NUTRITION AND SPORTS. WATER AND ELECTROLYTES
Water
The importance of water replacement during physical exercise is due to the fact that body cells function in an aqueous medium. Water transports nutrients nutrients and other cell waste products through the circulatory system. Also, a significant supply of blood to the tissues is required for the heat produced by exercise to evaporate through the skin. One of the most typical symptoms of physical exercise or sporting activity is sweating. Body water is lost through sweating through a mechanism that compensates body temperature.
The loss of liquids from the body takes place through sweating and respiration. The amount of water lost through sweating can be sufficiently significant to cause dehydration and if action to stop dehydration is not taken, a cardiovascular failure may occur. Even mild dehydration (3 to 4% water loss) can decrease muscle capacity in a 20-30%; if the water loss is more than 10% a circulatory collapse may occur. Thus, the replacement of liquids must be one of the most important concerns of physical trainers and sportsmen.
The amount of water that a sportsman looses depends on the intensity, duration of the exercise and the weather conditions. When not exercising a normal individual produces 500-700 ml of sweat per day, whilst the practice of prolonged physical exercise in a humid setting can cause a loss of water due to sweating of about 8-12 litres/day (some marathon runners may loose up to 5 litres during the event, which amounts to 6-10% of their body weight).
The replacement of liquids during physical exercise must start two hours before hard physical training or before the competition ingesting about 500 ml of liquids, followed by small amounts at short and regular intervals every half an hour. During competition or training, rehydration must be prescribed, mainly for those athletes that perform long events, especially in humid settings, even though the feeling of thirst may not be present. Hydrating the body during the event or training should avoid the loss of weight caused by dehydration, although this is not possible on many occasions. Rehydration after strenuous physical activity is always necessary.
Electrolytes
Sweat contains sodium, chloride, magnesium and potassium and is hypotonic with respect to other body fluids (lesser concentration), the more trained the athlete the more hypotonic the sweat is.
Electrolyte loss through sweating is not usually sufficiently significant to be life-threatening (water loss is though) as regulating mechanism are triggered when electrolytes are lost. Nonetheless, the loss of electrolytes can give rise to decreased resistance and muscle power.
Electrolytes must be replaced by ingesting isotonic liquids during training and competition or alternatively bi ingesting foods with the proper amounts of salts after the competition. The intake of salt tables in an isolated fashion is not recommendable.