A matter of life and taste

 Illustration by Nichla Smith

Illustration by Nichla Smith

Taste is one of our basic human senses, and arguably among the most important. When someone asks that dinner party conversation starter, “If you had to live without one of your senses, which would it be?” most of us would immediately rule out losing our sense of taste. There’s a very good reason for this: Taste is not a mere luxury for enjoying food. It's also about keeping us alive.

We have five basic tastes: sweet, sour, salt, bitter and umami. Sweet and umami are appetitive tastes; they detect the 'desirable' components of food, the carbohydrates and the proteins that give us vital energy. Sour and bitter are aversive tastes and important for defensive eating, since natural poisons and spoiled foods often have bitter or sour tastes. Salt can go either way: low concentrations are appetitive and high concentrations are aversive. This encourages us to maintain an appropriate electrolyte balance in the body.

“Our chemical senses are gatekeepers about what we should and shouldn’t put in our bodies,” said Alex Russell, a psychologist from University of Sydney and Southern Cross University. According to Dr Russell, taste combines with other senses to give us the perception of flavour in food and beverages.

“People often mix up taste and flavour,” he said. “We say we are tasting something when actually we’re looking at flavours. Flavour is the combination of taste with smell and other senses, such as irritation, temperature and texture.”

So when you eat a white jellybean, it doesn't really taste like vanilla. The taste is sweetness, and the vanilla component comes from the smell. Try pinching your nose next time you eat a jellybean, and the sweetness will become more apparent; it will be harder for you to tell which flavour you are eating. It’s a fun trick for dinner parties.

  Jellybeans — many flavours, but they all taste basically the same.   D. Davis/Flickr  (CC BY-NC-ND 2.0)

Jellybeans — many flavours, but they all taste basically the same. D. Davis/Flickr (CC BY-NC-ND 2.0)


If taste is the gatekeeper for what we put in our bodies, why do we all taste differently? The first point of variation is in our genes. Just as we all have a different combination of genes for our eye colour, hair type, and height, we also have different versions of all our taste receptor genes. How strongly you taste is further modified by the density of taste buds on your tongue.

Interestingly, there is less variance in the appetitive tastes — sweet and umami taste receptors are coded for by only three genes. This makes sense from an evolutionary point of view, said Eugeni Roura, a researcher in nutritional chemosensing from University of Queensland. “Sweet has evolved to identify and ‘like’ simple sugars as a good energy source,” he said. 

In prehistoric times when food was scarce, liking high energy foods helped us survive and would have been selected for. So while gene variations can produce differences in our sensitivity, we all have some liking for sweetness. Disliking sweetness would have no clear evolutionary benefit.

The aversive bitter taste is much more diverse; there are more than 30 different bitter taste receptors. Bitter taste may have evolved to protect us from poisons, and plants may have evolved to produce bitter compounds to stop us from eating them.

Despite this, some people inherit genes that classify them as bitter 'non-tasters.' These variations are more common in African populations where malaria is historically endemic. In those regions, bitter non-tasters might have a survival advantage, since they're more likely to consume bitter-tasting plants containing anti-malarial chemicals called quinines.

Differences in tastes are also seen across species and reflect the way they eat. For example, birds lack sweet taste receptors, since they swallow their food whole and so aren’t going to be encouraged to eat food by its sweet taste.

  The bitter-tasting broccoli is a divisive vegetable among children and adults alike.   Steffen Zahn/Flickr  (CC BY 2.0)

The bitter-tasting broccoli is a divisive vegetable among children and adults alike. Steffen Zahn/Flickr (CC BY 2.0)


But genes are only the start of the story. We also learn to like or dislike flavours throughout our lives. One of the most important things your mother ever did was teach you what foods are safe and good to eat. You might be surprised to learn that this started well before she gave you your first little spoonful of solids.

At about 12 weeks' gestation, foetuses start swallowing amniotic fluid. Taste and smell become well developed about 10 weeks later, well before the foetus can eat or drink anything on its own. Evidence suggests that flavours from the mother's diet get into this fluid. For instance, Julie Mennella and colleagues from the Monell Chemical Senses Centre gave pregnant women garlic or sugar capsules, then took samples of amniotic fluid for volunteers to smell. Volunteers were almost always able to pick out the garlic-eating mothers.

So the baby is certainly being exposed to flavours, but how do we know they are 'tasting' them?  In controlled experiments, researchers gave mothers certain foods during pregnancy, and then later tested the babies' reactions to these foods. You can tell how much a baby likes a particular food by monitoring how much they eat, how long they eat for, and their facial expressions while sampling flavours.

In 2006, Dr Mennella and colleagues gave one group of pregnant mothers carrot juice to drink regularly, while another group was told to avoid carrots in their diet. When their babies started eating solids, they were fed test preparations of cereal prepared with carrot juice. The babies whose mothers drank the carrot juice responded more positively to the carrot flavour than those who weren’t exposed before birth. Other research groups have used different foods and flavours with similar outcomes.

Pre-natal exposure to flavours could also influence the offsprings' behaviour later in life. For example, babies whose mothers reported drinking a moderate amount of alcohol during pregnancy responded more positively to the smell of alcohol than babies who were not exposed in utero. The same goes for salt. Mothers who reported eating lots of salty dry foods due to morning sickness had babies who preferred saltier solutions when offered different concentrations.

This training continues after birth. The carrot juice study also included a group of women who avoided carrots during pregnancy, but drank carrot juice while breastfeeding. Again, their offspring responded more positively when offered carrot flavour later. Similar research has shown that other flavours such as vanilla and mint are transferred in breast milk.

By sensing the dietary flavours in their mother's amniotic fluid and breast milk, said Dr Mennella, "the baby is learning the types of foods she is eating, what foods she has access to, and what foods she prefers.”

  Babies exposed to carrot juice early in development were later more agreeable to drinking carrot juice.   Mark Seton/Flickr  (CC BY-NC-ND 2.0)

Babies exposed to carrot juice early in development were later more agreeable to drinking carrot juice. Mark Seton/Flickr (CC BY-NC-ND 2.0)


Exposure to a diverse array of flavours in breast milk might also encourage a diverse diet. The longer children are breastfed, the greater variety of foods they have in their diet. One study showed that children who are breastfed are less fussy and more likely to try new foods. It's important to note that most of these experiments were performed with infants and toddlers, and it's still unclear how much these exposures change your tastes or dietary decisions in the long term. 

“The evidence suggests it is the first — but not the only — way we learn to like foods,” said Dr Mennella. “Infants are more accepting of flavours experienced in these first two foods, but the learning doesn't stop there. Regardless, if infants are breast or formula-fed, they learn through repeated exposure and variety”.

Taste preference is something we continue to learn throughout our lives. Researchers have shown that what toddlers eat depends more on their mother's current diet than what she ate during pregnancy. “It seems clear that flavour acceptance is mainly learnt," said Dr Roura. "Culture plays an important role."

One good example of an acquired taste linked to culture is the durian, a fruit popular in Southeast Asia. “To those who haven’t tasted durian, it smells revolting,” said Dr Russell. “To those who have grown up with it, it’s perfectly acceptable.”

For another example, look no further than your cup of coffee. Although a popular beverage for millions of people worldwide, few will confess to loving coffee at first sip. Over time, your tastes adjust as you learn to associate coffee's bitter aftertaste not with poisons, but with the good feeling that caffeine gives you.

Learnt tastes like these build throughout the years, decreasing the influence of genetics as we age. Think about the vegetables you hated when you were a kid, but now enjoy. This might be your body's way of protecting you. When you are young, your brain can’t tell the difference between the bitter flavours that are poisons and the bitter flavours that are healthy vegetables. Every time you eat them and don’t die, your brain learns the difference and that innate yuck response becomes less severe.  So if your kids won’t eat their veggies, don’t be cranky. Keep getting them to taste so they can learn. 

  Some people relish the potent flavour of the durian, whereas others are repulsed by it.   Mohd Hafizuddin Husin/Flickr  (CC BY-NC-ND 2.0)

Some people relish the potent flavour of the durian, whereas others are repulsed by it. Mohd Hafizuddin Husin/Flickr (CC BY-NC-ND 2.0)


The downside of ageing is that our sense of taste starts to decay. “The taste sensory cells in the tongue have a very short turnover,” said Dr Roura. “Age may slowly decrease the capacity to maintain the appropriate turnover, and so the number of taste sensory cells declines with age.”

Of course, the food choices we make are not all about taste. As we grow up, we learn lots of other things about food. We make our food choices based on perceived health benefits and habits, consciously choosing to eat comfort foods, social foods and traditional foods. So why does all this taste stuff matter? Other than your friends judging you for putting sugar in your coffee, or for your choice of beer, taste may also be linked to health outcomes. 

The strength of your tastes can influence your intake of certain foods. People who taste bitter less may benefit from eating more healthy bitter vegetables like broccoli and Brussels sprouts, or suffer from a higher likelihood of alcohol or tobacco consumption. If you are highly sensitive to sugar, you may consume fewer sweets. People with a high density of taste buds might avoid full fat foods, as they find them less pleasurable.

These habits, directed by your taste preferences, could affect your chance of getting diseases with diet-related risk factors, such as diabetes, cardiovascular disease and colon cancer.  How much genes matter compared to learning and conscious decisions is still an ongoing point of research. But our tastes may go some way towards explaining why some find it more difficult to follow a healthy diet. 

Our tastes likely evolved to help nourish and protect us, but the taste profiles that once gave us an evolutionary advantage might be mismatched to our modern environment. Mimicking your mother's diet is not ideal if it includes learning her bad habits, and our penchant for sweetness is not necessarily required in a calorie-abundant world. Next time you are eating, remember that it is your tastes that drive you, but we can continue to learn and adapt our tastes throughout life, so it is never too late to try those bitter green vegetables.