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Aging and the Sense of Smell

August 2014 - How does the sense of smell change as people age? Some studies show that sensory neurons in the olfactory system in people over 60 may show responses to odour that make it difficult to identify specific smells. This can create issues with identifying dangerous substances and promoting good nutrition.

Discussing research published in Neurobiology of Aging in 2011, Professor Diego Restrepo, Ph.D., director of the Center for NeuroScience at University of Colorado School of Medicine said:

"We found clear changes in olfactory sensory neuron responses to odours for those 60 and up. When we presented two different odours to the olfactory sensory neurons of younger people they responded to one or the other. The sensory neurons from the elderly responded to both. This would make it harder for the elderly to differentiate between them."

The study showed that people losing their sense of smell had a higher risk of malnutrition because taste and smell are closely related. Also they may be unable to detect spoiled food, gas leaks and other toxic vapours.

440 subjects in two age groups - 45-years-old and younger and people 60 and over - were tested for responses to two distinct odours and also subsets of those odours. The objective was to find out if age-related differences in the function of Olfactory Sensory Neurons (OSNs) might contribute to the impairment of the sense of smell. Cells from the two groups were biopsied in collaboration with Monell Chemical Senses Center in Philadelphia.

The researchers expected to find fewer OSNs in the older subjects and thought the neurons would be less likely to respond to stimuli. However, they found just as many neurons in the older group as the younger but the over 60s could not differentiate between two odours - they blended together.

According to Diego Restrepo:

"Whereas cells from younger donors were highly selective in the odorants to which they responded, cells from older donors were more likely to respond to multiple odor stimuli… suggesting a loss of specificity. The study suggests that changes in nose and the brain contribute to smell loss in the elderly."

In a study of mice reported in 2014 in the Journal of Neuroscience, Florida State University post-doctoral researcher Nicolas Thiebaud and colleagues showed a reverse link: that a bad diet impacted on the sense of smell. When mice were given a high-fat diet over a period of six months showed a significant reduction in their ability to distinguish odours.

Smell and Neurodegenerative Conditions

Research by Dr Amy Johnston, from Griffith University's School of Nursing and Midwifery and the Eskitis Institute of Cell and Molecular Biology, published in the journal Chemical Senses in 2007 found that normal aging processes have little detrimental effect on the sense of smell.

The study tested the ability of about 1000 Australian males and females of all ages to detect or identify a range of odours at different concentrations. The results showed that olfactory function deteriorates relatively slowly with age in the absence of other factors such as smoking, medication or history of nasal problems. Healthy women were found to have a more sensitive sense of smell than healthy men.

However, the sense of smell was found to be adversely affected by some medications and deterioration to be associated with a number of neurodegenerative illnesses. The study supported findings that common anti-cholesterol and antihypertensive medications, and chronic conditions such as Parkinson's disease and Alzheimer's disease were among those associated with impairment of the sense of smell.

The study underlined important consequences of losing the sense of smell with particular relevance for older people, for example the risk of poor appetite and nutrition and being unable to assess the fitness of food for consumption.

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