This will be long, but let me illustrate something with regares to what Artic Mama and JohnP advised mineral (sodium/potassium ratios and I presume magnesium to help retain?) balance is likely the culprit. I'm no expert and am only now learning about this myself....
two days ago i began suffering from dry mouth, a headache, lack of lucidity, and overall fatigue. On Tuesday I had a goal of running 8 miles, but had to stop at 6.7 because I had no power and just felt really weak and thirsty. Wednesday, I went to Moksha (hot yoga) and was going to hit the stationary bike for a bit, but just couldn't get the power. I thought (!) it was because I was tired, but no...
Get this: I always drink at least 2 liters of water/day. Easily. I have a black stainless steel bottle that contains 750ml. Drink three = 2250ml which is two liters and one cup. No prob. ...I never drink during my runs ,but on Tuesday I was SO THIRSTY! It was also hot out or hotter than usual (I live in northern Canada and summer is coming!!). I sweat a lot. I had Moksha the previous day as well. The following day, I did more Moksha and drank 6 of my 750ml bottles (4 1/2 liters). I was still so FRICKEN thirsty!! OMG. I've never been so thirsty...
On Tuesday I weighed in at 125...on Wednesday, 130. I have been eating at a deficit. I also only eat whole foods that I process/prepare myself. So VERY little added sodium. ....apparently one needs (as per canada food guide) a minimum of 1500mg of sodium/day to help maintain electrolyte balance.
WHO KNEW I needed more salt?! ....Long story short, I crashed, exhausted only to get up yesterday morning with dry mouth, groggy...I RAN to Mac's and bought two huge bottles of Powerade to help replenish my electrolytes and after pounding the first one back (1 liter) I started to feel like myself again within minutes. My mouth was no longer dry and I felt my energy levels power back.
I haven't weighed yet, but I suspect that I will be closer to 125 this morning...
all of this to say: its more than just drinking enough water like I always thought. The body is more complicated than this. That said, I will be spending more time researching mineral balance in the body now
good luck
