The Gatorade Sports Science Institute in Barrington, Illinois published a study showing that 46 percent of recreational exercisers are dehydrated (Applied Physiology, Nutrition and Metabolism, June 2006). However with good reason, the study does not say that they are harmed. There is no data anywhere to show that this mild dehydration affects health or athletic performance. Another study from the University of Connecticut shows that a person must lose a tremendous amount of fluid before it affects his performance (Medicine & Science in Sports & Exercise, October 2006).
When you exercise for more than an hour, you may need to take fluid, but not too much. Excessive fluid can cause a potentially fatal condition called hyponatremia. Normally, the amount of salt and other minerals in your bloodstream should equal the same total mineral content in every tissue in your body. If the mineral concentrations are not equal, they try to become equal. Fluid moves from the area of lower mineral content to that of the higher concentration. If you take in so much fluid that it lowers the mineral level in your blood, levels in your brain are higher than those in your bloodstream. This causes fluid to move from your bloodstream into your brain, which increases pressure in your brain and can cause seizures and unconsciousness. The swelling can cause permanent brain damage.
Hyponatremia is a disease seen almost exclusively in people who are not exercising near their maximum. The major risk factor is having more time to drink than to concentrate on pushing the pace, no matter what the sport or the duration of the event. Top athletes drink very little fluid during competitions such as bicycle racing, marathon running or cross country skiing, because it is so difficult to drink while you are exercising near your maximum. On the average, a world-classes marathon runner drinks less than a cup an hour during a race. This is far less than the amount recommended by the American College of Sports Medicine just a few years ago. On the basis of our present knowledge, it may not be safe for mediocre athletes to take in more than 800cc per hour (3.5 cups).
Recent studies show that fit humans can tolerate significant fluid loss before their performance suffers, and that most cases of muscle cramps are not caused by dehydration or salt loss. They are caused by muscle damage itself and can be controlled by stopping exercise and stretching the cramped muscle.