Does losing your sense of smell predict death risk?
"Sense of smell 'may predict lifespan'," BBC News reports. New research suggests people unable to smell distinctive scents, such as peppermint or fish, may have an increased risk of death within five years of losing their sense of smell.
The study found adults aged 57 or above who could not correctly identify five particular scents – peppermint, fish, orange, rose and leather – were more than three times as likely to die in the next five years.
The authors speculate loss of smell does not directly cause death, but it could be an early warning sign that something has gone wrong, such as exposure to toxic environmental elements or cell damage.
While this study is interesting, it does not prove that loss of sense of smell (anosmia) is a predictor of early death. Researchers used only five scents to identify people with anosmia and only tested people's sense of smell once, which makes the results less reliable.
There are many reasons for temporary loss of sense of smell, including viral infections, nasal blockage and allergy, so you shouldn't panic if you suddenly stop "smelling the roses". But you are advised to see your GP if there is no obvious reason for a sudden loss of smell.
Where did the story come from?
The study was carried out by researchers from the University of Chicago and was funded by the US National Institutes of Health, as well as other public bodies.
The study was published in the peer-reviewed journal, PLOS One. PLOS One is an open access journal, so the study is free to read online.
Many headlines were alarmist – for example, "Your nose knows death is imminent" in The Guardian and The Daily Telegraph's claim that, "A poor sense of smell could mean the end is nigh".
The Daily Mail took a more responsible approach by including comments from independent experts, who urged people with anosmia not to panic and said more research was needed on this topic.
What kind of research was this?
The research was part of a large US cohort study looking at health and social relationships in a large nationally representative sample of men and women aged 57 to 85. It is based on two surveys of about 3,000 participants – one carried out in 2005-06 and the second five years later.
The authors say sense of smell (olfactory function) plays an essential role in health and is also linked to key parts of the central nervous system. Normal olfactory function depends on cellular regeneration, which may be affected by the ageing process, they say.
They also say problems with sense of smell are already known as an early symptom of some major neurodegenerative diseases, including Alzheimer's and Parkinson's diseases.
Their hypothesis is olfactory dysfunction could be an early indicator of impending death. But because this was a cohort study, it is unable to prove cause and effect – in other words, that the loss of sense of smell led to death.
What did the research involve?
In 2005-06, researchers conducted interviews with 3,005 participants (1,454 men and 1,551 women) at home, assessing their ability to identify five common distinctive smells. These were, in order of increasing difficulty: peppermint, fish, orange, rose and leather.
Researchers used a validated odour identification test, presented using felt-tip pens. The five smells were selected and presented one at a time. Participants were asked to identify each by choosing from a set of four picture or word prompts.
The results were used to categorise olfactory function as:
- anosmic (loss of sense of smell) by 4 to 5 errors
- hyposmic (moderate loss of sense of smell) by 2 to 3 errors
- normosmic (normal sense of smell) by 0 to 1 error
In a second survey in 2010-11, researchers investigated which of the participants were still living. They did this either by speaking to the participants, family members or neighbours, or by examining public records or news sources.
They analysed their results using standard statistical methods and produced various models of their results, one of which adjusted for other factors that might affect mortality (confounders).
These included age, socioeconomic status, disease status, nutrition and body mass index. The researchers also controlled their results for frailty (measured by the inability to perform one or more of seven activities of daily living), cognitive function, smoking and drinking.
What were the basic results?
In 2010-11, 430 (12.5%) of the original 3,005 study subjects had died and 2,565 were still alive. In 10 cases it was unknown if the participant was alive or not, and these people were excluded from the analysis. A further 77 were excluded because of missing data.
Researchers found 39% of older adults with anosmia were dead at the time of the second survey, compared with 19% with hyposmia and 10% of those with a normal sense of smell. This pattern was seen in all age groups.
Once all other factors were taken into account, anosmic older adults had more than three times the odds of death at five years compared with those with a normal sense of smell (odds ratio 3.37, 95% confidence interval 2.04-5.57).
How did the researchers interpret the results?
The researchers say olfactory function is one of the strongest predictors of five-year mortality and may serve as a "bellwether" for slowed cellular regeneration, or as a marker for cumulative exposure to toxic environments.
They say loss of sense of smell was an independent risk factor stronger than several common causes of death, such as heart failure, lung disease and cancer.
Olfaction is "the canary in the coalmine of human health", they say, adding that "this evolutionarily ancient special sense may signal a key mechanism that affects human longevity".
A short olfactory test may be clinically useful in identifying patients at risk who might benefit from additional monitoring and follow-up.
Conclusion
This is an interesting study but it had limitations, including its use of only one short test and only five smells to identify people with anosmia. The diagnosis was not clinically verified and the test was performed in the person's home environment, rather than standardised across all participants in a clinic.
Although researchers tried to control for confounders, it is still possible that measured and unmeasured confounders played a role.
Even if the results of this study were robust, this study did not look at cause of death, so no preventative strategies were identified for people with anosmia.
Being told you have an increased risk of death is not particularly useful if there are no well-validated methods of reducing said risk. If anything, such news does more harm than good.
There are many reasons for temporary loss of sense of smell, including viral infections, nasal blockage and allergy. But anyone who suddenly loses their sense of smell is advised to see their GP as anosmia may be a sign of an underlying – and treatable – disorder.
