You know water’s great for you, but do you know when you’ve had enough?
08/11/2019 // Zoey Sky // Views

Staying hydrated is crucial for your overall health. But how does the human body know when you’ve had enough water?

According to a 2019 study published in the journal Nature, an element in the gut could have a role in predicting when you’re already hydrated.

"Thirst signals" in the brain

For the study, researchers ran tests on mouse models. Findings revealed that a unique element in the gut helps determine how much water your body needs. This element then alerts the brain, which decides how thirsty you should be.

In a previous study, researchers from the University of California, San Francisco, (UCSF) discovered that when mice drink liquids, their mouths and throats send signals to the brain. This shuts down the brain cells that dictate thirst.

These “thirst cells” are found in the hypothalamus, a region of the brain that regulates thirst, blood pressure, and other bodily processes. Thirst cells are also present in the subfornical organ, which sits at the top of the hypothalamus.

The mouth and throat send these signals a few seconds after you drink something. However, it often takes 10 minutes to one hour before the water enters your bloodstream and reaches the thirsty cells throughout your body.

The brain is tasked with turning off the signals at the right moment so you have enough time to get hydrated.

Zachary Knight, an associate professor of physiology at UCSF and one of the authors of the study, says that the brain knows how to strike a balance so that people can quickly drink the right amount of water to satisfy their body's needs.


Brain function and thirst neurons

For the 2019 study, Knight and his fellow researchers implanted optical fibers and lenses near the hypothalamus of mice.

Using these optical fibers, the scientists observed and measured when thirst neurons were turned on and off. When the mice received saltwater, the thirst neurons stopped firing almost immediately. However, after a minute, the neurons switched back on.

The throat and mouth send signals to the brain to begin quenching thirst, regardless of the type of liquid. However, since salty liquids can dehydrate the body, the researchers posited that the signal which turned the thirst neurons back “on” came from somewhere else, after the throat and mouth have already turned them “off.” (Related: Healthy and hydrated: How much water should you drink daily?)

To confirm their hunch, the scientists directly infused both salty and fresh water into the mice's stomachs to avoid the mouth and throat signals.

They found that while fresh water made the neurons stop firing, saltwater didn't. When saltwater-infused mice received fresh water to drink, the thirst neurons first switched off then quickly switched back on. These imply that certain molecules in the gut sense the salt content in liquids.

After measuring their salt content, it’s possible that these molecules can predict how much a drink will hydrate the body.

This system only seemed to work when the mice were truly dehydrated. The researchers believe that the system sends this information to the brain within a minute, making the thirst neurons quickly turn on and off.

Knight noted that aside from sodium, other compounds could also set off these gut molecules. Anything that changes the osmolarity of the blood is detected by this unique system. Osmolarity refers to how concentrated a liquid is.

Further studies are needed to confirm these findings on humans. If confirmed, they can help many individuals, especially the elderly, who have trouble staying properly hydrated. Dehydration among the elderly causes medical problems, particularly during extremely hot weather.

Drinking enough water helps the heart pump more blood through the blood vessels to the muscles. This helps the muscles work properly. Dehydration, on the other hand, is linked to various problems, such as headaches, swollen feet, and heatstroke.

The study helped the researchers learn more about the control of thirst. While mice and humans differ in certain brain structures, they have similar hypothalami. Since the results are consistent with data obtained from human brain scans, there’s a chance that some of the findings are also applicable to humans.

Another finding that the researchers reported is that thirst signals travel through the vagus nerve, the main signal highway between the brain and the gut. After cutting the vagus nerve in a different experiment, the researchers found that the thirst neurons didn't turn back on even when the mice started drinking.

This led them to hypothesize that the signal comes from the small intestine, which is closely connected to the vagus nerve. The small intestine is also in the right timespot in the digestive process to activate those thirst nerves within a minute or so of drinking water.

The research team said they will continue their research to discover the origin of this thirst signal.

Sources include:

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