Why does drinking water taste so good when you’re thirsty?
Imagine you are exercising outside on a hot day. You are drenched in sweat, and the feeling of thirst begins to overwhelm you. You take out your water bottle and take your first big whiff – and your body fills with relief and happiness.
“There’s a hedonic response to it,” Patricia DiLorenzodistinguished professor of psychology at Binghamton University in New York, told Live Science. “When you’re really thirsty and you drink water, it’s great.”
But why does drinking water taste good when you’re thirsty?
We get thirsty when we exercise a lot, because the more we sweat, the less our blood volume. Many places of brain they are separated by the blood-brain barrier, a layer of cells that prevents harmful toxins and pathogens from infecting the brain. But some parts of the brain fall outside this barrier, which makes it possible to quickly detect changes in our blood. When we lose blood due to exercise or eating salty foods, neurons in these parts of the brain send a signal to trigger the feeling of thirst.
“This quick response is very important for survival,” Yuki Okabiology professor at Caltech, told Live Science. “If it takes that long, then you might be dehydrated.”
Related: How much water do you really need to drink?
Three parts of the brain: subfornical organ (SFO), organum vasculosum lamina terminalis (OVLT) and the middle preoptic nucleus (MnPO). Both the SFO and OVLT are located outside the blood-brain barrier. In 2018 study in mice, Oka revealed that although all three areas contain neurons that drive drinking when those nerve cells are excited, MnPO is at the center of this process. It relays thirst messages from the SFO and OVLT to other parts of the brain to prompt drinking.
It takes about 30 minutes after you’re dehydrated for water to enter and circulate through your body, Oka said. But your body starts sending messages to your brain that you are getting enough water before you are fully rehydrated. With just the first drink, your brain releases a rush of the neurotransmitter dopamine. Many scientists believe that dopamine is involved in it seeking reward, movement and purpose. Basically, dopamine motivates animals to use energy in activities that give us a reward or that help us survive, including eating and drinking.
If dopamine is released when they perform a certain behavior, “animals tend to repeat that behavior,” Oka said. “That’s a good sign.”
Exactly how drinking water causes the release of dopamine is not yet known. But in a 2019 study published in the journal NeuronOka and his colleagues found that thirsty mice that drank water released dopamine, while thirsty mice that received water directly from their intestines did not. This suggests the act of drinking – not thirst – releases the neurotransmitter. Oka says this explains why dehydrated patients given IV fluids don’t get the same reward as drinking a glass of cold water.
Conversely, the act of swallowing also sends a message to neurons in the MnPO that the body is getting water, according to the study. The MnPO then blocks the thirst neurons in the SFO, signaling satiety.
However, swallowing is not the only way to stop thirst. After water passes down the intestines, the body detects a drop in salt and water balance. This leads to elevated levels of a hormone called vasoactive intestinal peptide (VIP). This hormone, rather than water itself, helps send signals to the brain that the body is satisfied. Much about how this process works is a mystery; researchers still don’t know where VIP comes from and what causes its release.
“We don’t even know what osmolality is [concentration of dissolved particles in the blood] it’s detected by these intestinal cells,” Oka said. “We’re still working on that.”
Fast-acting thirst quenchers help prevent excessive dehydration, Oka noted. But he also wonders if they exist to help not only the survival of the individual but also the survival of the group. When essential resources such as water are scarce, quenching thirst quickly can help preserve some form of life. The theory has yet to be tested, but Oka is intrigued by the idea.
“That’s a very interesting test of the sharing method,” he said. “If that’s true, the neurocircuit has evolved to think about… others, not just.”
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