ARI SHAPIRO, HOST:
Food and drinks that are really salty can be appealing one day and off-putting the next. Now scientists think they know why. NPR's Jon Hamilton reports on a study that found two separate brain circuits that influence our taste for salt.
JON HAMILTON, BYLINE: Our relationship with salt is complicated. Yuki Oka, a scientist at Caltech, says sodas, sports drinks and even tap water all contain a little salt, also known as sodium chloride.
YUKI OKA: You enjoy low sodium water, but if you imagine very high concentration of sodium like ocean water, you really hate it.
HAMILTON: Unless your body is really low on salt. That's pretty rare in people these days. But Oka says experiments with animals show that when salt levels plummet, the tolerance for salty water goes up.
OKA: If your body needs sodium, then animals immediately start liking ocean water.
HAMILTON: They crave sodium, and they can tolerate it in high concentrations they would normally avoid. Oka wanted to know how this system works in the brain, so he and a team of scientists studied mice. They showed that one set of neurons toward the back of the brain regulates the craving for salt.
OKA: If you stimulate these neurons, then animals run to sodium source and then start eating.
HAMILTON: Another group of neurons toward the front of the brain normally sets an upper limit on salt tolerance, but when salt levels get low enough, Oka says, these neurons get switched off.
OKA: This means that the sodium craving and the sodium tolerance are controlled by completely different types of neurons.
HAMILTON: The finding, which appears in the journal Cell, is part of a growing field of study called interoception. It deals with internal sensations like hunger and pain. Stephen Liberles, a cell biologist at Harvard Medical School, says scientists already know a lot about how the brain deals with sensory information coming from the eyes, ears, nose and skin.
STEPHEN LIBERLES: The brain also receives tons of sensory information from the body, from the heart, the lungs, the stomach, the intestine, and how these work has remained more mysterious.
HAMILTON: The new study suggests that brain cells involved in salt tolerance are controlled by hormone-like substances called prostaglandins. These substances, which circulate in the bloodstream, are best known for their role in causing inflammation, fever and pain. Liberles says it now appears that prostaglandins also play a role in salt tolerance.
LIBERLES: So the question is, how is the same chemical, the same prostaglandin molecule re-used across biological systems in different contexts?
HAMILTON: Answering that question might make it possible to develop a prostaglandin drug to discourage salt overconsumption. Nirupa Chaudhari of the University of Miami says we tend to eat too much salt because evolution prepared our bodies for a world in which salt is scarce.
NIRUPA CHAUDHARI: Wars were fought over salt just a few centuries ago. So we think of sodium chloride - table salt - as so plentiful in our diet and in our environment but it wasn't always.
HAMILTON: Chaudhari says too much salt can lead to high blood pressure and heart disease.
CHAUDHARI: Salt ingestion is a major issue. Calorie ingestion is a major issue. So it becomes really critical to understand how all of these different systems work.
HAMILTON: She says understanding how the brain processes saltiness could help food companies develop a palatable salt substitute. At least one previous effort failed badly.
CHAUDHARI: It tasted really foul, so people didn't want to use it.
HAMILTON: Chaudhari says finding a better option may require more research on not only how the brain monitors salt intake but how it interacts with our taste buds. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.
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