There are two kinds of people in the world: Those who can smell stinky fish, and those lucky enough not to. A new study published in Current Biology on Thursday shows how a rare genetic mutation makes some people immune to the nauseating aroma of rotten fish, Nature News reports.
About 11,000 people participated in the study by providing DNA samples and putting their noses to the test. When presented with a each of six odors, each participant took a whiff and tried to identify it. For many people, the rotten fish smell was easy to identify and incredibly unpleasant, but a small group labeled the scent as something neutral.
A look in their DNA revealed that the group shared a genetic mutation in common. They all had at least one broken version of a gene called TAAR5.
“I can assure you I do not have this mutation,” neurologist and co-author of the study Kári Stefánsson, of deCODE Genetics in Reykjavik, tells the New York Times’ Katherine Wu. “I tend to get nauseated when I get close to fish that is not completely fresh.”
The gene makes the tool that cells in your nose use to identify a rank chemical called trimethylamine, or TMA, which is also found in faeces, blood and bad breath, Donna Lu writes for New Scientist. TMA is a red flag for iffy food, and people’s disgusted reaction to its sickening smell helps them avoid danger.
“TAAR5 is a very conserved gene, so it’s very similar across species, probably because it has been important to protect us against harmful microorganisms,” says deCODE neuroscientist Rosa Gisladottir to New Scientist.
The researchers asked the study participants to smell samples with synthetic odors of cinnamon, peppermint, banana, licorice, lemon and rotten fish. Success with identifying each scent tended to decrease with age, but younger people also sometimes confused the smells of banana and lemon with other sugary sweets, the New York Times reports. Each participant also rated the pleasantness of each smells, and rotten fish was overwhelmingly the worst.
But for people with broken TAAR5 genes, the rotten fish smell sample seemed relatively neutral. Some couldn’t smell it at all, while others identified it as potato, caramel, ketchup or roses — and a few even ranked it as relatively pleasant.
The Reykjavik-based company deCODE has been studying people’s DNA for decades, but it might be key that this study was conducted in Iceland. About two percent of Icelanders have the broken TAAR5 genetic mutation, an unusually high percentage compared to other regions of the world. In Europe overall, 0.8 percent of people have the same mutation, and only 0.2 percent of people in Africa have the same mutation.
“If they hadn’t looked at this population, they might not have found the variant,” says Bettina Malnic, who studies olfaction at the University of São Paulo, to the New York Times. Another sensory science researcher, Paule Joseph, tells the Times that a future study with a more diverse study population could show whether different diets affect the prevalence of the mutation.
Iceland has a largely fish-based cuisine. For example, the country’s national dish, hákarl, is made of dried, fermented Greenland shark that can smell like urine and taste like pungent cheese.
“It seems to be some sort of local selection,” Stefansson tells New Scientist. “In Iceland, we have been living on fish mostly for thousands of years.”