The brain rewards you for behaviors that keep you alive. When the brain notices that you need water, it triggers the “thirsty” signal. As soon as you drink something, the brain rewards you for what you did and gives you that “ahh, hydration” feeling. What matters to the brain is that you actually did the life-preserving action, no need to wait and see whether the drink actually hydrated you.
There are sensors in your mouth that detect when you consume something. You and your brain know that water has entered and doesn’t need to send thirst signals.
People use this trick to knock thirst while wrestling by spitting water out so they can knock thirst and not have water sploshing around in their stomachs.
Here’s something cool to know even if it’s a tangent from the poster’s question: hydration already began before you even drank the water.
Water balance in the body is mainly controlled by a hormone called Vassopressin [or AVP]. Basically, this hormone is released from the brain when the body is either low in volume (hypovolemic or hypotensive) or too “salty” (or high in electrolytes). What it does is it reabsorbs water in the kidneys to combat dehydration. Since kidneys function 24/7, there’s always water to reabsorb.
The brain releases this hormone at a lower threshold compared to the brain recognizing the body is thirsty. So, even without realizing you’re thirsty, the body is already reabsorbing the water your kidneys are filtering.
I’ve also read something (no sources, so take this as you will, maybe someone can corroborate) that talked about the difference between feeling instantly satisfied when drinking, but not when eating
The difference was you can die from drinking too much water too quickly, but not from eating too much food too quickly, so the body has an instant response for fluid intake to stop you over drinking and potentially dying
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