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  • Many people report a disruption in their ability to perceive the basic tastes of sweet, sour, bitter, salty, and umami after recovering from COVID-19.
  • Experts have assumed that most of these individuals have actually only lost their sense of smell, which plays a major role in the overall taste experience.
  • However, a new study suggests that at least one-third of such people have genuinely lost some of their ability to perceive these basic tastes.
  • The cause of basic taste loss after COVID-19 remains a mystery, although scientists have proposed several possible mechanisms.

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Loss of taste and smell is a common symptom of COVID-19. Research suggests it may be a better predictor of whether someone has a SARS-CoV-2 infection than other common symptoms, such as cough, fever, and fatigue.

People whose symptoms persist for at least 4 weeks after COVID-19, popularly known as long COVID, often also report losing their “gustatory” sense.

This is the basic ability to taste sweet, sour, bitter, salty, and umami — the savory flavor of glutamate.

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However, experts have assumed that most of these individuals have not actually lost their basic taste perception. This is because smell, or olfaction, plays a significant part in the overall flavor of food and drink.

Sense organs and tissues in the tongue, the roof of the mouth, throat, and nose all contribute to taste perception.

But a new study has found that around one-third of people who report taste loss after COVID-19 may have genuinely lost some of their basic sense of taste.

Chewing food releases odors, which sense receptors in the back of the nose detect. This is known as retronasal olfaction.

Additionally, the tongue contains taste buds that perceive the basic tastes of sweet, sour, bitter, salty, and umami, adding to the overall taste sensation.

Complete loss of the ability of the tongue to perceive these basic tastes is known as ageusia, while decreased sensitivity is known as hypogeusia.

A third condition, dysgeusia, involves a persistent, unpleasant taste in the mouth — such as sour, metallic, or rancid — that can taint the flavor of all food and drink.

The technical name for the loss of the ability to smell, as opposed to taste, is anosmia.

“There is some difficulty in self-rating true taste loss versus reduced flavor perception, which is a consequence of smell loss — as usually, we smell flavor through a process of retronasal olfaction: chewing breaks up food, releasing odors that are forced into the nose as we exhale,” said Claire Hopkins, professor of rhinology at Guy’s Hospital, London, in the United Kingdom, and one of the authors of the new study.

“This leads to over-reporting of taste loss, and it has been largely assumed that this accounts for most reported alterations in taste disturbance after COVID,” she told Medical News Today.

“This study highlights that true taste disturbance is, however, also more common than we had thought,” she added.

Researchers led by the University of Trieste in Italy set out to discover what proportion of people who say they have trouble with basic taste sensation after COVID-19 have possible damage to their taste buds.

They identified 105 patients at the university’s ear, nose, and throat outpatient clinic who reported a disruption of their ability to taste sweet, sour, salty, or bitter more than 3 months after a SARS-CoV-2 infection.

They gave them a range of validated taste and smell tests, including a Sniffin’ Sticks test of olfaction and a Taste Strips test of basic taste perception.

More than half of the participants who reported problems with basic taste perception actually had typical gustation.

However, the Taste Strips test confirmed hypogeusia — a loss of basic tastes — in 42% of these individuals.

The taste buds of some participants may have already lost some of their ability to discriminate between basic tastes simply due to normal aging.

“Taste and smell disorders increase in incidence with aging, and therefore age adjustments allow us to better compare with ‘normal’ function,” Prof. Hopkins told MNT.

However, even after adjusting for the possible contribution of age, 29% of the patients still appeared to have hypogeusia as a result of COVID-19.

The study appears in JAMA Otolaryngology — Head & Neck Surgery.

“The mechanism is unclear but could involve direct damage to the taste buds, reduction of saliva production (a dry nose and mouth is commonly reported after [COVID-19]), or a more central process (although this is less likely),” said Prof. Hopkins.

Taste bud cells carry the ACE2 receptor in their membrane, which the virus uses to infect cells. But inflammation or increased blood clotting as a result of the infection, for example, could also indirectly harm the cells.

“Loss of smell and taste are a feature of long COVID but not as common as some of the other symptoms, such as fatigue and breathlessness, and more people report these symptoms earlier in their symptom trajectory,” a spokesperson for the patient advocacy group LongCovidSOS told MNT.

All the same, a survey by the organization of more than 800 people with long COVID found that more than 65% reported problems with their smell and taste.

The survey did not differentiate between the two symptoms. But the spokesperson said a larger survey, which appears in The Lancet, found no significant difference between rates of smell and taste loss.

“However, they do describe something we come across quite often, which is alterations in perception, whereby things smell and taste wrong, often very unpleasant — these changes can lead to people becoming malnourished because food tastes so bad,” said the spokesperson.

The authors of the new study report several limitations of their research.

For example, they write that they evaluated participants at different time points after their infection, and there was no age-matched control group for comparison.

In addition, they say most of the participants were women who had experienced only mild symptoms of COVID-19, which could limit the generalizability of the findings.

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