Imagine you are exercising outside on a hot day. You start to sweat and get thirsty. When you pull out your water bottle and take your first big sip, your body immediately fills with relief and elation.
“There’s a hedonistic response to that.” Patricia Di LorenzoProfessor Emeritus of Psychology at Binghamton University in New York told Live Science. “If you drink water when you’re thirsty, it tastes really good.”
But why does drinking water feel so good when you’re thirsty?
When you exercise intensely, you sweat and your blood volume decreases, which makes you thirsty. In most areas, brain They are separated by the blood-brain barrier, a layer of cells that prevents harmful toxins and pathogens from infecting the brain. However, certain parts of the brain lie outside this barrier, allowing changes in the blood to be detected quickly. When your blood volume decreases due to exercise or eating salty food, neurons in these parts of the brain send signals that cause the sensation of thirst.
“This quick response is critical for survival.” Yuki Okaa professor of biology at the California Institute of Technology told Live Science. “If it takes that long, you might become dehydrated.”
Related: How much water do you really need to drink?
Thirst is processed in three parts of the brain: the subconus organ (SFO), the organ of the terminal vascular layer (OVLT), and the emmetropic preoptic nucleus (MnPO). Both SFO and OVLT are located outside the blood-brain barrier. in 2018 study Using mice, Dr. Oka found that while there are neurons in all three regions that prompt drinking when excited, MnPO is intermediate in this process. Thirst signals from SFO and OVLT are sent to other parts of the brain to encourage drinking.
According to Oka, it takes about 30 minutes after swallowing water for it to be absorbed and circulated through the body. But long before you’re fully hydrated, your body starts sending signals to your brain that you’re drinking water. Just the first bite causes your brain to release a flood of the neurotransmitter dopamine. Most scientists agree that dopamine is involved. Reward pursuit, exercise, motivation. Importantly, dopamine prompts animals to exert energy in acts that reward us and help us survive, such as eating and drinking.
When dopamine is released when performing a certain behavior, “animals tend to repeat that behavior,” Oka says. “That’s a positive signal.”
We still don’t know exactly how drinking water triggers the release of dopamine. However, a 2019 study published in the journal neuronOka and his colleagues found that mice that were thirsty and drank water released dopamine, but thirsty mice that received water directly into their intestines did not. This suggests that the act of drinking, rather than quenching thirst, releases neurotransmitters. Oka said this explains why dehydrated patients receiving IV fluids don’t see the same benefit from drinking a glass of cold water.
In another process, the study says, the act of gulping sends a message to MnPO neurons that the body is receiving water. MnPO then inactivates thirst neurons in the SFO, giving a feeling of satiety.
However, gulping is not the only mechanism to quench thirst. As water travels to the intestines, the body detects a decrease in the salt-to-water ratio of the blood. This increases levels of a hormone called vasoactive intestinal peptide (VIP). This hormone, not the water itself, helps activate neurons that tell your brain that your body is satisfied. There is much mystery about how this process works. Researchers still don’t know where VIP comes from or how its release is triggered.
“I don’t even know what the osmotic pressure is. [concentration of dissolved particles in the blood] “It’s detected by these intestinal cells. We’re working on that,” Oka said.
Oka noted that the process of quickly quenching thirst helps prevent overhydration. But he also wonders if they evolved to help not just the survival of each individual but the survival of the group. When critical resources like water are limited, quickly quenching thirst can help species survive. Although this hypothesis has not yet been tested, Oka is intrigued by the idea.
“This is a really interesting experiment in how to share,” he says. “If that’s true, our neural circuits have evolved to think about others, not just ourselves.”
