The coronavirus can have a symptom other respiratory infections do—loss of smell and taste, BBC News reported. However, researchers said it’s not common enough to add to a list of key symptoms, nor should those with diminished senses of taste or smell assume they have the coronavirus. Both senses function biochemically.
According to BBC News, people suffering from COVID-19 may find themselves with subdued senses of smell and taste, according to a new survey. “A team at King’s College London looked at responses from more than 400,000 people reporting suspected COVID-19 symptoms to an app,” the article said. “Of these 400,000 people, 1,702 said they had been tested for COVID-19, with 579 receiving a positive result and 1,123 a negative one. Among the ones who had coronavirus infection confirmed by a positive test, three-fifths (59%) reported loss of smell or taste.”
The article stressed that “continuous cough or high temperature” remain the most serious symptoms for which to look and that loss of these senses is not a specific indicator of the coronavirus—other respiratory infections share them as well. Coronavirus or not, it raises the question: How do these senses function on a biochemical level?
Sensory Cells and Taste
Humans detect five main types of taste: salty, sweet, sour, bitter, and savory. What purposes do they serve in the survivalist sense?
“Sweet tastes signal detection of energy-rich carbohydrates, which is why we are drawn to sweet food,” said Dr. Kevin Ahern, Professor of Biochemistry and Biophysics at Oregon State University. “Salty tastes encourage us to obtain the sodium and potassium necessary to maintain water and salt balances. Sour tastes may indicate spoilage for detecting foods past their prime.”
Dr. Ahern said that many poisonous substances are bitter, so we may have such a strong reaction to bitter foods from a life-saving trait of evolution. Finally, savory foods taste as such due to the presence of amino acids like glutamate and aspartate, which indicate nutrients that are rich in protein.
Contrary to previous thought, each taste bud can sense multiple flavors. This is because each taste bud houses 50 to 100 sensory cells. When it comes to how sensory cells work, salty and sour tastes are the easiest to explain.
“For detecting salt, sensory cells with a sodium channel known as ENaC depolarize when sodium hits them,” Dr. Ahern said. “This in turn opens voltage-gated calcium channels that initiate the action potential. As for all the senses, an action potential sends a message to the brain, which responds by giving you the sensation—in this case, a salty taste.”
Similarly, sour tastes result from acidity. The acid’s positively-charged protons enter the sensory cells and set off a similar action potential that’s sent to your brain, which responds that something sour is happening.
Passing the Smell Test
The sense of smell is known formally as olfaction. It serves a number of purposes, with or without food involved.
“It occurs as a result of binding of odorant molecules to olfactory receptor cells in the nasal cavity,” Dr. Ahern said. “Smells reach receptors by two routes. The primary one is via the nostrils; a second one is located where the roof of the mouth connects the throat to the nose.”
The receptor cells that help us smell things are located in our olfactory epithelium, which the National Center for Biotechnology Information defines as “the sheet of neurons and supporting cells that lines approximately half of the nasal cavities.” Dr. Ahern said it’s about the size of a postage stamp.
“Each olfactory neuron expresses a single kind of receptor,” Dr. Ahern said. “But each receptor can bind several different odorants—some weakly and others more tightly. A particular odorant may bind more than one kind of receptor—again, with different affinities.”
It’s because of this eclectic receptor-odorant binding that we can identify so many different smells.
Respiratory infections from the common cold to the coronavirus interfere with the biochemical machinations of these senses, inhibiting our abilities to properly smell and taste the world around us.
Dr. Kevin Ahern contributed to this article. Dr. Ahern is a Professor of Biochemistry and Biophysics at Oregon State University (OSU), where he also received his Ph.D. in Biochemistry and Biophysics. He has served on the OSU faculty in Biochemistry/Biophysics since the mid-1990s.