Water and Electrolyte Balance

Few situations represent such a challenge to the body's homeostatic mechanisms as that posed by prolonged strenuous exercise in a warm environment. Only about 20-25% of the energy available from substrate catabolism is used to perform external work, with the remainder appearing as heat. At rest, the metabolic rate is low: oxygen consumption is about 250mlmin~1, corresponding to a rate of heat production of about 60 W. Heat production increases in proportion to metabolic demand, and reaches about 1 kW in strenuous activities such as marathon running (for a 70-kg runner at a speed that takes about 2.5 h to complete the race). To prevent a catastrophic rise in core temperature, heat loss must be increased correspondingly and this is achieved primarily by an increased rate of evaporation of sweat from the skin surface. In hard exercise under hot conditions, sweat rates can reach 3lh-1, and trained athletes can sustain sweat rates in excess of 2lh-1 for many hours. This represents a much higher fractional turnover rate of water than that of most other body components. In the sedentary individual living in a temperate climate, about 5-10% of total body water may be lost and replaced on a daily basis. When prolonged exercise is performed in a hot environment, 20-40% of total body water can be turned over in a single day. In spite of this, the body water content is tightly regulated, and regulation by the kidneys is closely related to osmotic balance.

Along with water, a variety of minerals and organic components are lost in variable amounts in sweat. Sweat is often described as an ultrafiltrate of plasma, but it is invariably hypotonic. The main electrolytes lost are sodium and chloride, at concentrations of about 15-80 mmoll-1, but a range of other minerals, including potassium and magnesium, are also lost, as well as trace elements in small amounts. Some athletes may lose up to 10 g of salt (sodium chloride) in a single training session, and may train in these conditions twice per day. These substantial salt losses must be replaced from foods and drinks, though the use of salt supplements is seldom necessary.

Failure to maintain hydration status has serious consequences for the active individual. A body water deficit of as little as 1-2% of total body mass can result in a significant reduction in exercise capacity. Endurance exercise is affected to a greater extent than high-intensity exercise, and muscle strength is not adversely affected until water losses reach 5% or more of body mass. Hypohydration greatly increases the risk of heat illness, and also abolishes the protection conferred by prior heat acclimation.

Many studies have shown that the ingestion of fluid during exercise can significantly improve performance. Adding carbohydrate to the fluid confers an additional benefit by providing an energy source for the working muscles. Addition of small amounts (perhaps about 2-8%) of carbohydrate in the form of glucose, sucrose, or maltodextrin will promote water absorption in the small intestine as well as providing exogenous substrate that can spare stored carbohydrate. The addition of too much carbohydrate will slow gastric emptying and, if the solution is strongly hypertonic, may promote secretion of water into the intestinal lumen, thus delaying fluid availability. Voluntary fluid intake is seldom sufficient to match sweat losses, and palatability of fluids is therefore an important consideration. It is not necessary to consume enough fluid during exercise to match sweat losses, as a body mass deficit of 1-2% is unlikely to have adverse consequences. If exercise is prolonged and sweat losses high, the addition of sodium to drinks may be necessary to prevent the development of hyponatremia. Ingestion of large volumes of plain water is also likely to limit intake because of a fall in plasma osmolality leading to suppression of thirst.

Replacement of water and electrolyte losses incurred during exercise is an important part of the recovery process in the postexercise period. This requires ingestion of fluid in excess of the volume of sweat lost to allow for ongoing water losses from the body. Re-establishment of water balance requires replacement of solute, especially sodium, losses as well as volume replacement. If food containing electrolytes is not consumed at this time, electrolytes, especially sodium, must be added to drinks to prevent diuresis and loss of the ingested fluid.

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