Imagine you're exercising outside on a hot day. You're drenched in sweat, and the feeling of thirst begins to overwhelm you. You take out your water bottle and swallow your first big gulp — and your body immediately fills with relief and elation.
"There's a hedonic reaction to it," Patricia Di Lorenzo, a professor emeritus of psychology at Binghamton University in New York, told Live Science. "When you're really thirsty and you drink water, it just tastes so good."
But why does drinking water feel so pleasurable when you're thirsty?
We get thirsty when we exercise heavily, because as we sweat, our blood volume decreases. Most areas of the brain are separated by the blood-brain barrier, a layer of cells that prevents harmful toxins and pathogens from infecting the brain. But certain parts of the brain fall outside this barrier, allowing for rapid detection of changes in our blood. When we lose blood volume from exercising or eating salty foods, neurons in these parts of the brain send a signal to trigger the feeling of thirst.
"This rapid response is very important for survival," Yuki Oka, a biology professor at Caltech, told Live Science. "If it takes so long, then you might get dehydrated."
Related: How much water do you really need to drink?
Three parts of the brain process thirst: the subfornical organ (SFO), the organum vasculosum lamina terminalis (OVLT) and the median preoptic nucleus (MnPO). Both the SFO and OVLT are located outside the blood-brain barrier. In a 2018 study in mice, Oka uncovered that while all three areas have neurons that drive drinking when those nerve cells are excited, the MnPO is in the middle of this process. It transmits thirst signals from the SFO and OVLT to other parts of the brain to prompt drinking.
It takes about 30 minutes after you swallow water for it to be absorbed and circulate in your body, Oka said. But your body begins to send signals to your brain that you are receiving water well before you are fully rehydrated. With just the initial sip, your brain releases a rush of the neurotransmitter dopamine. Most scientists agree that dopamine is involved in reward-seeking, movement and motivation. Critically, dopamine prompts animals to exert energy on acts that give us a reward or help keep us alive, including eating and drinking.
If dopamine is released when they do a specific behavior, "animals tend to repeat that behavior," Oka said. "That's a positive signal."
Exactly how drinking water triggers the release of dopamine is still unknown. But in a 2019 study published in the journal Neuron, Oka and his colleagues discovered that thirsty mice that drank water released dopamine, whereas thirsty mice that received water directly to their gut did not. This suggests the act of drinking — and not thirst satiation — releases the neurotransmitter. Oka said this explains why dehydrated patients who are administered IV fluids don't experience the same reward they do from drinking a cold glass of water.
In a separate process, the act of gulping also sends a message to neurons in the MnPO that the body is receiving water, per the study. The MnPO then deactivates thirst neurons in the SFO, giving a feeling of satiation.
Yet gulping isn't the only mechanism that helps to halt thirstiness. After water travels down to the gut, the body detects a drop in the blood's salt-to-water ratio. This leads to a rise in the levels of a hormone called vasoactive intestinal peptide (VIP). This hormone, rather than the water itself, helps to activate neurons that signal 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 or how its release is triggered.
"We don't even know how osmolality [concentration of dissolved particles in the blood] is detected by these intestinal cells," Oka said. "We're working on that."
The processes that quickly relieve thirst help prevent overhydration, Oka noted. But he also wonders whether they evolved to aid the survival of not just each individual but also the survival of a group. When critical resources such as water are limited, a quick stop to thirst may help keep a species alive. The hypothesis has yet to be tested, but Oka is intrigued by the idea.
"That's a very interesting experiment in how to share," he said. "If that's really true, the neurocircuit has evolved to thinking about … others, not just the self."
