Scientists have identified the ‘trigger molecule’ that causes pleasant aromas, such as burning garbage or sewage, to be smelled in people whose sense of smell is impaired by Covid.
Loss of smell is a hallmark symptom of Covid-19, estimated to affect around 18% of UK adults. Some people also suffer from an olfactory disorder — a condition known as parosmia — but the biological basis for this has remained a mystery.
Now scientists have identified a powerful odor molecule that appears to be a trigger for the disgust felt by many people with parosmia. The molecule called 2-furanmethanethiol found in coffee has been described as coffee- or popcorn-like by people with a normal sense of smell, but people with parosmia typically described its smell as gross, repulsive, or dirty.
dr Jane Parker, director of the Flavor Center at the University of Reading and co-author of the study, said: “This is solid evidence that not everything is in the mind and that feelings of disgust may be related to the compounds in the distorted foods. The central nervous system is certainly also involved in interpreting the signals it receives from the nose.”
According to a recent international survey, around 10% of people with Covid-related smell loss experienced parosmia immediately after becoming ill, and that figure rose to 47% when respondents were re-interviewed six or seven months later.
Some of the most common triggers for parosmia are coffee, chocolate, meat, onions, and toothpaste. The latest study looked at whether certain compounds within these substances are responsible.
By capturing the coffee aroma, the team was able to test individual coffee compounds on volunteers with parosmia and compare their response to those who didn’t. Of the hundred or so aromatic compounds found in coffee, people with parosmia could point to those responsible for the feeling of disgust. Among the 29 volunteers, scientists found 15 commonly identified compounds that trigger parosmia, with the main culprit being a chemical called 2-furanmethanethiol, which 20 of the volunteers said had a terrible smell.
The nose has more than 400 different types of olfactory receptors in the nose, each sensitive to different aromas. The chemical 2-furanmethanethiol has an exceptionally low detection threshold, making it potentially one of the first chemicals to return to a person’s radar after they lose their sense of smell. Parker said the brain appeared to miscategorize smell, but that more work was needed to understand this element of the condition. The paper noted that people with parosmia might perceive even the most unpleasant odors differently, for example describing the smell of feces as “less unpleasant or biscuit.”
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 to understand this condition, but this research is the first to look at the mechanism in the nose. We now know that this must have something to do with the nerves and their receptors, which is how these molecules are recognized.”
The results are published in the journal Communications Medicine.