Summary: Study reveals the mechanism behind why certain food and drink aromas smell disgusting to people with parosmia.
Source: University of Reading
For people with parosmia, or distorted sense of smell, the aroma of freshly ground coffee can be as disgusting as burning rubbish.
Now, researchers have discovered the secrets of why certain food and drinks smell (and likely taste) disgusting to people with parosmia.
In new research published in Communications Medicine, a team of scientists have found that certain highly potent odour molecules found in coffee trigger the sense of disgust which is associated with parosmia. Take 2-furanmethanethiol – the most potent aroma molecule that you’ve never heard of before. It is one of those molecules that has been driving people to their wit’s end.
By trapping the aroma of coffee, the team were able to test coffee compounds on volunteers who had parosmia and compare their reaction with those who didn’t.
From the hundred or so aroma compounds present in coffee, people with parosmia could point to those responsible for the sense of disgust. Among the 29 volunteers, scientists found 15 commonly identified compounds that triggered parosmia.
Dr Jane Parker, Associate Professor of Flavour Chemistry and Director of the Flavour Centre at the University of Reading said:
“This is solid evidence that it’s not all “in the head”, and that the sense of disgust can be related to the compounds in the distorted foods. The central nervous system is certainly involved as well in interpreting the signals that it receives from the nose.
The parosmic experience is a combination of the two mechanisms which produces the distorted perception of everyday foods, and the associated sense of disgust.
“We can now see that certain aroma compounds found in foods are having this particular effect. It will, we hope, be reassuring for those with parosmia to know that their experience is “real”, that we can identify other foods which may also be triggers and, moreover, suggest “safe” foods that are less likely to cause a problem. This research provides useful tools and strategies for preventing or reducing the effect of the triggers.”
Mr Simon Gane, one of the researchers, from the Royal National Ear, Nose and Throat and Eastman Dental Hospital said: “We still have a long way to go in understanding this condition, but this research is the first to zoom in on the mechanism in the nose. We now know this has to be something to do with the nerves and their receptors because that’s how these molecules are detected.”
Some of the most cited food and drinks that set off parosmia in sufferers include:
The study used a technique called GC-Olfactometry. The aroma from the coffee is introduced into one end of a very long and narrow pipe called a capillary. Some of the aroma compounds travel through the capillary faster than others. The fastest ones come out of the other end first and the slower ones come out later, thereby separating the compounds out so that the volunteers could smell them and describe them one by one.
Prior to the global pandemic caused by Covid-19, parosmia was a rare condition known to occur after infections such as cold, flu or sinus infections, with very little awareness about the causes and treatments for the disease.
During the pandemic Covid-19 symptoms included loss of smell and taste in 50-60% of cases, of which about 10% developed parosmia. While, since the Omicron variant, loss of smell and taste has become a less common symptom (estimated to occur in about 10-20% of cases) and parosmia cases are likley to be fewer in number, parosmia is still estimated to affect 2 million people in Europe.
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Insights into the molecular triggers of parosmia based on gas chromatography olfactometry
Parosmia is a debilitating condition in which familiar smells become distorted and disgusting, with consequences for diet and mental health. It is a feature of post-infectious olfactory loss, particularly resulting from COVID-19. There is currently little understanding of its pathophysiology, and the prevailing hypothesis for the underlying mechanism is aberrant growth of regenerating olfactory sensory neurons after damage.
We use gas-chromatograph olfactometry to individually present components of a complex olfactory mixture as a rapid screening tool for assessment of both quantitative and qualitative olfactory dysfunction in those with and without parosmia. This allows them to report the associated sensory effects and to identify those molecules which are altered or parosmic in nature.
Here we show 15 different molecular triggers of this symptom. These trigger molecules are common to many in the parosmic volunteer group and share certain characteristics such as extremely low olfactory threshold and common molecular structure
We posit that specific highly odour-active molecules are the cause of the parosmic symptom in most cases and initiate the sense of disgust, suggesting that parosmia is, at least in part, a receptor-level phenomenon.