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That's too salty! How the tongue knows

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Eating salt, like many things, is best done in moderation. And now scientists have found how taste receptors can tell the difference between pleasantly savory and too salty.

Humans and other animals can detect five basic tastes — sweet, sour, bitter, salty and umami (a savory or meaty flavor). Of these, sweet and umami flavors are innately appetizing, whereas sour and bitter ones are instinctively unappetizing. Saltiness is enticing up to a certain amount, and beyond that it becomes repulsive. 

Large amounts of sodium trigger the same avoidance mechanism as bitter and sour foods do, helping prevent animals from the health effects of too much salt, researchers report Feb. 14 in the journal Nature.

"All animals need to have sodium — it's an essential ion, used in just about every cell in the body," study co-author Nicholas Ryba, a biochemist at the National Institutes of Health, told LiveScience. But, "too much salt is bad for you, so the taste system appears to have evolved so that low concentrations of salt are attractive, whereas high concentrations become extremely unattractive," Ryba said.

Here's how the taste system works: Teensy taste receptors on an organism's tongue contain ion channels, which are porelike structures that let charged particles such as sodium and chloride in and out. These receptors activate taste nerves, which carry the "taste" information to the brain.

While previous studies have shown that animals have special taste-receptor cells to detect low levels of salt (sodium chloride), how they detect excessive amounts of salt was a puzzle. [The 7 Other Flavors Humans May Taste]

To investigate this in mice, the researchers blocked different taste receptors and measured how taste nerves responded to salt overload. When the scientists blocked the bitter taste receptors (using a compound found in mustard oil), the nerves responded less to high salt, too; in other words, the nerves were less likely to carry the salty taste information to the brain.

Similarly, in genetically engineered mice that lacked the ability to detect bitter taste, the animals couldn't detect high salt very well, either. The same response to high salt was observed in mice that lacked a sense of sour taste.

The findings suggest that both bitter- and sour-sensing mechanisms are involved in sensing high sodium levels.

Next, the researchers gave a "licking test" to the mice lacking bitter and sour taste receptors. When given water with varying amounts of salt, these genetically engineered mice readily licked the very salty water as well as the low-salt water. Meanwhile, normal mice licked the lightly salted water but avoided the more salty water.

Animals may have evolved to use sour- and bitter-sensing pathways to make sure they avoid the health consequences of eating too much salt, the researchers say.

Of course, mice are only "models" for human taste. The findings are certainly intriguing, neuroscientist Robert Contreras of Florida State University, who was not involved in the study, told LiveScience. "The interesting problem is, if high salt is part of an aversion pathway, why do humans like high salt?"

More studies are needed to answer that question.

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