There is a common perception that we should drink as much water as we can while we are exercising. However, excessive water can adversely affect exercise performance and cause problems like headache, altered levels of consciousness, and swelling of the hands legs and feet. Why would these changes occur in what could be done to prevent them?

During exercise we sweat. Sweat contains both water and electrolytes. If we replace the fluid with pure water we decrease the concentration of electrolytes in the blood. The most important electrolyte controlling the movement of water between the intracellular and extracellular fluid is sodium. Excessive intake of pure water during exercise results in hyponatraemia.
hypo - means low
-aemia - means in the blood

The best way to protect against the development of exercise induced hyponatraemia is to drink the appropriate amount of fluid.Because fluid losses vary significantly from person to person it's not appropriate to simply recommend a required fluid intake. A more sensible way to approach the problem is to use thirst as a guide to fluid intake-it will prevent both dehydration and overhydration. There is no evidence that sports drinks are protective because they contain low concentrations of sodium. Although excessive water intake is essential contributing factor to exercise induced hyponatraemia, it is not the only important factor. Other contributing factors will be discussed in later questions.