We've been told it's important to stay well-hydrated when we work out, but there's growing research that shows athletes, specifically endurance athletes such as marathoners and long-distance cyclists, can harm themselves by drinking too much water during their competitions. The condition, called hyponatremia, is caused by abnormally low sodium levels in your blood from drinking too much water. And while it's rare, it can be deadly if it happens. Even more surprising, a study published in the British Journal of Sports Medicine showed that athletes perform perfectly.