This has been an issue for me for as long as I can remember. It seems that water goes right through me and I had have to work harder at staying hydrated compared to those around me. If I've been slipping up on water consumption I will start to feel it, when most won't. (headache, slightly dizzy) When I was in high school, I passed out a few times after being out in the sun most of the day on a hawaii vacation (and getting a bad sunburn.) I literally have to alternate sips of water and coffee when I drink coffee, as to not feel instantly dehydrated. When I was pregnant (3rd tri) , I drank a cup of coffee 2 days in a row, and on the 3rd day I ended up in the hospital with some preterm labor contractions due to hydration. I feel like I can drink larger amounts of a water at once, compared to other people...and I pee more frequently too.
I've been tested for diabetes numerous times, especially since it runs in my family.
I just thought I'd put it out here in case anyone else shares these symptoms and/or has any idea of what I should look into.