It sounds a little hard to swallow, but a new study suggests that how people react to bitter flavors correlates with the severity of their COVID-19 infection.
It’s an exciting insight because during the past 16 months, it’s become clear that people don’t respond to SARS-CoV-2 predictably. It’s been impossible to determine whether someone will experience mild symptoms or develop life-threatening respiratory disease. Imagine if a simple taste test could indicate a person’s risk of developing severe COVID-19.
Henry Barham, a rhinologist at the Baton Rouge General Medical Center, in Louisiana, published a study in the medical journal JAMA Network Open on May 25 that analyzed nearly 2,000 patients and found that “supertasters”—individuals who are overly sensitive to some bitter compounds—were less likely to test positive for the virus. If this association holds true, it implies, for example, that people who don’t find broccoli too bitter are in a higher risk group for severe COVID-19.
“This is a very interesting study that suggests that receptors on our tongue that allow us to sense bitter flavors are also linked to our vulnerability to respiratory infections like COVID-19,” says David Aronoff, director of the division of infectious diseases at Vanderbilt University Medical Center, in Nashville, Tennessee, who was not involved with this research. That taste receptors may also be involved with immunity is surprising, he says.
Do “supertasters” have super powers?
At Yale University in the 1990s, psychologist Linda Bartoshuk pioneered the study of genetic variations in taste perception. She coined the term “supertaster” to describe the 25 percent of people who are intensively sensitive to bitter flavors. Another 25 percent of people are “nontasters” who barely detect bitter flavors, and the remaining 50 percent are just “tasters”—those who register the bitterness but not to the degree that it’s distasteful.
Supertasters are more sensitive to bitter flavors because they have up to four times as many taste buds on their tongue. Bitter compounds in certain foods and drinks are recognized by type 2 taste receptors, which are made by a family of genes called T2R. The T2R38 gene is among the best studied of these. Variations in the structure of the T2R38 protein the gene encodes correlates with a person’s tolerance of bitter compounds—such as phenylthiocarbamide and propylthiouracil—which are abundant in many vegetables, including broccoli, cabbage, and brussels sprouts.
This is not the first time being a supertaster had been linked to a medical condition. Supertasters have a higher likelihood of having polyps in their colon, a risk factor for cancer associated with lower intake of those bitter vegetables.
But supertasters can experience physiological advantages as well. As it happens, the T2R38 proteins are found in places other than the tongue. These “extraoral” areas include the epithelial cells that line the nose and upper respiratory tract, where they respond to invading pathogens.
A 2012 study led by Noam Cohen, a rhinologist at the University of Pennsylvania, in Philadelphia, found that bacteria responsible for sinus infections activate the T2R38 protein receptors on cells lining the respiratory tract, causing them to produce nitric oxide. Nitric oxide is a key component of our immune response, the first line of defense against invading pathogens. It stimulates hair-like structures called cilia in the respiratory passageways that remove foreign particles and pathogens from the body. Consequently, supertasters experience fewer bacterial sinus infections.
Barham, who studies T2Rs as they relate to innate immunity within the respiratory tract, was also aware that nitric oxide could poison SARS-CoV, a coronavirus (related to SARS-CoV-2 that causes COVID-19) first reported in Asia in 2003 that caused a respiratory illness that spread to 22 countries before it was contained. This prompted him to investigate whether a link exists between COVID-19 and supertasters.
Tasteful study bitter pill for nontasters
Barham’s team studied 1,935 adults, 266 of whom tested positive for SARS-CoV-2. Nontasters were significantly more likely than tasters and supertasters to test positive for SARS-CoV-2, to be hospitalized once infected, and to suffer symptoms for a longer time. Eighty-six percent of those with severe COVID-19 requiring hospitalization were nontasters. Less than 6 percent of supertasters tested positive for SARS-CoV-2.
Barham also speculates that the possible connection between T2Rs and COVID-19 might be linked to why children generally are less susceptible. The numbers of “taste receptors decrease with age, which potentially explains why the elderly population seems to do worse than their younger counterparts,” Barham says. Conversely, most children, who have more T2Rs, suffer less severe symptoms or illness when infected with SARS-CoV-2. “The 25 percent of children who are nontasters show few to none of these T2Rs, resulting in potentially more severe symptoms,” he says.
According to Aronoff, the study has limitations. The relatively small number of adults examined were in a fairly narrow age range, so it’s not known whether the correlation between taste preferences and COVID-19 severity exists in children or the elderly. In addition, he says, the population studied may differ in unknown ways that influenced the results.
Are COVID-19 taste tests coming?
Being able to determine quickly who is most at risk from SARS-CoV-2 would be a valuable tool as society emerges from quarantine. Barham’s findings suggest that taste-testing could provide a safe, fast, and inexpensive way to categorize people into risk groups for COVID-19 and other infections.
“At this point, the results of this work are premature to help us manage COVID-19 in the clinic,” Aronoff cautions. “But the results could impact our understanding of what leads people to be more or less vulnerable to infections like COVID-19.” Aronoff emphasizes that supertasters shouldn’t overinterpret these conclusions: “People who hate broccoli should not avoid vaccination,” he says.
Danielle Reed, associate director of the Monell Chemical Senses Center, in Philadelphia, also warns against overinterpreting the findings. Reed, who studies genetic differences in taste and smell, performed the genetic testing in the Barham study but declined to be named as an author because she interpreted the results differently.
Reed points out that Barham’s analysis didn’t account for “general taste loss, which is an early and cardinal feature of COVID-19.” As a result, she believes that some patients “were categorized incorrectly as nontasters.” Furthermore, T2R genes were not identified as being involved with COVID-19 severity in an independent genomic analysis.
Reed says taste tests to help guide medical care are “a goal we can work toward. But the first step is to make screening for taste and smell a regular part of health care, like we do with vision and hearing. As we add taste and smell checks to routine health care, how these senses predict health and disease may emerge and be useful tools.”
Barham agrees that more research is needed and says his team has continued gathering data to “explore the relationship” between taste receptors and COVID-19. He expresses optimism about extending the work to other infectious diseases. “We are also studying this family of receptors as they influence innate immunity to influenza, along with other upper respiratory infections.”
nationalgeographic.com, 29 May 2021