Why does salt taste so good?

10:11George:expression, a spoonful of sugar makes the medicine go down or helps the medicine go down. That's referring to taste, right? So sugar will help something bitter feel less bitter because of the interaction of those two things. And salt probably works pretty similarly, where it's enhancing certain qualities to make something feel more delicious than just salt or

10:33Marnie Chesterton:just say umami on its own. So do we know how salt does that, what the mechanism is for how that works?

10:40George:The easy answer is no, not at all. The more complicated answer is we think that there is some evidence that taste cells talk to one another, which would affect how much they're responding to a certain stimulus that's in the mouth, whether it be sugar or bitter or salty. So the addition of salt might affect the response of the taste bud to another quality. But it also could be happening more downstream in that pathway of information. So it could be happening at the level of the brainstem when the communication, the information gets there. It could also be happening in your taste cortex where the information gets there and cells could be interacting to modulate our perception.

11:27Marnie Chesterton:So the magical transformative power of salt remains a bit of a mystery. It could change the way our tastebud cells behave, or it could change the way we perceive their signals in the brain. But salt isn't just a seasoning. As Courtney says, it's also vital for our body to function properly. Could that be part of the reason why we find it so Moorish? Before we get to that, let's first return to our journey into the salt mine in Austria. The cart we rattled in on takes us almost two kilometres into the mountain. We then squeeze into a lift and ride up to a series of tunnels supported by wooden beams.

12:08Marnie Chesterton:Our guide is archaeologist Daniel Bradner from the Natural History Museum in Vienna.

12:13George:We are standing right now in the oldest salt mine of the world which is still producing. Salt mining began here 7 ,000 years ago, already in the Stone Age, and salt is being mined up to this day.

12:27Marnie Chesterton:Daniel takes us through more tunnels, deeper into the mine. I've completely lost my bearings at this point, and it's pitch black in here. But from the light of my head torch, I start to notice the walls glinting back at us.

12:40George:So this is salt? This is salt? Yeah, this is rock salt, like me.

12:44Marnie Chesterton:So if I would lick this, it would taste like salt?

12:46George:Yes, but down to this here, down to this here, maybe we'll find another place. Specialities of the salt deposit here, we call it Haslgebirge, and it's a mixture of salt, clay and anhydride. It's very inhomogeneous and has different colours, different textures.

13:12Marnie Chesterton:It takes me a while to realise we're standing inside huge veins of solid salt that are running through the mountain rock. So how did it all end up here?

13:23George:The salt we got here was formed due to evaporation of sea salt. So 250 million years ago, this place here was on the shallow side of a big sea. And parts of this shallow area were disconnected from the water supply from the sea. and in this hot and arid climate, the water began to evaporate, leaving the salt. Over thousands of years, large layers of rock salt accumulated here. So you got tectonic activities during that the Alps form. And so you got the limestone that's now shifted above these salt layers, deforming it, overlaying it, and sometimes, because the salt is soft in contrast to the quite hard limestone, it's pressed up to the surface.

14:25George:Prehistoric people could discover the salt deposit and mine it from here.

14:30Marnie Chesterton:Wow. So it was originally sea salt that is basically turned into rock salt and kind of melded with the rock of the seabed. Wow. Today, this landscape is no longer at the bottom of the ocean. In fact, Austria is a landlocked country and Halstatt is 200 kilometres from the sea. Which means the vast natural deposits of salt here have been an incredibly valuable resource for people over thousands of years. But why is that exactly? What makes salt such a vital commodity? That's what we'll be finding out next.

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16:12Marnie Chesterton:You're listening to CrowdScience from the BBC World Service. I'm Anand Jagatia, and this is the show that pays my salary to answer your science questions. That's not a weird brag, by the way. The word salary originally referred to an allowance given to Roman soldiers to purchase salt. And salt is what this episode is all about. Namely, why do we find it so delicious? Well, part of the answer is that we've evolved to crave it because it's so fundamental to our metabolism. Animals, including fish in the sea and land animals, including us, use sodium for a variety of purposes. It's critical for life.

16:52Marnie Chesterton:This is neuroscientist Joel Geerling from the University of Iowa in the United States.

16:57George:Somewhere on the order of a third of our daily energy expenditure is dedicated to pumping sodium out from the inside of a cell to the outside of the cell. Every cell in your body has a sodium-potassium pump in the outer lining of the cell, chugging away all day, pumping sodium ions out from the inside of the cell, and makes sodium want to rush back in. Creates this concentration gradient that works kind of like charging a battery.

17:21Marnie Chesterton:Having all that sodium outside our cells means that it naturally wants to rush back in, a bit like water being held back behind a dam. But our cells control the movement of sodium through special channels. When they open, the sodium floods in and our cells can harness the energy of that movement.

17:39George:And the cell can use this energy gradient to piggyback all sorts of other processes, transporting molecules into the cell, changing the voltage temporarily on the membrane. That's how neurons work and muscle cells work.

17:52Marnie Chesterton:Remember that sodium ions are charged. Well, this means that their movement into or out of cells can also be used to generate spikes of electrical activity, which is what happens every time a neuron fires in your brain.

18:06George:Sodium ions rush in and it causes a rapid and dramatic change in the membrane voltage that we call a spike or a firing event in the neuron. And that's just your brains, but also the muscle cells in your heart that keep you alive from beat to beat.

18:20Marnie Chesterton:So, I mean, that's incredible. So basically, if we didn't have sodium, our cells basically just wouldn't work. Is that the reason then why, as humans anyway, we have evolved to find salt so delicious because it's so important that we really need to make sure that we have enough of it?

18:38George:That's a very interesting question. I like how you phrased it too. Animals that live in the sea have plenty of sodium around them. And in fact, they have the opposite problem that land animals, including humans, have. They need to keep some of the sodium out and maintain an internal balance.

18:54Marnie Chesterton:So for land-dwelling animals, do different kinds of species have different appetites for salt?

19:00George:Yeah, that's a great question. I suspect that different kinds of animals do indeed have different kinds of salt appetite. So sodium is fairly scarce on land. If you live away from the ocean, especially if you don't eat meat, you'll have very little sodium in your diet. If carnivores eat other animal tissue, and animal tissue has about 0.9 % sodium chloride in it, just like we do, carnivores tend to get plenty of salt. But herbivores get near zero or zero sodium in their diet if you're eating just plants, just fruits and vegetables, which would be very high in potassium and have virtually zero sodium.

19:38George:So elephants are a famous example. There are elephant tribes in Africa that remember the locations of caves that have salt on the walls, and they'll tusk it off the walls. It's pretty well known that deer will be attracted to salt licks, and hunters use this to attract them. People who race horses keep salt licks around to maintain the health of their horses, and so on. So animals that eat a purely vegetarian diet need to have a source of salt and they'll seek it out and they tend to exhibit a stronger salt appetite in the wild even.

20:13Marnie Chesterton:That's so interesting. I mean, humans are omnivores, I guess, right? So we wouldn't, if we just ate whatever we were going to eat, we wouldn't get enough salt from our diet. And so that's maybe why we have this real appetite and craving for it.

20:26George:Yeah, there's a lot of salt in the human diet nowadays because it's added. We've established and procured ways of getting salt out of the ocean and out of the ground and adding it to our food and adding it as flavour. So the bigger problem in a lot of human societies now is moderating the amount of salt in the diet.

20:48Marnie Chesterton:So salt is vital for life. Without it, our cells would grind to a halt. As humans, we've evolved mechanisms to make us crave it and search it out so that we get enough of it. Today, most people have plenty of salt in their diet, but for our ancestors, finding enough salt would have been essential. And just like the elephants mining salt with their tusks, ancient humans would have been drawn to places like Hallstatt in Austria, which are natural sources of this precious mineral. Our guide, Daniel, has now taken us into one of the archaeological excavation sites, where they've been looking for traces of people mining here millennia ago.

21:27George:We are standing in part of a big mining chamber which was opened 3200 years ago in the late Bronze Age. Salt mining began here 7000 years ago in the Neolithic time. So we have early farmers and settlers who are going for the salt deposit here. They discover this salt deposit by natural brine springs on the surface, and then they start to dig deeper. In the Bronze Age, about 3 ,500 years ago, we find a completely evolved deep mining, which goes down more than 250 meters below surface, has organisatory structures, has special inventions for their tools. They have developed a transport system here and they're really going for big scale rock salt mining and are main supplier of a large part of Central Europe.

22:35George:Basically salt is a necessity just for surviving on one spot for a longer time. Being able to settle in the Alps all year round really needs salt to be able to get food in stock for the long winters, sometimes more than half a year.

22:53Marnie Chesterton:So even as long ago as that, salt was something that was really valuable and was traded across quite big distances between different people.

23:02George:So salt is a necessity there. And Hallstatt really plays the role of some kind of a monopoly in the brass age for a large part of Central Europe, supplying them with rock salt.

23:17Marnie Chesterton:So just how much of a force has salt been throughout history in this region?

23:29George:A very big one. Yeah, you need this. And the salt production here in this area transformed all this region.

23:42Marnie Chesterton:Prehistoric people living in this area and beyond would have used salt to preserve their food and provide salt licks for their animals to last them through the winter. And if they ran out of salt, the consequences wouldn't have been pleasant.

23:54George:The organs, all the cells will swell. And this is a particular problem in the brain because if the brain swells too much, it's stuck in the skull and it starts oozing out the bottom, the hole in the bottom of the skull called the foramen magnum, the big hole, which is very dangerous. So you can't let the sodium concentration get too low.

24:12Marnie Chesterton:Neuroscientist Joel, who we heard from earlier, researches how we regulate salt concentration. Part of that is to do with controlling the body's water content.

24:21George:This is something regulated by a hormone that's released called antidiuretic hormone, and that tells the kidneys how much water to hold on to. That's very tightly regulated from hour to hour, minute to minute, throughout the day.

24:33Marnie Chesterton:But that's not the only way our body can control salt levels. In fact, in his work, Joel has discovered mechanisms within the brain that drive salt-seeking behaviour. In my lab, we study a particular group of neurons.

24:46George:We call them the HSD2 neurons that detect the levels of a hormone called aldosterone. It's made in the adrenal glands when the volume of salt and water in the body is low enough that the heart is struggling a little bit to keep the blood pressure up. This hormone aldosterone goes up, and at least in rats and mice, this triggers the neurons to drive the animal to seek salt and consume more salt. So far, we've identified them in mice, in rats, in pigs and humans. And we haven't done a deliberate and careful survey of other species, but it appears that they're in mammals generally.

25:27Marnie Chesterton:Okay. And that's fascinating that we actually have neurons in our brain that are kind of dedicated to monitoring how much salt we are, but then also driving or initiating this salt-seeking behavior, right?

25:41George:Yeah, I think it's fascinating too. It's a highly specific appetite of behaviour. We haven't been able to find anything else that these neurons do as of yet. We'll keep looking. They don't affect your blood pressure or your breathing or anything else that we can measure. They seem to specifically drive the animals to consume more salt.

26:05Marnie Chesterton:So, George, you wanted to know why salt tastes so delicious. On one level, it's because it changes the way things taste. We don't exactly know how, but it could be that our salt receptors alter the behaviour of other taste cells, or the perception of taste in the brain. On another, it's delicious because it's vital for our cells, so we've evolved to crave it and find it tasty, in the right amounts. In fact, we even have neurons tuned to it in the brain that drive us to go and find it. Our hankering for salt is a deeply human desire, and it's one that shaped civilization. Back in Austria, at the mine, Daniel leads us back up to the surface on foot, to where people would have first discovered the salt hidden there and started digging for it.

26:51Marnie Chesterton:They probably had no idea that we'd still be doing that 7 ,000 years on.

26:57George:This now here is the high valley above Hallstatt. Down there, you can see the shoreline of Lake Hallstatt at the very bottom of the valley where we started from. And now here we are in the Ha Valley where the salt deposit could be accessed from the surface in prehistoric times.

27:18Marnie Chesterton:It's amazing coming through that tunnel into the blinking into the daylight because it's really hard to sort of conceptualize when you're underground where you actually are.

27:30George:And if you look to your left, to your right, you see these limestone rocks to the both sides here. And it's only in this small space in between where the rock salt hasn't been found.

27:45Marnie Chesterton:I'm never going to be able to put salt on my chips again without thinking of this place. Mission accomplished.

28:00Marnie Chesterton:George, I hope you felt this episode was worth its salt. Over to you for the credits.

28:06George:You've been listening to CrowdScience from the BBC World Service. The question was for me, George. The presenter was Anand Jagatia. The producer was Robbie Wojciechowski. If you've got a question you'd like the CrowdScience team to find the answer to, you can email crowdscience at bbc.co.uk. Thanks for listening. Bye.

28:34George:The United States is about to mark its 250th anniversary. And so on the Global Story podcast from the BBC, we're telling surprising tales of American influence on the world stage and in ordinary people's lives all across the globe. We have this ability to export our story and a lot of people have bought it. I feel like the American dream is alive, but not well. From the BBC, it's the United States at 250. Listen on BBC.com or wherever you get your podcasts.

29:04Marnie Chesterton:Hi, I'm Simon Jack. I'm Zing Zing. And together we host Good Bad Billionaire. The podcast exploring how some of the wealthiest people on the planet made their money. And we are back with a new season. From sporting superstars to music moguls and celebrity CEOs. We'll be taking a closer look at the lives and fortunes of some of the world's richest people. And asking you to decide if they're good, bad or just another billionaire. Good Bad Billionaire from the BBC World Service. Listen now or search for Good Bad Billionaire wherever you get your BBC podcasts.

29:37Thank you.