However, since most fluid lost contains salt (sweat), drinking plain water does not replace electrolytes and can even make you hyponatremic. Hyponatremia, or low blood sodium, can make you feel dizzy, disoriented, nauseated, and lethargic, and in extreme cases can be deadly.
Most endurance athletes these days do no drink plain water during exercise. A mixture of sugar, water, and salt is best. Consider adding 1/2 tsp of salt for every 16 ounces of fluid, in addition to 2-4 tbsp of sugar (equivalent to 120-240 calories). The juice of a fresh-squeezed lemon can really improve the taste. This is a homemade Gatorade without the artificial colors and other junk. You'll find that your body will naturally drink more of a mixture containing energy and electrolytes than it would drinking plain water alone.