight subjects had died. The greatest predictor of death? Smell.
In all, nearly 40 percent of those who had failed the smell test died, while 19 percent of those who showed signs of moderate loss of smell died, and just 10 percent of those who had a healthy sense of smell died. In addition, when adjusting for other risk factors like age, socioeconomic status and overall health, the group who had lost their sense of smell had an even higher likelihood of dying within a five-year span.
“Olfaction is a critical, if underappreciated, component of human physiology,” write the authors, led by UC Professor Dr. Jayant Pinto. “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 team points 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 potential pathology, and while it may seem an unusual sort of medical test, smell tests have been used in other diagnostic ways as well. For instance, a 2013 study found that sniffing peanut butter through one nostril at a time could be used to predict the future onset of Alzheimer’s. And just earlier this year, researchers tested a device they say could detect lung cancer, just by smelling a person’s breath.
Due to its links to diverse physiological processes, the authors behind the new study hypothesized that it could also potentially be an indicator of mortality. The new analysis, published in the journal PLoS ONE, is part of the National Social Life, Health and Aging Project, a nationally representative longitudinal investigation of the health and well-being of older adults.
Smell tests
From 2005-2006, the researchers interviewed 3,005 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, at which point 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, the researchers found. And after adjusting for confounding variables such as race, age and socioeconomic status, the analysis revealed 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 eithe94r 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.