The saltiness is surprisingly mysterious

We've all heard of the five tastes our tongue can detect: sweet, sour, bitter, salty, and umami. But it's actually six, because we have two different systems for detecting salt. One of them detects the appealing relatively low levels of salt that make potato chips taste delicious. The other has high levels of salt—enough to make foods that are too salty offensive and discourage us from consuming too much.

Exactly how our taste buds perceive the two types of salinity is a mystery that has taken some 40 years of scientific research to unravel, and researchers still haven't worked out all the details. In fact, the more they study the salt sensation, the stranger it becomes. In the last 25 years, many other details of taste have been unraveled. In the case of sweet, bitter and umami, it is known that the molecular receptors of certain cells in the taste buds recognize food molecules and, when activated, set in motion a series of events that end up sending signals to the brain.

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Acidity is slightly different: It's detected by cells in the taste buds that respond to acidity, researchers recently discovered.

In the case of salt, scientists know many details about the low-salt receptor, but the full description of the high-salt receptor still lags behind, as does understanding which cells in the taste buds house which detector.

"There are still many gaps in our knowledge – especially in the taste of salt. I'd say it's one of the biggest gaps," says Maik Behrens, a taste researcher at the Leibniz Institute for Food Systems Biology in Freising, Germany. "There are always pieces missing from the puzzle."

A Delicate Balance

Our double perception of saltiness helps us walk the tightrope between the two sides of sodium, an element crucial for the functioning of muscles and nerves, but dangerous in high amounts. To tightly control salt levels, the body manages the amount of sodium it expels in the urine and controls how much goes in through the mouth.

"It's the Goldilocks principle," says Stephen Roper, a neuroscientist at the University of Miami Miller School of Medicine in Florida. "You don't want too much; you don't want too little; you want just the right amount."

If an animal ingests too much salt, the body tries to compensate by retaining water so that the blood is not too salty. In many people, that extra volume of fluid raises blood pressure. Excess fluid overloads the arteries and, over time, can damage them and create the conditions necessary for heart disease or stroke.

But some salt is necessary for body systems, for example, to transmit the electrical signals that underlie thoughts and sensations. A lack of salt can lead to muscle cramps and nausea — that's why athletes take Gatorade to replenish the salt they lose through sweat — and, if long enough passes, shock or death.

Scientists looking for salt taste receptors already knew that our bodies have special proteins that act as channels to allow sodium to cross nerve membranes in order to send nerve impulses. But the cells in our mouths, they reasoned, must have some additional special way of responding to sodium in food.

A key clue to the mechanism emerged in the 1980s, when scientists experimented with a drug that prevents sodium from entering kidney cells. This drug, applied to the rats' tongues, impeded their ability to detect salty stimuli. It turns out that kidney cells use a molecule called ENaC to absorb extra sodium from the blood and help maintain proper blood salt levels. The finding suggests that cells in the taste buds that detect salt also use ENaC.

To prove this, the scientists created mice without the ENaC channel in their taste buds. These mice lost their normal preference for slightly salty solutions, the scientists reported in 2010, confirming that ENaC was, indeed, the receptor for the good salt.

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So far, so good. But to really understand how saltiness works, scientists also needed to know how the input of sodium into the taste buds translates into a "yummy, salty!" feeling. "It's what's sent to the brain that's important," says Nick Ryba, a neuroscientist at the National Institute of Dental and Craniofacial Research in Bethesda, Maryland, who was involved in the research linking ENaC to saltiness.

And to understand that signal transmission, scientists needed to find where in the mouth the signal started.

The answer might seem obvious: the signal would come from the specific set of cells in the taste buds that contain ENaC and are sensitive to tasty levels of sodium. But it wasn't easy to find those cells. It turns out that ENaC is made up of three different pieces, and while the individual pieces are found in various places in the mouth, scientists struggled to find cells that contained all three.

In 2020, a team led by physiologist Akiyuki Taruno of Kyoto Prefectural University of Medicine in Japan announced that they had finally identified sodium-sensing cells. The researchers assumed that sodium sensing cells would emit an electrical signal when salt was present, but not if the EnaC blocker was also present. They found a population of such cells inside taste buds isolated from the middle part of the tongue of mice, and it turned out that these made all three components of the ENaC sodium channel.

Scientists can now describe where and how animals perceive desirable levels of salt. When there are enough sodium ions outside of those key taste bud cells in the mid-tongue area, the ions can enter these cells using the three-part ENaC gateway. This rebalances sodium concentrations inside and outside the cells. But it also redistributes the levels of positive and negative charges across the cell membrane. This change activates an electrical signal within the cell. The taste cell then sends the message "yummy, salty!" to the brain.

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Too salty!

But this system doesn't explain the "ugly, too much salt!" signal that we can also feel, usually when we taste something that has more than twice as much salt as our blood. In this case, the story is less clear.

The other component of salt — chloride — may be key, some studies suggest. Remember that the chemical structure of salt is sodium chloride, although when dissolved in water it separates into positively charged sodium ions and negatively charged chloride ions. Sodium chloride produces the saltiest sensation, while sodium combined with other larger, multiatomic ions tastes less salty. This suggests that the sodium partner may be an important contributor to the sensation of saltiness, and that some pairs taste saltier than others. But as for exactly how chloride can cause the salty taste, "no one has a clear idea," Roper says.

One clue comes from Ryba and his colleagues' work with an ingredient in mustard oil: In 2013, they reported that this component reduced the high-salt signal on the tongue of mice. Interestingly, the same mustard oil compound also almost completely eliminated the tongue's response to bitter taste, as if the high salt detection system was leaning on the bitter taste system.

Stranger still, acidic taste cells also seem to respond to elevated levels of salt. Mice that lacked one or the other of the bitter or acidic taste systems were less discouraged by the extremely salty water, while those that lacked both happily sipped the salt water.

Not all scientists are convinced, but the findings, if confirmed, raise an interesting question: Why don't things that are too salty taste bitter and acidic as well? It could be because the overly salty taste is the sum of multiple signals, not just one entry, says Michael Gordon, a neuroscientist at the University of British Columbia in Vancouver, co-authored, with Taruno, of an analysis of the known and unknown of saltiness in the 2023 Annual Review of Physiology.

Despite the mustard oil clue, attempts to find the receptor molecule responsible for the high-salt taste sensation have been inconclusive so far. In 2021, a Japanese team reported that cells containing TMC4—a molecular channel that allows chloride ions to enter cells—generated signals when exposed to high levels of salt in laboratory dishes. But when the researchers created mice without the TMC4 channel anywhere in their bodies, there wasn't much difference in their aversion to extremely salty water. "There's no definitive answer at the moment," Gordon says.

As an added complication, there's no way to be sure that mice perceive salty flavors exactly the same way people do. "Our knowledge of the taste of salt in humans is quite limited," Gordon says. To be sure, people can distinguish desirable, lower levels of salt from the unpleasant, high-salt sensation, and the same ENaC receptor used by mice appears to be involved. But studies of the sodium channel blocker ENaC in people vary in confusing ways, sometimes appearing to decrease taste for salt, but other times boosting it.

One possible explanation is the fact that people have an extra fourth piece of ENaC, called the delta subunit, which rodents lack. This subunit may take the place of one of the others, which could result in a version of the channel that is less sensitive to the ENaC blocker.

Forty years after researching the taste of salt, researchers are still wondering how people perceive salt on people's tongues and how the brain sorts those sensations into "fair" or "excessive" amounts. There's more at stake than satisfying a scientific curiosity: Given the cardiovascular risks that a high-salt diet poses to some of us, it's important to understand the process.

Researchers even dream of developing better salt alternatives or enhancers that create that delicious taste without health risks. But it's clear that they still have work to do before they invent something that we can sprinkle on our plates without worrying about our health.

This article originally appeared in Knowable en español, a non-profit publication dedicated to making scientific knowledge available to everyone.

Article translated by Debbie Ponchner.

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