Sense of smell: Death does not stink

22. December 2014
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Anyone elderly and with a poor sense of smell is supposed to have a higher risk of dying within the next five years than people whose sense of smell is good. At least this is what a survey of around 3,000 Americans between 57 and 85 years of age would appear to have demonstrated.

When inhaling, various molecules from the source of a fragrance make their way to the upper nasal cavity. These molecules deposit on receptors which are found on the cells in the olfactory epithelium and activate a signalling cascade. This releases an action potential which is forwarded to the olfactory bulb in the brain, bulbus olfactorius. For the purpose of storage the stimulus is transmitted from there to the hippocampus, the learning and memory centre of the brain. The fragrance is identified in the thalamus and orbitofrontal cortex and emotional responses get triggered in the basal forebrain and orbitofrontal cortex. Alongside the numerous tasks and functions performed by the sense of smell through these processes, according to US scientists there now one more has made its way to the list: the olfactory perception of older people is supposed to be an indicator of expected lifespan.

From sense of smell to mortality risk

In order to come to this realisation, Pinto and his group analysed the National Social Life, Health and Aging Project (NSHAP). This study collected data on approximately 3,000 Americans between 57 and 85 years of age. The selection of participants was representative in relation to this age group. In 2005-06 the participants were required to recognise peppermint, fish, orange, rose and leather by smell and were then divided into three groups, according to the number of correct matches. In this study 78 percent detected four or five odours. This is equivalent to normal olfactory performance. Just 3.5 percent were able to categorise only one or none of the fragrances properly.
It remains unclear whether people with a poor sense of smell detected nothing or whether they simply did not identify the scents correctly. Within five years 430 study participants had died, representing 12.5 percent of all participants. What is striking is that many of the dead individuals had failed in the smell test, because in the small group that had correctly detected a maximum of one aroma, 39 percent died and thus four times more than among the “normal smellers”. However, in consideration of age, it was shown that study participants between 75 and 85 years with normal ability to smell were about twice as likely to die within the next few years as those 20 years younger with a normal olfactory function. For people with a poor sense of smell, in contrast the probability of death in all three age groups was similar. If we then consider only the results for the 75 to 85 year old participants in the group that had a maximum detection of one odour, only twice as many died after five years when compared to age-matched “normal smellers”. According to researchers, poor olfactory perception better indicates approaching death than do diseases such as cancer, heart failure or lung disease.

The canary in the coal-mine of health?

According to Jayant Pinto the sense of smell has a similar role in human health to that of the canary in the coal mine for coalminers, in that the canaries, as experience showed, were sensitive to carbon monoxide and so made miners aware of the dangerous gas. A poor smell perception will also only warn of the impending danger, but will not in itself be the cause of it. The mechanism, however, is unclear and has not been examined by the researchers. They do however suggest that air pollution could be the reason, since the olfactory nerve is the only cranial nerve that is directly exposed to the environment. Through this, pathogens, toxins and particulate matter could find their way into the central nervous system and cause damage to bodily functions. A poor capacity to smell would also indicate a reduced regenerative capacity in the body, the researchers say, because the smelling function in the nose constantly needs to be renewed via new stem cells. If these and other adult stem cells die off within the body, this might be the cause of the increased risk of death.

Previous study results

This study published by Jayant Pinto and his group is not the first to occupy itself with the issue of whether a poor sense of smell may indicate approaching death. Already in 2011 Robert Wilson published similar results. He analysed data from a study involving over 1,100 participants who were on average almost 80 years old. The selection of participants was however not representative. The subjects were required to identify twelve different fragrances in an olfactory test. The result: the risk of death was reduced by six percent for every smell correctly categorised. However, since poor olfactory perception is often associated with neurodegenerative diseases such as Alzheimer or Parkinson, according to Robert Wilson a poor performance in the test only indicates a potential illness which may not yet have been recognised.
Bamini Gopinath also dedicated his time a year later, in 2012, to this issue. He evaluated data from more than 1,500 participants over 60 years of age who were required to define eight fragrances in an olfactory test. It was shown that people with a poor sense of smell had a 67 percent higher chance of dying within the next five years. If the researchers however took into account cholesterol levels or cognitive impairments, this association no longer existed. “People with a poor sense of smell were generally older, male, and had vision and memory disorders, diabetes. angina pectoris, were stroke victims, had low BMI, ill health or high cholesterol”, says Bamini Gopinath in his publication.

Taking advantage of the observational study

From previous studies it is already known that olfactory dysfunction may indicate neurodegenerative diseases. These diseases increase the risk of death. There are however also other reasons for a poor sense of smell. In addition to age-related decline, poor olfactory perception can be genetic or impaired due to chronic rhinitis, craniocerebral trauma or infections of the upper respiratory tract. The reasons why people performed poorly in the smell test were not specified by Jayant Pinto, nor was consideration made as to whether the increased risk of death of the “bad smeller” is due to pre-existing disease. The proposed mechanism is merely a guess. Before things therefore go so far that people with a poor sense of smell be recommended to get as soon as possible onto the task of writing a will, numerous investigations will have to deliver the necessary clarity to the picture.
Original publication of the article: http://www.plosone.org/

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