What we usually think of as salty taste is actually two flavors – low-concentration salty taste and high-concentration salty taste. Scientists are also exploring the mechanism of sensing high-concentration salty tastes. In fact, the more they looked into saltiness, the weirder it seemed.
Compiled | Chen Qiang
We generally think that our tongues can perceive 5 tastes: sweet, sour, bitter, umami and salty, but in fact, there are 6 types of taste because we have two independent salty systems. One system senses the low concentrations of salt that make foods like potato chips delicious; the other senses high concentrations of salt — enough to make the food disgusting and prevent a person from consuming it further.
Over the past 20 years, we have solved many taste perception mechanisms. For sweetness, bitterness and umami, the sensing mechanism is that receptor molecules on certain taste bud cells will recognize specific molecules in food, and when activated, will trigger a series of reactions, ultimately transmitting the signal to the brain, allowing us to Taste the difference. The sour taste is slightly different. Scientists recently learned that it is detected by taste bud cells that respond to acidity.
Saltiness is the most complex. Scientists have basically mastered the secrets of the low-salt taste system, but they know less about the high-salt taste system and which taste bud cells are responsible for detecting high salt. However, studying saltiness is not only a matter of scientific curiosity, but also because a high-salt diet can pose cardiovascular risks for some of us, so understanding this process is important.
Which molecule makes us feel salty?
When we consume too much salt, the body attempts to dilute the salinity in the blood by retaining water. For many people, extra water can cause an increase in blood pressure. Excess water can also put pressure on arteries, which can damage them over time and increase your risk of heart disease or stroke.
However, our bodies require a certain amount of salt to maintain the normal functioning of tissues such as muscles and nerves. For example, eating too little salt can cause nausea and muscle twitching, and if continued for too long, can lead to shock or death. This is why athletes often drink drinks containing electrolytes to replace lost salt.
To precisely regulate sodium levels in the body, the body controls how much sodium is excreted in the urine and how much sodium is taken in through the mouth. Our two systems for sensing salty taste can help us maintain a balanced intake of sodium.
Scientists have discovered that there are “ion channels” (pore-forming proteins) in various parts of our bodies that allow sodium ions to pass through nerve cell membranes, thereby generating nerve impulses. But they speculate that the taste bud cells in our mouths must have some specialized mechanism to sense salty tastes.
Scientists discovered an important clue to the salty perception mechanism in the 1980s. At that time, they used a drug that blocks sodium from entering kidney cells. The drug, when applied to mice’s tongues, blocks their ability to detect salty tastes. Research shows that kidney cells use an ion channel called “ENaC” to absorb excess sodium from the blood, helping to regulate blood salt levels. This finding suggests that taste bud cells that sense salty taste also use ENaC.
To test this hypothesis, some scientists used genetic engineering in 2010 to make the taste bud cells of some mice lack ENaC. Experiments showed that these mice no longer had their normal preference for low-salt solutions, confirming ENaC as a receptor for low-salt taste.
Which taste buds are responsible for detecting moderate saltiness?
But to truly understand how low-salt taste occurs, scientists also need to know how sodium enters the taste buds and is converted into neural signals, giving us the feeling of “yum, salty!” To uncover how this signaling occurs, scientists need to find where the signal begins in the mouth.
The answer seemed obvious: the signal should come from a type of taste bud cell that contains ENaC and is sensitive to moderate sodium levels. But these cells are hard to find. Research shows that ENaC is made up of three different components, and although individual components of ENaC can be found throughout the mouth, scientists have had a hard time finding cells that contain all three components.
In 2020, a team at Kyoto Prefectural University of Medicine in Japan announced that they had found sodium-sensing cells. They first hypothesized that sodium-sensing cells would produce electrical signals in the presence of salt, but not in the presence of ENaC blockers. Eventually, they found a group of cells in the taste buds in the middle of the mouse tongue that were able to make all three components of ENaC.
Specifically, when there are enough sodium ions on the outside of these taste bud cells, these ions can enter the taste bud cells through the 3-part ENaC channel. This rebalances the concentration of sodium ions inside and outside the cell, but it also changes the distribution of positive and negative charges on both sides of the cell membrane. This change triggers electrical signals within the cells, and the taste bud cells send a message to the brain: “Yum, salty!”
The origin of the overly salty taste remains an unsolved mystery
However, this system cannot explain why we sometimes think, “Bah, that’s too salty!” This feeling usually occurs when we taste something that is more than 2 times saltier than blood.
Some studies suggest that another component of salt — chloride ions — may be the key factor. We all know that the chemical composition of table salt is sodium chloride, but when dissolved in water, it breaks down into positively charged sodium ions and negatively charged chloride ions. Chlorine combined with sodium creates the perception of a high-salt taste, while sodium combined with other larger, polyatomic anions produces a lighter taste. This suggests that chloride ions may be an important contributor to the salty taste, but scientists don’t yet have a clear answer as to how it causes the salty taste.
In 2013, scientists at the National Institute of Dental and Craniofacial Research discovered a clue while studying mustard oil. They report that a compound in mustard oil reduces the sensitivity of mice to high salt levels. Strangely, the mustard oil compound also nearly eliminated the mice’s perception of bitter taste, as if the high-salt system was attached to the bitter system.
Even stranger, the study also found that cells that sense sour taste also appear to respond to high salt. If mice lack either of the bitter or sour taste systems, they are less repelled by extremely salty water. If both systems are missing, rats happily drink extremely salty water.
The findings aren’t agreed upon by all scientists, but if they are confirmed, it raises an interesting question: Why don’t extremely salty things also taste bitter or sour? Some scientists think this may be because the salty taste is the sum of multiple signals, rather than a single signal.
Although research on mustard oil has provided some clues, scientists have so far not found the receptor molecule responsible for sensing the taste of high salt. In 2021, a Japanese team reported that cells containing TMC4, an ion channel that lets chloride ions enter cells, produced signals when exposed to high salt levels in lab dishes. But when scientists created mice without TMC4, their aversion to extremely salty water did not change significantly.
Even more troubling, we can’t be sure whether rats perceive salty taste the same way humans do. Humans can apparently distinguish between pleasant low-salt tastes and offensive high-salt tastes, and, like mice, the ENaC receptor appears to be involved. But studies of ENaC blockers in humans have been confusing—sometimes the blockers seem to diminish salty taste, and other times they enhance it.
One possible explanation is that humans have a fourth ENaC part that mice don’t have, called a delta subunit. It could replace one of the other components, perhaps forming an ion channel that is less sensitive to ENaC blockers.
In short, until now, scientists haven’t figured out how our tongues detect salty tastes. Scientists hope to solve this mystery in the near future. They even hope to develop better salt flavor enhancers or substitutes that can create a “tastier” taste without compromising health. But obviously, scientists still need a lot of research to do this.