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FOOD

How taste and smell work (and why you really don’t want COVID-19 to take these senses away)

The nature and fragility of taste and smell is now top of mind for many people, including scientists.

For most people, taste and smell return once they have recovered from COVID-19.
For most people, taste and smell return once they have recovered from COVID-19.Stevica Mrdja/Microgen - stock.adobe.com

The same way eyesight, hearing, and physical strength vary among us, so does the ability to taste and smell. These senses, precious not just for eating but enjoying other facets of life, are some of the many wonders of the human body. And as many people now know from personal experience, COVID-19 can dull these senses. For some, the loss is temporary. For others, the loss lasts much longer.

“Some people are losing their sense of taste and smell with COVID and not getting it back, or their taste is distorted,” says Dr. Danielle Reed, associate director of the Monell Chemical Senses Center in Philadelphia. While it’s not uncommon to lose one’s sense of taste and smell with certain viruses, chemotherapy, surgery, and accidents, she says, “What’s different now is that over a million people are going to be left permanently with a sense of taste and smell that doesn’t recover.”

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The nature — and fragility — of taste and smell is now top of mind for many. We thought we’d take a quick look at how these senses work.

We smell by sniffing with our nose. The cells that respond to odors are located way up at the back of the nasal passage. These cells have cilia, which are microscopic, hair-like structures on the surface of the cells, that help communicate sensory information to the brain. “With COVID, the cilia fall off, like the decimation of a forest,” says Reed. “We know olfactory cells with cilia can return,” she says. “It can happen and does happen for many people. With COVID, this process can be very slow or not happen at all.”

Smelling also occurs as we chew and swallow foods and the odors travel up the rear nasal passage to olfactory receptors, which send messages to the brain. This dual taste-and-smell process is how we discern flavor.

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Taste comes from your tongue. The tiny, mushroom-shaped bumps on the tongue called fungiform papillae contain taste buds. Inside the taste buds are taste receptor cells, which respond to sweet, salty, sour, bitter, and umami (savory), and communicate that information to the brain. Taste cells in the papillae are constantly dying and being regenerated, although this process slows with age.

The ability to taste is affected by both biology and experience. About 30 years ago, Dr. Linda Bartoshuk, Bushnell Professor of Food Science and Human Nutrition at the University of Florida (who was Reed’s Ph. D adviser at Yale) observed and coined the phrase “supertasters.” These folks are extremely sensitive to bitter tastes, as depicted by their ability, a genetic trait, to taste a very bitter chemical (6-n-propylthiouracil or “PROP”). About 25 percent of people are extremely sensitive to PROP. “Everything is turned up for them,” says Reed. Non-tasters (25 percent) can’t taste the compound’s bitterness at all. The rest of the population falls into the medium (or average) range and perceive only a slight bitterness.

If you’re deemed a non-taster — both Reed and Bartoshuk are non-tasters — there’s no need to worry. “Your general sense of taste can be just fine,” says Reed. Supertasters generally don’t eat spicy or overly sweet foods because of their heightened sensitivity. The hot sauce lover you know is probably a non-taster.

Gender and age also play a part. Women generally have a better sense of smell and taste than men. Children are more sensitive than adults. A “picky” child may simply have a greater sensitivity to certain foods and odors. Compared to adults, children prefer extreme sweet tastes and dislike bitter ones (a biological trait alerting humans to avoid toxic substances). In a taped interview a few years ago, Bartoshuk suggested that parents of picky eaters should try masking bitter vegetables like broccoli and Brussels sprouts with a little sugar, butter, or cheese sauce to make these important foods more tolerable. Both children and adults can learn to acquire new food preferences over time.

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Chefs generally have more discerning palates than the average person. They’re also more open-minded and adventurous when it comes to eating.

“There are things I don’t like, but there’s nothing I won’t eat,” says Jay Murray, chef at Ellis Square Social in Beverly and former chef of Grill 23 & Bar in Boston. Hazelnuts, sweet potatoes, winter squash, and eggplant (“except when I cook it,” he says) are foods he doesn’t care for. “I can’t stand the smell of flavored coffee, specifically hazelnut coffee, and I have the same response to truffle oil,” he says. Vinegar, on the other hand, is one of his favorite ingredients to balance savory foods. “It brightens dishes without making them sour,” he says.

Murray has been treated twice for pancreatic cancer. “I was warned that I might lose both taste and smell with chemo,” he says, but luckily neither was affected. Sometimes intense aromas bothered him. While he was getting chemotherapy, his wife used to bring him noodle soups from a Chinese noodle shop near the hospital. After a while, he says, “It was all I could smell, and it started to really disgust me.”

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Scientists have studied and affirmed the relationship between our sense of taste and smell and health outcomes. Sensitive tasters typically avoid bitter foods, such as cruciferous vegetables (e.g. broccoli, Brussels sprouts, cabbage, and kale) which are powerhouses of nutrition and fiber. Studies show that supertasters are at greater risk for colon cancer, partly because they don’t eat enough (bitter) vegetables. On the flip side, supertasters don’t drink much alcohol or smoke due to the bitterness.

People with low taste sensitivity may over-salt foods and prefer very sweet and high-fat items, which can lead to diabetes, obesity, and high blood pressure. Of course, there are other factors that influence what we eat, including family favorites, negative food experiences, and what we can afford.

Sensory loss is poised to be a far-reaching impact of COVID going forward. “We’re going to be very unpleasantly surprised by the large number of those who’ve lost their sense of taste and smell due to COVID, and it’s going to be challenging to figure out how to feed people if they’ve lost pleasure in eating by not being able to taste and smell,” says Reed. (In March 2020, the Global Consortium for Chemosensory Research was founded to conduct worldwide research on COVID-19 and its impact on smell and taste.) “This is a new phenomenon; we don’t have the knowledge or experience yet to deal with it.”

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Lisa Zwirn can be reached at lisa@lisazwirn.com