ARI SHAPIRO, HOST:
And now we have an update for you on the science of thirst. NPR's Jon Hamilton reports on special brain cells that appear to tell us when we want just plain water and when we might crave one of those fancy sports drinks.
JON HAMILTON, BYLINE: When you're sweating through a long workout, your body loses minerals as well as fluid. And Yuki Oka of Caltech says that produces a special kind of thirst.
YUKI OKA: Pure water doesn't do it. It's not enough. You need water and salt to recover. And we can easily imagine that under such condition, we crave for sports drink.
HAMILTON: Which is basically a mix of water, salt and sugar - Oka wanted to know what triggers this craving. So he and a team studied the cells in two regions of mouse brains.
OKA: So those regions contain neurons that drive thirst.
HAMILTON: Oka's team induced two kinds of thirst in the mice. One was caused by a simple lack of fluid in the body. The other stimulated the loss of fluid and minerals that occurs during a sweaty workout. And Oka says each type of thirst produced a different response in the brain.
OKA: Our brain can detect these two distinct stimuli with different cell types.
HAMILTON: One type of brain cell was responding to a need for water alone. The other detected a need for salt as well. To confirm the finding, Oka's team did an experiment with mice.
OKA: We presented two bottles, one with pure water, another one with water plus minerals.
HAMILTON: Then the team used a technique called optogenetics to stimulate each type of thirst cell in the mice. Oka says when they stimulated the water-only thirst cells...
OKA: Then animal immediately went to water and it started drinking vigorously.
HAMILTON: But only from the bottle of pure water - next, the team stimulated the thirst cells that respond to salt levels.
OKA: Now mice start drinking both pure water and minerals.
HAMILTON: The mouse version of reaching for a sports drink - Sean Stocker of the University of Pittsburgh says the experiments, which appear in the journal Nature, represent a scientific tour de force.
SEAN STOCKER: They bring the most cutting-edge techniques to bear on this question that we've been trying to ask for decades and decades and decades.
HAMILTON: Stocker says the question goes beyond thirst. The cells in these brain regions appear to be involved in a wide range of bodily functions.
STOCKER: This study has implications not simply for fluid intake but clearly for other things such as blood pressure regulation.
HAMILTON: Too much salt and fluid in the body can cause high blood pressure, and Stocker says many of today's blood pressure drugs appear to be affecting cells in the same areas of the brain that regulate thirst. Claire Gizowski is a researcher at the University of California, San Francisco. She says this new understanding of how thirst works could lead to better health.
CLAIRE GIZOWSKI: Fluid balance is one of the top 10 factors that cause you to be admitted into a hospital when you show up to the ER.
HAMILTON: Some of those trips might be avoided if vulnerable people had some sort of sensor that worked the same way the brain does to monitor fluid and salt levels. Also, Gizowski says...
GIZOWSKI: This would be super-helpful for athletes to, you know, know how much Gatorade they're supposed to drink to, like, have optimal performance.
HAMILTON: Gizowski thinks the next frontier in thirst science is figuring out how the brain anticipates the need for fluid or salt.
GIZOWSKI: Sometimes, you drink, and you don't even realize that you're drinking or that you're even thirsty because your body is just doing these things without you knowing.
HAMILTON: She says that's probably to protect you from a fluid imbalance later on.
Jon Hamilton, NPR News.
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