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The link between poor sense of smell and diseases from obesity to Alzheimer's

NOTED logo NOTED 26/12/2018 Ruth Nichol
a woman holding a banana: Photo/Getty Images © Bauer Media Photo/Getty Images

As Mei Peng and other researchers are now finding, our ability to smell doesn’t just make life better – it’s also closely linked to both our physical and mental health.

If University of Otago researcher Mei Peng had to choose between losing her sense of smell or losing her sense of taste, she would opt for the latter.

Peng, who specialises in sensory science, says many of us believe that taste is more important to our quality of life than smell. However, she disagrees, pointing out that taste has only five dimensions – sweetness, sourness, saltiness, bitterness and umami (or savouriness) – whereas we can detect millions of different smells.

“Smell is much more complicated than taste,” she says. “Take a cup of coffee – we think it has a ‘coffee smell’, but it has more than 300 volatile compounds that combined give us that smell. Every single one of the odour receptors in our nose is working to detect those combinations.”

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Someone with no sense of taste is unable to detect the sweetness of a strawberry or the sourness of a lemon, but thanks to their sense of smell they can still enjoy the strawberry or lemon flavours.

“People think you eat lemon and you get a lemon flavour in your mouth, but it’s actually in your nose. That’s why I would choose to lose the sense of taste, because it’s easier to live without.”

As she and other researchers are now finding, our ability to smell doesn’t just make life better – it’s also closely linked to both our physical and mental health.

It’s been known for some time that losing your sense of smell is an early symptom of both Parkinson’s and Alzheimer’s diseases. This is thought to be caused by changes in the olfactory bulb – the part of the brain that gives us our sense of smell – and occurs long before other parts of the brain are affected.

Changes to the olfactory bulb are also associated with depression. Many people with depression have smaller olfactory bulbs and are less sensitive to odours than those who aren’t depressed.

More recently, Peng has found a link between a poor sense of smell and obesity. She was lead author of a study published in the November issue of Obesity Reviews that found that the better a person can smell, the more likely they are to be slim – and vice versa.

Peng says a possible explanation is that a poor sense of smell makes us more likely to choose high-kilojoule, sweet or salty foods rather than blander, lower-kilojoule foods – opting for bacon and maple syrup for breakfast rather than low-fat, low-sugar cereals.

“A declining sense of smell means less satisfaction from the flavour side of the food, which may lead to more reliance on taste.”

She says the effect is most noticeable in those who are already obese, which suggests that carrying extra weight causes metabolic changes that affect the ability to smell, increasing the difficulty of making healthy food choices.

Obesity is just one reason people lose their sense of smell. Other causes include using zinc-based nasal sprays, taking certain drugs, or a blow to the head. A few unlucky people lose their sense of smell permanently after having a cold or the flu.

In about 60% of cases, the reason is that they are developing either Parkinson’s or Alzheimer’s. Maurice Curtis of the University of Auckland’s Centre for Brain Research describes changes to the olfactory bulb that occur at the very beginning of these two diseases as the “canary in the coal mine”.

He says the reason the olfactory bulb is affected first may be that it’s the part of the brain most exposed to the outside world. It’s thought that an as-yet unidentified “smoking gun” – possibilities include bacteria, viruses or environmental toxins – enters the olfactory bulb via the nose and triggers neurodegenerative changes that eventually spread to other parts of the brain.

In most cases, says Curtis, the change is so gradual people don’t notice it. However, he believes that simple “smell tests” may eventually make it possible to detect people in the early stages of Parkinson’s and Alzheimer’s and start treating them before they develop more significant problems such as memory loss.

“That would be the dream, but we also need better treatment. That’s very much what we’re working towards.”

This article was first published in the December 15, 2018 issue of the New Zealand Listener.

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