Hydration Before Exercise
Fluid replacement following exercise represents hydration prior to the next exercise bout. Any fluid deficit prior to exercise can potentially compromise thermoregulation during the next exercise session if adequate fluid replacement is not employed. Water loss from the body due to sweating is a function of the total thermal load that is related to the combined effects of exercise intensity and ambient conditions temperature, humidity, wind speed. In humans, sweating can exceed min-kg h-. Water lost with sweating is derived from all fluid compartments of the body, including the blood hypovolemia, thus causing an increase in the concentration of electrolytes in the body fluids hypertonicity. People who begin exercise when hypohydrated with concomitant hypovolemia and hypertonicity display impaired ability to dissipate body heat during subsequent exercise. They demonstrate a faster rise in body core temperature and greater cardiovascular strain. Exercise performance of both short duration and high power output, as well as prolonged moderate intensity endurance activities, can be impaired when individuals begin exercise with the burden of a previously incurred fluid deficit an effect that is exaggerated when activity is performed in a hot environment.
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During exercise, humans typically drink insufficient volumes of fluid to offset sweat losses. This observation has been referred to as voluntary dehydration. Following a fluid volume deficit created by exercise, individuals ingest more fluid and retain a higher percentage of ingested fluid when electrolyte deficits are also replaced. In fact, complete restoration of a fluid volume deficit cannot occur without electrolyte replacement primarily sodium in food or beverage Electrolytes, primarily sodium chloride, and to a lesser extent potassium, are lost in sweat during exercise. The concentration of Na in sweat averages ~ mmol L- but can vary widely – mmol L- depending on the state of heat acclimation, diet, and hydration. Despite knowing the typical electrolyte concentration of sweat, determination of a typical amount of total electrolyte loss during thermal or exercise stress is difficult because the amount and composition of sweat varies with exercise intensity and environmental conditions. The normal range of daily U.S. intake of sodium chloride NaCl isto .g ~- mmol and potassium K is -g – mmol. Exercise bouts that produce electrolyte losses in the range of normal daily dietary intake are easily replenished within following exercise and full rehydration is expected if adequate fluids are provided. When meals are consumed, adequate amounts of electrolytes are present so that the composition of the drink becomes unimportant. However, it is important that fluids be available during meal consumption since most persons rehydrate primarily during and after meals. In the absence of meals, more complete rehydration can be accomplished with fluids containing sodium than with plain water.
To avoid or delay the detrimental effects of dehydration during exercise, individuals appear to benefit from fluid ingested prior to competition. For instance, water ingested min before exercise will enhance thermoregulation and lower heart rate during exercise. However, urine volume will increase as much as times that measured without preexercise fluid intake. Pragmatically, ingestion of- ml of water before exercise should allow renal mechanisms sufficient time to regulate total body fluid volume and osmolality at optimal preexercise levels and help delay or avoid detrimental effects of dehydration during exercise.