Dehydration, hyponatremia, too much, too little. Hyponatremia (over-hydration) is really not about drinking too much water but about not have enough electrolyte while dehydration is about not having enough water at the cellular level regardless of how much you drink. When we talk hydration we are talking water and electrolyte (WE).
Dehydration of 3% of body weight causes a reduction in performance, makes tissues less pliable and more open to aches, pains and injury. The end result is it will make your race or training run seem a lot harder than it needs to. How much should you drink? Half of your body weight in ounces on a daily basis and on average 1 liter per hour when exercising. Your chosen mode of transport for your water bottle can be: 1) Hold it in your hand, remembering to switch hands periodically. 2) A running vest that takes between 1 liter and 2.5 liters and also has tons of storage for nutrition. 3) Purchase a water system from Nathan or Camelbak. 4) On longer runs you can stash bottles on your run route the night before. 5) Take money and buy en-route. What you carry can be: 1) Water. A partially frozen bottle keeps your water cold on hot days. 2) Fluid replacement drinks such as E Load, Tailwind or Infinit or 3) water with Nuun or Precision (electrolyte no calories). To drink while moving, regardless of the type of delivery system your bottle has, DO NOT drink and swallow at the same time. Take fluid into your mouth and hold it while you breath through your nose, then swallow.
If you experience post-run headache or cough, are red faced, or take a long time post-run to pee or your pee is very dark in colour you are most likely dehydrated. Water follows sodium into the cell so if you just drink water without electrolyte it will pass right through the system and not reach your cell, basically like watering a dry plant; in the top, out the bottom. You know you need more sodium if you pee more frequently than every 2.5 hours, if your urine is clear or if you get swollen digits when exercising.