This study was conducted by researchers from Michigan State University over a 13-year period. Their study involved 2,289 adult men and women who were tasked to complete a smell test of 12 common odors. Depending on the results, the researchers rated the participants as having good, moderate or a poor sense of smell.
By the 13th year of the study, nearly half of the participants (1,211) had already died. Participants who were rated with poor olfactory functions had a 46 percent higher cumulative risk for death by the 10th year of the study and a 30 percent higher risk by year 13.
Sex, race and many other lifestyle and demographic factors didn't affect the results of this research. However, the scientists did find that many participants who were healthy at the start of the 13-year long study were found to be largely responsible for the higher risks.
Poor sense of smell can be an indicator of underlying concerns
The researchers, whose findings were published in the journal Annals of Internal Medicine, said that a poor sense of smell is already a possible early indicator of weight loss, dementia and Parkinson's disease. However, they stipulated that this only explained 28 percent of the increased mortality risk – 22 percent for neurodegenerative diseases and six percent for weight loss – leaving most of the uptick unexplained.
"We don't have a reason for more than 70 percent of the increased risk," said Honglei Chen, an epidemiologist and one of the researchers. "We need to find out what happened to these individuals." Chen said they plan to pursue this mystery in the future with more studies. "It tells us that in older adults, impaired sense of smell has broader implications of health beyond what we have already known."
Chen further stated that including a screening test for sense of smell should become routine for checkups, as it may give health practitioners a better understanding of the condition of their patients.
Robert Howard, an expert in old age psychiatry at the University College London, remarking on the study, concurred with Chen's conclusion that loss of smell can be a marker for generalized aging and that olfactory deterioration should be taken seriously both by older people and their attending health practitioners.
However, Howard also said that "the trouble with any observational research like this is that it's impossible to be sure about what's causing what." This echoes Chen's own sentiment about how more than 70 percent of the increased risk in deteriorating sense of smell is still unaccounted for.
Howard further remarked that there are many differences between people with a poor sense of smell and those whose olfactory systems were working perfectly and that this may not be the only factor influencing the higher mortality rates in the former. This means there's a possibility that a poor sense of smell isn't the cause of higher death rates at all.
He believes there are two possibilities at play here. First is that a poor sense of smell is a factor and that it is an early sign of an underlying illness that contributes to the mortality rate. Second is that having a poor sense of smell is what leads to illnesses, "perhaps because poor smell affects how food tastes, and therefore might contribute to poor nutrition which could lead to bad health." (Related: Humans' sense of smell is actually just as strong as dogs, but people dull their senses with carcinogenic artificial fragrance.)
Whatever the case may be, both Chen and Howard seem to agree that more research needs to be done to investigate just how big or small of a role a poor sense of smell plays in elderly mortality rates.
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