I get that our body needs water. But why does the thirst response almost always overestimates the amount of water our body actually needs, thereby leaving excess water that comes out as urine? How come our body doesn’t tell us to drink just the right amount of water that it needs? Why does yellow pee mean dehydration when the fact that there was enough excess water to produce urine means that you’re probably not dehydrated anymore?
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Basically it’s because your “reservoir of ‘spare’ water” is your blood, but holding water is at the bottom of the list of things that you blood has to do, and it’s a long list with some very critical things on it (oxygen, nutrients for cells, immune system, etc.). So “how much you drink” is controlled in a “better be safe and drink extra than sorry and die of dehydration” way.
It’s also because your kidneys are responsible for removing toxins from your blood, not just water, and they can’t function without also removing water. You can’t pee solids; doing that is called passing kidney stones, and it’s very painful. Yellow pee means your kidneys are forced to concentrate a lot of toxins into very little water to work with, and that’s bad for you (kidney stones again).
Your body IS telling you to drink the right amount of water. If you’re thirsty you will drink; if you’re not thirsty you may drink some (and the body can deal with it), but trying to drink a lot will feel uncomfortable to you as you’re drinking.
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