Researchers studying olfactory dysfunction and aging in thousands of people say that for older adults, being unable to identify scents may be a predictor of mortality within five years. These startling results are published in PLOS ONE this week.
“We think loss of the sense of smell is like the canary in the coal mine,” University of Chicago’s Jayant Pinto says in a news release. “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.”
As part of the National Social Life, Health and Aging Project (NSHAP), Pinto and colleagues surveyed 3,005 men and women ages 57 to 85 in 2005 through 2006. They assessed their ability to identify five distinct common odors -- using odor-dispensing devices called Sniffin’ Sticks (pictured right) -- one smell at a time from a set of four choices. Here they are in order of increasing difficulty: peppermint, fish, orange, rose, and leather. Almost three-fourths of the participants were able to identify at least four of the five correctly.
The team checked in on the participants with a second survey during 2010 and 2011. During that five-year gap, 430 of the original 3,005 study subjects had died.
Of the participants who failed the first smelling test, 39 percent had died before the second survey -- compared with 19 percent of those with moderate smell loss and 10 percent with a healthy sense of smell. For those already at high risk, lacking a sense of smell more than doubled the probability of death. This was more or less the case even after the researchers adjusted for age, gender, socioeconomic status, overall health, and race. And as expected, performance on the scent test declined steadily with age.
However, it’s unclear precisely how smell loss contributes to mortality. “Obviously, people don’t die just because their olfactory system is damaged,” says study author Martha McClintock from University of Chicago. Although, olfactory dysfunction was better at predicting mortality than a diagnosis of heart failure, cancer, or lung disease.
Although they didn’t investigate the link specifically, the team does have several ideas about why smell loss signals mortality. The olfactory nerve (the only cranial nerve directly exposed to the environment) may serve as a conduit, exposing the central nervous system to pollution, airborne toxins, and pathogens. Additionally, the olfactory system also has stem cells that regenerate, so a decreased ability to identify odors may indicate the body's inability to rebuild key components that decline with age.
“Of all human senses,” Pinto says, “smell is the most undervalued and underappreciated -- until it’s gone.” They hope the findings will lead to a quick and inexpensive way to identify patients most at risk.