Dehydration

The human body is about two-thirds (approximately 70%) water. This water is present in all of our body tissues (even our teeth are 5% water), and it is vital this water level is maintained.

Dehydration occurs when the normal water content of your body is reduced. This change upsets the vital balance of chemical substances in your body, most notably sodium and potassium. In order to function properly, many of the body’s cells depend on these substances being maintained at the correct levels. It is common for endurance athletes to suffer cramp in the latter stages of their events due to dehydration reducing their sodium levels.
Such is the body’s dependency on water, a drop of just a few percent in total body hydration can cause very severe effects. In some extreme cases, the effects of dehydration can be fatal.