The Science of Taste and Smell & the Covid-19 Connection
What can your sense of taste and smell tell you about your health? The answers may surprise you.
OSU Food Science Professor Juyun Lim studies how people experience taste, smell, and other related chemosensory stimuli. Her research focuses “primarily on understanding how human taste and smell work and the role of underlying human taste and smell perception and the role of human sensory perception in food preference,” she said.
Like many scientists at Oregon State University and beyond, the COVID-19 pandemic has both impacted her research and also opened up additional areas of study.
The COVID-19 Connection
Sometimes a person with COVID-19 experiences a sudden loss or reduction in smell or taste. It may be an early symptom, or even the only noticeable symptom. While smell or taste loss may seem minor compared to other symptoms, like extreme shortness of breath, many still suffer as a result.
“Those people are miserable,” said Lim. “Imagine you feel barely anything while eating meals or drinking something, let’s say a coffee. Imagine you cannot smell a rose in your garden or your loved ones.”
In addition to the loss of pleasure, there are potential safety implications. Imagine you are cooking and forgot to turn off the burner before serving your meal. Most people would notice the smell of something burning before a fire started, opposed to someone who’s lost their sense of smell.
Lim recommended that people pay attention to the tastes and scents they encounter every day. Some people already track their daily temperature – taking a moment to focus on daily tastes and scents is another self-tracking technique.
However, COVID-19 symptoms vary and some people experience neither fever nor loss of olfactory sensations
COVID-19 & Chemosensory Systems
Early on, individuals and doctors reported instances of smell or taste loss in patients, but the CDC and other health authorities were slow to officially add it to the list of recognized symptoms. For example, the CDC added these to the symptom list around April 22, whereas the United Kingdom’s Department of Health and Social Care added it around May 17.
Lim said colleagues in the field reached out to her in March, inviting her to join an international consortium of clinicians and research scientists who are working together to better understand how COVID-19 impacts smell and taste. This group is known as the Global Consortium of Chemosensory Research (GCCR) and consists of 484 members from 54 countries.
The GCCR created an international questionnaire which is now available in 30 languages. Lim led the team who translated the survey into her native language, Korean.
“Within a month, over 100 people worked together and published one paper,” said Lim. “Every day is a new day – it is hard to keep up sometimes… This is the fastest moving research I’ve participated in.”
Although a study like this typically takes over a year before researchers submit their findings to peer reviewed publications, the GCCR is practicing “open science” and has already submitted one manuscript with preliminary results to a journal, and are working on a second manuscript at present.
The GCCR is still seeking respondents who have or recently had a respiratory illness of any kind, including a cold, COVID-19, or the flu. If you fit this category and wish to participate, visit the GCCR page and choose your preferred language.
So far, more than 30,000 people worldwide have completed the survey. The information collected is powerful, thanks to the large and growing number of responses. Previously, Lim explained, researchers had anecdotal evidence based on small populations.
According to Lim, the exact mechanism for COVID-19 smell or taste loss is still unknown. One hypothesis is that COVID-19 may disrupt sensory-neural mechanisms. To understand why, it helps to first learn the basics.
People with COVID-19 may experience anosmia (loss of smell), ageusia (loss of taste), hyposmia (reduction in smell), or hypogeusia (reduction in taste).
Some people with a sensory dysfunction experience a distortion in smell (parosmia) or taste (dysgeusia). For example, a person might sniff a rose and perceive a completely different scent. Lim said people with COVID-19 don’t seem to experience such distortions in smell or taste. A range of other conditions, including head injury, radiation for treatment of head and neck cancers, and neurological conditions such as Parkinson’s disease or Alzheimer’s disease can alter taste or smell in various ways.
Over time, people who experience taste or smell loss from COVID-19 appear to recover their senses as they heal.
Lim said that recovery times vary widely. Some report recovering within a week, while others may require a couple of months or longer. Individuals may even start to recover, then relapse.
The Sixth Taste Sense
Another area of Lim’s research has attracted nationwide attention in recent years. Through her research, she identified a potential sixth sense of taste: starch. Yes, for those attracted to carbs, there may be an evolutionary reason.
Lim explained that taste helps animals and humans identify nutrients and discriminate against consuming toxins. For example, many natural poisons taste bitter so people starting in infancy are typically repulsed when they first experience unfamiliar, bitter tasting foods. This finely tuned survival instinct may explain why many children don’t like vegetables such as brussel sprouts.
There are currently five recognized taste categories: sweet, salty, sour, bitter, and umami. The newest among these recognized tastes, umami is often described as “a pleasant savory taste.”
Some examples include foods high in the amino acid glutamate, like Parmesan cheese and miso soup. The existing tastes are well-studied and scientists have already identified the exact receptor for each.
The recognition of starch as a potential taste is not yet universally accepted. However, Lim’s previous research attracted interest from mainstream media outlets like the BBC, OPB’s “Think Out Loud,” and German National Public Radio – in addition to her scientific journal publications. She’s also received a $1.3 million National Institutes of Health (NIH) grant for this line of research.
“Scientists thought people cannot taste starch,” said Lim. “That is kind of absurd since starch is the biggest proportion for our caloric intake.”
Lim theorizes that survival instincts cause humans to detect and enjoy the taste of starch. She cited National Geographic’s “What The World Eats” project. To varying degrees, worldwide grains – including complex carbohydrates – make up the largest portion of calorie intake.
However, Lim didn’t set out with the intention of discovering a new type of taste. She said several years ago, she was studying how people learn to like or dislike certain vegetable flavors. This research involved human subjects who participated in flavor learning tests.
At some point, her research group wanted to see whether adding calories would enhance this learning process. They added maltodextrin, which was considered “a tasteless caloric substance.”
“While preparing for that study, we noticed it has a taste,” she said, “so we dug into it.”
A New Type of Taste
Discovering a new type of taste involves uncovering first whether people can taste starch-based materials and if so, what are the mechanisms?
Compared to sugar, starch is a large molecule. However, an enzyme in human saliva quickly starts breaking it down into shorter chain saccharides. This happens almost the moment starch enters the mouth, said Lim.
Within Lim’s lab, human subjects participate in modulated and controlled tests, without knowing what researchers are looking for. Subjects describe the taste of starch-based molecules as bread-like or potato-like.
Unfortunately, this area of Lim’s research has been impacted by COVID-19. Her new NIH funded research was going to start on April 1, but is delayed as she figures out how to conduct the tests safely, while honoring social distancing standards.
Learning Alters Preference
Food preferences change as a person matures. As mentioned, many children originally dislike bitter tasting foods. From a survival point of view, this makes sense because bitterness can indicate that a plant is not safe to eat. The flavor serves as a natural protection for the plant.
Family preferences, exposure, and culture are among many factors that influence which foods an individual learns to enjoy. For example, a child who is regularly served kale and brussel sprouts may become an adult who craves these vegetables. Over time, they build positive associations with the flavors, after realizing they can eat these vegetables safely and even feel good afterwards.
Lim explained that vegetables have beneficial micronutrients like vitamins and minerals.“In the long term that is very good for us. People feel good about [eating them] and we learn to like it. But, it’s a learned liking, it’s not innate,” said Lim. “Sugar is an innate liking. A one day old baby would love to drink sugar water but not vegetable juice.”
This learning process may even begin while a fetus is still in the womb. Lim mentioned numerous studies by scientists that point to the existence of prenatal flavor learning. The studies indicate that when a pregnant woman regularly eats a specific vegetable, the child often learns to enjoy it later, after they are mature enough to eat solid foods. The theory is that some flavors and scents are in the amniotic fluid swallowed by the fetus. Similar studies over the years have found that young children tend to like the vegetables their mothers consumed while nursing.
Lim explained this learning process happens throughout an individual’s life. One common example is how often a teen or young adult tries black coffee for the first time, and dislikes it due to the bitter flavor. If they continue to drink it, however, they often start to enjoy it, as they associate the scent and flavor with the sensation of alertness.
Of course, this over-simplifies the complicated combination of factors that influence food preference.
“There are many possible implications and various layers. Understanding what we taste and how we taste itself is hugely important,” said Lim.
Learning more about how our senses influence our enjoyment of foods has other potential health implications, such as preventing or managing conditions like obesity and type two diabetes.
“Our taste and smell systems are designed to help us get foods that provide nutrients,” said Lim. “Sugar, carbs and fat all taste pretty good… We need to understand how it works first in order to have strategies to modulate consumption.”