The phenomenon may be due to alterations in cells responsible for perceiving sweet, bitter and umami tastes.
“Just a few days after contracting Covid, I completely lost my sense of smell and taste, hopefully temporarily, but I am worried about this sudden and total loss.” This is just one of the messages we have received in recent years from readers concerned about no longer being able to smell or taste after infection with Sars-CoV-2. Researchers at the Swedish University of Agricultural Sciences, Uppsala University, and the University of Colorado have now demonstrated, with a modern study, that the loss of taste may be due to alterations in the cells responsible for perceiving sweet, bitter, and the so-called “umami” tastes.
Taste Buds
Throughout history, tastes have been divided into four groups – salty, sour, sweet, and bitter – to which a fifth was added in the early 20th century, umami, described as the flavor of Japanese miso soup. Sars-CoV-2 can, in some cases, suppress one or more of the fundamental taste qualities, or even all five. In most patients, the loss is short-lived, but it is not uncommon for taste disturbances to persist after the infection has ended. This is not a minor issue, as not being able to taste flavors can cause weight loss and health problems. Understanding the mechanisms behind these changes is crucial for developing strategies to support patients regain their sense of taste and improve their quality of life.
Signal Transmission
In the study, led by Göran Hellekant of the Swedish University of Agricultural Sciences, researchers investigated how infection affects the ability of taste cells to transmit signals to nerves and how the loss of taste related to sweet, bitter, and umami flavors could be explained. “We recruited 28 people who tested positive for Sars-CoV-2 and who reported taste disturbances for more than 12 months,” said Göran Laurell, of Uppsala University. To determine which flavors were impaired and measure the extent of taste loss, the researchers used a specific kit (Wett, waterless empirical taste test). The examination confirmed that all participants except one had a taste disorder.
Sweet, Bitter, and Umami
The next step was to examine the taste buds on the tongue in detail. Twenty of the participating subjects volunteered to allow researchers to take tissue samples (biopsies of 5-8 papillae) from the tip of the tongue. The so-called “fungiform” papillae are visible as small dots on the tip and sides of the tongue. “When the papillae were examined under a microscope, few damages were found to the overall structure or nerve connections – said Tom Finger of the University of Colorado. Both the papillae and the nerves appeared substantially normal.” Although, molecular analyses of the different types of taste cells showed how the sense of taste was altered. The anomaly was linked to the tastes of sweet, bitter, and umami, the same ones that were found to be “defective” in the Wett test.
Low Levels of mRNA
“We were able to link the reduced ability to perceive sweet, bitter, and umami to low levels of mRNA encoding a protein called PLCβ2 in specific taste cells – explained Göran Andersson, a researcher at the Swedish University of Agricultural Sciences. PLCβ2 is necessary to amplify the signal that taste cells send to nerves, which then transmit the information as electrical impulses to the taste centers of the brain, where flavors originate.” the long-term loss of the ability to perceive sweet, bitter, and umami flavors after Covid may be due to changes in some cells of the taste buds. These are the cells that normally transmit nerve impulses to specific areas of the brain, giving rise to the sensations of sweet, bitter, and umami.
