How are taste and smell connected?

Have you ever thought about what it would be like to lose your sense of taste and smell?

Imagine waking up not being able to taste your breakfast, or smell the grass that's just been mowed outside. It may be that you wouldn't notice it as quickly as you would if you lost your sight or hearing, but your experience of many things you know and love would be different.

Well, for some Covid-19 sufferers, this became their reality when they contracted the virus.

The loss of smell and taste is now included in the WHO's and UK government's top three warning signs that indicate you may have the novel coronavirus.

The other two symptoms to look out for are a fever and a new, continuous cough

It’s a very common symptom among sufferers - one study in the US found that Covid-19 patients are 27 times more likely than people without the virus to lose their sense of smell, which would make it a better predictor of the illness than fever.

And it doesn't just happen to Covid sufferers - it is a common symptom of a number of viruses, and the loss of both smell and taste seem to often come hand in hand.

So why are these two senses linked in a way that touch and sight aren’t, for example? We spoke to some people in the know to find out.

Similar but different

Abigail Walker is an ear, nose, and throat specialist, and co-wrote a paper on the loss of smell and Covid-19 with senior author Claire Hopkins and other specialists. Abigail explained that smell (olfaction) and taste (gustation) are two separate processes.

“Taste sensation comes from special sense cells called taste buds that are scattered around the tongue, roof of the mouth, and back of the throat. These taste buds can detect five things: whether something is sweet, salty, bitter, sour, and umami (a kind of rich meaty taste).

Taste and smell are separate chemical processes, but are very closely linked

"The smell sense comes from little nerve branches that are hidden away deep inside the roof of the nose and at the back of the nose. These tiny nerve endings are incredibly sensitive and can detect a wide range of different smells. The nerves then deliver the impulse inside the brain where the message is then relayed to many different centres such as appetite and pleasure.”

But we all know that when we hold our nose when we’re trying to eat something, it usually tastes very different, if of anything at all. So why is that?

Nina Hill can answer that. She works at charity Fifth Sense, which helps people affected by smell and taste disorders:

“The nose enables you to smell smells in the air, but it's connected to taste because without the nose, flavour is missing."

So if you eat some chocolate but your smell is impaired, you'll be able to detect that what you are eating is sweet, but you won't be able to make out its chocolatey flavour. It will be a similar sensation to if you ate honey, or anything else sugary.

“Essentially, the nose, the airflow and the olfactory bulb (a section at the front of your brain that detects smell) enable the eater to interpret flavour.”

Hand in hand

This is why when you hold your nose, your food tastes strange - you’re experiencing one of the five tastes, but without any distinguishing flavour.

But why has the brain evolved to do this? Why has it established this link between these two otherwise separate senses?

Charles Spence, a professor in experimental psychology at the University of Oxford can explain this for us.

“Ultimately it’s tastebuds that tell us whether something is sweet, energy dense, bitter or potentially poisonous. Our taste buds are the ultimate arbiter of what we should be eating.

“And yet we don't have time (and it's too dangerous) to go around putting everything in our mouth and see what it tastes like. What our brain is doing is trying to predict how things are going to taste.”

Through experience, we come to learn that bad smells usually means bad food

Essentially, we learn through experience. Charles says that we aren’t born thinking things smell good or bad, and therefore taste as such, but we learn as we go along.

It’s not just negative experiences that cement this in our psyche either; positive ones have the same effect. This is why, for example, we may hate the taste of coffee when we’re younger but grow to like it when we get older. At first, it’s bitter and there doesn’t seem to be much point to it. However, when you eventually experience the effects of caffeine, you might look forward to having it. From there, the smell and taste might begin to be a lot more pleasant.

Lost but not forgotten

Because of this, losing your sense of smell (or anosmia as it’s officially called), and therefore your perception of flavour, can negatively impact your mental wellbeing.

Nina from Fifth Sense explains:

“You think it's a sense that you're always going to have, and it's not always as obvious as potentially losing the sense of sight suddenly. The impact can be more slow growing… you don’t realise how powerful it was and how important it was until you don’t have it anymore.

“Because eating and drinking, and being able to enjoy a bunch of flowers, and the fragrance from our favourite perfume… they're all life enhancing. They all make life enjoyable and not being able to do that anymore could be really distressing for people.”

Eating is also a very social activity – it’s not uncommon, if you’re eating something yummy, to encourage your pals to try a bite. But it can be quite upsetting if one of you can’t taste any flavours. If that occurs, Nina suggests experimenting with texture and colour to keep mealtimes enjoyable.

Meal times can be difficult if you can't enjoy your food in the same way as everyone else

You’re also still able to technically taste food, in the five ways we mentioned earlier. The loss of taste, called aguesia, is very rare, and has no effect on your ability to smell. In fact, while people who contract Covid-19 usually report a loss of both taste and smell, Charles says it’s thought it’s actually exclusively the latter.

Aguesia can still be very scary and upsetting though - if you can smell garlic bread cooking but can’t make out the taste when you eat it for example, this can be quite a shock.

While some Covid sufferers are now reporting long term disruption to smell, there are also some more encouraging stories. You can quite often get your sense of smell back after Covid, or any other illness, Abigail says.

“You can actually train your nose to start smelling again by using essential oils to try and trigger the nerves to regrow. This may help some patients to recover their sense of smell.

“As with all symptoms of Covid, it seems that younger children are less prone to severe problems compared to older adults.”

And those who are born with no smell (which is called congenital anosmia) find ways to experience the flavour of food over time, with experience - just in a different way to others with a sense of smell.

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