Physiology and Performance
Individuals can become dehydrated while performing physical activity (Table 2), and prior to emphasis on rehydration during exercise, larger fluid deficits may have been more common.[23,101,149] Individuals often start an exercise task with normal total body water and dehydrate over an extended duration; however, in some sports the person might initiate the exercise task dehydrated such as when the interval between exercise sessions is inadequate for full rehydration or when initial body weight is an issue. For example, in weight-class sports (e.g., boxing, power lifting, wrestling) individuals may purposely dehydrate to compete in lower weight classes. In addition, some individuals undertaking twice a day training, or prolonged daily sessions of exercise in hot conditions, may also carry a fluid deficit from their previous workout into the next. Finally, individuals medicated with diuretics may be dehydrated prior to initiating exercise. Water deficit without proportionate sodium chloride loss is the most commonly seen form of dehydration during exercise in the heat. If large sodium chloride deficits occur during exercise then the extracellular fluid volume will contract and cause "salt depletion dehydration." Regardless of the dehydration method, for any water deficit, there is similarity in altered physiologic function and performance consequences.
Dehydration increases physiologic strain as measured by core temperature, heart rate and perceived exertion responses during exercise-heat stress. The greater the body water deficit, the greater the increase in physiologic strain for a given exercise task.[2,96,97,122] Dehydration >2% BW degrades aerobic exercise and cognitive/mental performance in temperate-warm-hot environments.[27,33,72] Greater levels of dehydration will further degrade aerobic exercise performance. The critical water deficit (>2% BW for most individuals) and the magnitude of performance decrement are likely related to the environmental temperature, exercise task, and the individual's unique biological characteristics (e.g., tolerance to dehydration). Therefore, some individuals will be more or less tolerant to dehydration. Dehydration (3% BW) has marginal influence on degrading aerobic exercise performance when cold stress is present. Dehydration (3–5% BW) probably does not degrade either muscular strength[54,68,72] or anaerobic performance.[30,72,74]
Physiologic factors that contribute to dehydration-mediated aerobic exercise performance decrements include increased body core temperature, increased cardiovascular strain, increased glycogen utilization, altered metabolic function, and perhaps altered central nervous system function.[106,118,121] Though each factor is unique, evidence suggests that they interact to contribute in concert, rather than in isolation, to degrade aerobic exercise performance.[32,118,121] The relative contribution of each factor may differ depending on the specific activity, environmental conditions, heat acclimatization status and athlete prowess, but elevated hyperthermia probably acts to accentuate the performance decrement. Cognitive/mental performance, which is important where concentration, skilled tasks and tactical issues are involved, is also degraded by dehydration and hyperthermia.[69,116] The evidence is stronger for a negative effect of hyperthermia than that of mild dehydration on degrading cognitive/mental performance, but the two are closely linked when performing exercise in warm-hot weather.
Hyperhydration can be achieved by overdrinking combined with an agent that "binds" water within the body.[58,66] These binding agents include glycerol and hypertonic drinks that can induce hyperhydration for varied durations. Simple overdrinking will usually stimulate urine production and body water will rapidly return to euhydration within several hours;[58,107,128] however, as previously discussed this compensatory mechanism (urine production) is less effective during exercise and there is a risk of dilutional hyponatremia. Likewise, over consumption of fluids with most hyperhydration binding agents will still elevate urine output well above normal levels. Hyperhydration does not provide any thermoregulatory advantages, but can delay the onset of dehydration, which may be responsible for any small performance benefits that are occasionally reported.[67,77]
Evidence Statement. Dehydration increases physiologic strain and perceived effort to perform the same exercise task, and this is accentuated in warm-hot weather. Evidence Category A. Dehydration (>2% BW) can degrade aerobic exercise performance, especially in warm-hot weather. Evidence Category A. The greater the dehydration level the greater the physiologic strain and aerobic exercise performance decrement. Evidence Category B. Dehydration (>2% BW) might degrade mental/cognitive performance. Evidence Category B. Dehydration (3% BW) has marginal influence on degrading aerobic exercise performance when cold stress is present. Evidence Category B. Dehydration (3–5% BW) does not degrade either anaerobic performance or muscular strength. Evidence Categories A and B. The critical water deficit and the magnitude of exercise performance degradation are related to the heat stress, exercise task, and the individual's unique biological characteristics. Evidence Category C. Hyperhydration agents can be achieved by several methods, but provides equivocal benefits and has several disadvantages. Evidence Category B.
Health problems in individuals can result from dehydration or overdrinking (consuming volumes greater than sweat losses). In general, dehydration is more common, but overdrinking-with symptomatic hyponatremia-is more dangerous. Dehydration can impair exercise performance and contribute to serious heat illness, and exacerbate symptomatic exertional rhabdomyolysis; while exercise-associated hyponatremia can produce grave illness or death.
Heat Illnesses. Dehydration increases the risk for heat exhaustion[2,91,123] and is a risk factor for heat stroke.[25,53,63,113] Heat stroke is also associated with other factors as lack of heat acclimatization, medications, genetic predisposition, and illness.[25,51] Dehydration was present in ~17% of all heat stroke hospitalizations in the U.S. Army over a 22-yr period. In a series of 82 cases of heat stroke in Israeli soldiers, dehydration was present in ~16% of the cases. Consistent with this association, team physicians providing medical support for American football players during summer practice have observed that dehydration-sometimes aggravated by vomiting-is associated with the development of heat stroke.[51,115] In addition, dehydration has been associated with reduced autonomic cardiac stability, altered intracranial volume and reduced cerebral blood flow velocity responses to orthostatic challenge.
Skeletal muscle cramps are believed associated with dehydration, electrolyte deficits and muscle fatigue, and they are common in non-heat-acclimatized American football players (early summer practice sessions), tennis matches, long cycling races, late in tropical triathlons, soccer and beach volleyball. Muscle cramps can also occur in winter activities-in cross-country ski-racers and ice-hockey goalies. Persons susceptible to muscle cramps are believed to be often profuse sweaters with large sweat sodium losses.[14,141] Triathlon athletes experiencing muscle cramps, however, have been reported to not have clinically significant different serum electrolyte concentrations than counterparts without cramps.
Rhabdomyolysis. Rhabdomyolysis (syndrome causing release of skeletal muscle contents) is most often observed with novel, strenuous, overexertion and clinical evidence suggests that dehydration can increase the consequences of rhabdomyolysis. For example, it appears that dehydration increases the likelihood or severity of acute renal failure associated with rhabdomyolysis.[19,124] Among U.S. soldiers who were hospitalized for serious heat illness, and thus likely experienced large fluid and electrolyte fluxes, 25% had rhabdomyolysis and 13% had acute renal failure.
A cluster of exertional rhabdomyolysis cases provides evidence that dehydration, combined with heat stress and novel training, can induce serious health problems. In 1988, at a Massachusetts State Police training academy, 50 cadets performed numerous calisthenics and running drills in hot weather during the first days of training, with limited water intake. One cadet who collapsed from exertional heat stroke while running, was hospitalized and required dialysis for acute renal failure caused by rhabdomyolysis. He later died from the complications of heat stroke, rhabdomyolysis, renal failure, and hepatic failure. Thirteen other cadets were hospitalized for severe dehydration, rhabdomyolysis, and acute renal insufficiency, and six were hemodialyzed for acute renal failure. In fact, all 50 cadets had some rhabdomyolysis (as defined by serum creatine kinase >10 times normal).
Exercise-associated Hyponatremia. Exercise-associated hyponatremia was first reported at the comrades marathon. Later, exercise-associated hyponatremia was reported in endurance runners, and since that time a number of participants from a variety of occupational and recreational activities have been hospitalized for this condition, with several having died.[8,82,100,108] Symptomatic hyponatremia can occur when plasma sodium rapidly drops to ~130 mmol·L−1 and below. The lower the plasma sodium falls, the faster it falls, and the longer it remains low, the greater the risk of dilutional encephalopathy and pulmonary edema. Some individuals have survived plasma sodium levels as low as 109 mmol·L−1 and others have died with initial (in hospital) levels over 120 mmol·L−1. With plasma sodium < 125 mmol·L−1 and falling, symptoms become increasingly severe and include headache, vomiting, swollen hands and feet, restlessness, undue fatigue, confusion and disorientation (due to progressive encephalopathy), and wheezy breathing (due to pulmonary edema). When plasma sodium falls well below 120 mmol·L−1, the chances increase for severe cerebral edema with seizure, coma, brainstem herniation, respiratory arrest, and death.
Contributing factors to exercise-associated hyponatremia include overdrinking of hypotonic fluids and excessive loss of total body sodium. In marathoners, symptomatic hyponatremia is more likely to occur in smaller and less lean individuals who run slowly, sweat less, and drink heavily-water and other hypotonic fluids-before, during, and after the race.[4,46,71] In tropical triathlons (e.g., Kona, HI), some participants may have been both dehydrated and hyponatremic based upon clinical observations. Individuals with genes for cystic fibrosis may be prone to salt depletion and exercise-associated hyponatremia. In general, symptomatic hyponatremia in events that last < 4 h is from overdrinking before, during and sometimes even after the event. In longer ultra-endurance events, sodium losses can induce hyponatremia to levels associated with the onset of symptoms regardless if the individual is over- or underdrinking, so replacing some of the sodium losses is warranted.
Exercise-associated hyponatremia occurs occasionally in American football and tennis players who drink too much water to treat or try to prevent heat cramps, or when a cramping player is given hypotonic fluid intravenously.[48,70] Consistent with this, hyponatremia hospitalizations have been associated with soldiers who were misdiagnosed as suffering from dehydration (similar symptoms such as light headedness, fatigue) and subsequently directed to drink large volumes of water.
Evidence Statement. Dehydration is a risk factor for both heat exhaustion and exertional heat stroke. Evidence Categories A and B. Dehydration can increase the likelihood or severity of acute renal failure consequent to exertional rhabdomyolysis. Evidence Category B. Dehydration and sodium deficits are associated with skeletal muscle cramps. Evidence Category C. Symptomatic exercise-associated hyponatremia can occur in endurance events. Evidence Category A. Fluid consumption that exceeds sweating rate is the primary factor leading to exercise-associated hyponatremia. Evidence Category A. Large sweat sodium losses and small body mass (and total body water) can contribute to the exercise-associated hyponatremia. Evidence Category B.
Cite this: Exercise and Fluid Replacement - Medscape - Feb 01, 2007.