Water is the main need for the body' life-supporting functions. Water is its number one nutrient. Body functioning is compromised by loss of too much water.

Sweat is the obvious manner in which the body loses water. Exhaling leads to loss of water via vapor. During physical exercise, both routes have a purpose, and dehydraytion becomes a a problem. Fatigue is the first symptom of dehydration: a water loss of even 1 to 2 percent of body weight can lessens a person's capacity to do muscular activity. A 7% loss can lead to a likely collapse.

Fluid losses via Sweat
Working muscles produce heat as a by-product of ATP breakdown. The body cools itself via sweating. Each liter of sweat dissipates about 600 kcalories of heat, preventing a rise in body temperature of almost 10° C. The body routes its blood supply through the capilaries just under the skin, and the skin secretes sweat to evaporate and cool the skin and the underlying blood. The deeper body chambers are cooled when the blood flows back.

Hyperthermia
In hot, humid weather, sweat doesn't evaporate well because the surrounding air is already laden with water. Body heat builds up and triggers maximum sweating, without sweat evaporation, little cooling takes place. In such conditions, active people must take precautions to prevent heat stroke. The only way to prevent heat stroke is to drink sufficient fluid before and during the activity, rest in the shade when tired, and wear lightweight clothing that allows evaporation. [Hence the danger of rubber or heavy suits that 'supposedly promote weight loss during physical activity - they promote profuse sweating, prevent sweat evaporation, and invite heat stroke] If you ever experience any of the symptoms of heat stroke listed under Explanations in the left column, stop your activity, sip fluids, seek shade, and ask for help. Heat stroke is dangerous and can be fatal, these symptoms demand attention.

Hypothermia
Endurance athletes can easily lose 1.5 liters or more of fluid during each hour of activity. To prepare for fluid losses, a person must hydrate before activity. To replace fluid losses, the person must rehydrate during and after activity. Even then, in hot weather, the GI tract may not be able to absorb enough to keep up with sweat losses, and some degree of dehydration may be inevitable.

Athletes preparing for competition are often advised to drink extra fluids in the days immediately beforehand, especially if they are still training. The extra water is not stored in the body, but drinking extra water ensures maximum hydration at the start of an event.

Some coaches and athletes withold water during practice because they mistakenly believe the body adapts to use less water and that this will somehow be beneficial. This false and dangerous idea has cost some athletes their health, and some their lives. [as well as poor performances] Full hydration is imperative for every athlete both in training and in competition. The athlete who arrives at an event even slightly dehydrated, arrives with a disadvantage. Drinking extra water does no harm and may be protective.

What is the best fluid for an exercisng body? For noncompetitive, everyday active people, plain cool water is recommended, especially in warm weather, for two reasons: it rapidly leaves the digestive tract to enter the tissues where it is needed, and it cools the body from the inside out. For endurance athletes, other purposes for replenishing water lost through sweating and providing a source of carbohydrate to supplement the body's limited glycogen stores. Carbohydrate depletion brings on fatigue in the athlete, but as already mentioned, fluid loss and the accompanying buildup of body heat can be life-threatening. Thus the first priority for endurance athletes should be to replace fluids. Many good-tasting drinks are marketed for active people.

Electrolyte losses and replacement
When a person sweats, small amounts of electrolytes - the electrically charged minerals soduim, potassuim, cholride, and magnesium - are lost from the body along with water. Losses are greatest in beginners; training improves electrolyte retention.

To replenish lost electrolytes, a person ordinarily needs only to eat a regular diet that meets energy and nutrient needs. In extremely demanding endurance events lasting more than 3 hours, electrolyte replacements may be needed. Electrolyte or salt tablets always cause water to flow out the tissues into the GI tract, they also increase potassium losses, irritate the stomach, and cause vomiting. Thus these tablets worsen dehydration and impair performance in several ways.