The inability of older adults to identify scents is a strong predictor of death within five years, according to a recent study.

Researchers from the University of Chicago Medical Center found that olfactory dysfunction was better at predicting mortality than a diagnosis of heart failure, cancer or lung disease. Only severe liver damage was a more powerful predictor of death. For those already at high risk, lacking a sense of smell more than doubled the probability of death.

­ "We think loss of the sense of smell is like the canary in the coal mine," Jayant M. Pinto, lead author of the study, said in a statement. "It doesn't directly cause death, but it's a harbinger, an early warning that something has 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."

The study was part of the National Social Life, Health and Aging Project (NSHAP), the first in-home study of social relationships and health in a large, nationally representative sample of men and women ages 57 to 85.

In the first wave of NSHAP, conducted in 2005-06, professional survey teams from the National Opinion Research Center at the University of Chicago used a well-validated test -- adapted by Martha K. McClintock, the study's senior author -- for this field survey of 3,005 participants. It measured their ability to identify five distinct common odors.

The modified smell tests used "Sniffin'Sticks," odor-dispensing devices that resemble a felt-tip pen but are loaded with aromas rather than ink. Subjects were asked to identify each smell, one at a time, from a set of four choices. The five odors, in order of increasing difficulty, were peppermint, fish, orange, rose and leather.

In the second wave, during 2010-11, the survey team carefully confirmed which participants were still alive. During that five-year gap, 430 (12.5 percent) of the original 3,005 study subjects had died; 2,565 were still alive.

When the researchers adjusted for demographic variables such as age, gender, socioeconomic status (as measured by education or assets), overall health, and race, those with greater smell loss when first tested were substantially more likely to have died five years later. Even mild smell loss was associated with greater risk.

"This evolutionarily ancient special sense may signal a key mechanism that affects human longevity," noted McClintock.

Age-related smell loss can have a substantial impact on lifestyle and wellbeing. As expected, performance on the scent test declined steadily with age; 64 percent of 57-year-olds correctly identified all five smells. That fell to 25 percent of 85-year-olds.

"Smells impact how foods taste. Many people with smell deficits lose the joy of eating. They make poor food choices, get less nutrition," Pinto said. "They can't tell when foods have spoiled or detect odors that signal danger, like a gas leak or smoke. They may not notice lapses in personal hygiene."

Precisely how smell loss contributes to mortality is unclear.

The research team, which includes biopsychologists, physicians, sociologists and statisticians, is considering several hypotheses.

The findings were published in the journal PLOS ONE.