Originally written by Kelly Doyle, MS, CSCS July 25, 2005
Dehydration vs Hyponatremia in Endurance Events
Hydration status, fluid consumption, and the effects on health and performance during endurance activities is a complex topic that continues to be investigated. Exercise induced dehydration is defined as the loss of body water that occurs during the course of an exercise session, and is most likely to occur in endurance activities lasting longer than 60 minutes (ACSM, 1996). Water is primarily lost from the body via urine, gastrointestinal tract, evaporation from skin, and respiration and is exacerbated by heat, humidity, or fluid restriction during exercise (Barr, 1999). Fluid losses occur in both intracellular and extracellular compartments and can cause different effects on the body depending on whether the type of water loss is hypotonic, isotonic, or hypertonic (Cheuvront & Haymes, 2001; Shirreffs, 2005).
In contrast, hyponatremia occurs when plasma sodium levels drop below normal as a result of hypotonic fluid replacement greatly exceeding fluid losses. Similar to dehydration, exercise induced hyponatremia is most likely to occur in endurance activities that take place in hot environments. For that reason, concerns about dehydration or heat related illnesses may lead to excessive fluid intake by some athletes (Davis, Videen, Marino, Vilke, Dunford, Van Camp, & Maharam, 2001). The purpose of this paper is to examine the effects of dehydration and hyponatremia on exercise performance and health in individuals who participate in endurance events lasting longer than 60 minutes, and to determine which of the two an athlete should be more concerned about.
Some level of dehydration is likely in endurance events, particularly under hot environmental conditions (Schnirring, 2003). A hot environment can complicate the ability of the body to regulate temperature during exercise. When air temperatures are lower than body temperature, a gradient for heat loss occurs as blood flow carries core body heat to the skin surface, where excess heat is then emitted by convection and radiation. In an environment warmer than body temperature, this process is reversed, causing the body to become more heated. At this point, sweat evaporation is the primary means of heat dissipation, with average sweat rates estimated at 1.2 liters/hour (Cheuvront & Haymes, 2001). If a person is unable to replace lost body fluid, they become dehydrated and blood volume is diminished. This results in greater cardiac strain as well as a higher risk of developing heat illness (ACSM, 1996). The need to maintain blood pressure supersedes heat dissipation, resulting in a peripheral vasoconstriction that restricts blood flow to the skin. This in turn causes decreased heat loss through convection or radiation, and increases risk of hyperthermia (Cheuvront & Haymes, 2001). The most severe form of hyperthermia is heat stroke, which can be a life threatening condition and often accompanies dehydration, but is extremely rare compared to heat exhaustion or hyponatremia. Body mass losses of two to eight percent as a result of dehydration during exercise have not been shown to have any lasting health consequences (Noakes, 2000).
Endurance performance is affected by dehydration as the lower blood volume creates cardiovascular strain, which is exhibited by an increased heart rate and decreased stroke volume (ACSM, 1996). Dehydrated individuals show increased muscle glycogen utilization during prolonged exercise, possibly as a result of higher body temperatures, increased catecholamine levels, or a combination of the two (Barr, 1999; Shirreffs, 2005). It has also been proposed that when a critical core body temperature is reached, the central nervous system reduces the drive to continue exercising as a means to reduce heat production (Shirreffs, 2005).
The degree of fluid deficit that can be tolerated appears to be somewhat controversial, and may depend on an athlete’s heat acclimatization status, training status, and exercise environment. An athlete that can tolerate a certain degree of dehydration at cool temperatures may not be able to tolerate the same fluid deficit in a hot environment, and dehydration combined with hyperthermia is worse than either dehydration or hyperthermia alone. Furthermore, deconditioned people may not be able to tolerate the same degree of fluid loss as a trained athlete (Barr, 1999). While some authors postulate that the degree of fluid deficit is directly proportional to the degree of performance impairment (Barr, 1999), others concluded that the extent of performance decrement was highly variable (Shirreffs, 2005). All concluded that dehydration of 2% body mass consistently showed diminished endurance performance in both temperate and hot climates, especially when the duration of exercise was greater than 90 minutes (Barr, 1999; Shirreffs, 2005).
Because of fear of dehydration, performance deficits, or heat injury, many endurance athletes have been overly vigilant about hydration practices during endurance activities (Davis et al., 2001; Hsieh, 2004). As a result, those who take in more hypotonic fluids than they lose during exercise are at risk of developing hyponatremia. Other risk factors for hyponatremia include female gender, hot environment, the use of NSAID medications, excessive and salty sweat, and endurance exercise lasting longer than four hours (Ayus, Varon, & Arieff, 2000; Davis et al., 2001; Hsieh, 2004; Venables, 2003). The incidence of hyponatremia is estimated to be anywhere from 1 percent to 27 percent of triathlon competitors, with a greater occurrence during ultradistance events such as the Ironman (Dallam, Jonas, & Miller, 2005). Another study estimated that 18 percent of marathon runners developed hyponatremia, the majority of who were healthy women (Ayus, Varon, & Arieff, 2000). Once thought to be a rare condition, hyponatremia appears to be a more prevalent problem during endurance sports than previously thought (Venables, 2003).
Currently, the prevailing theory behind hyponatremia is that athletes take in more hypotonic fluid than they lose through sweat, resulting in a dilutional effect that lowers plasma sodium levels (Speedy, Noakes, & Schneider, 2001). Other possible mechanisms for developing hyponatremia include third space sodium shifts with fluid accumulation in the intestinal lumen, or inappropriate secretion of antidiuretic hormone (Hsieh, 2004; Shopes, 1997). The maximum capacity for urine production may not be able to keep up with fluid ingestion, leading to a progressive fluid overload. During exercise, urine production decreases from 20 to 60 percent because of decreased blood flow to the kidneys (Venables, 2003). Non steroidal drugs may play a role by causing decreased renal prostaglandin production, which leads to a slower glomerular filtration rate (Speedy, Noakes, & Schneider, 2001).
Females are at greater risk for developing hyponatremia and suffering more severe symptoms than males (Davis et al., 2001; Venables, 2003). In rats, estrogen induces renal oxytocin receptor mRNA production and affects regulation of osmolality. In humans, pregnancy hormones lower the thirst threshold, and estrogen augments antidiuretic hormone secretion in post menopausal females (Davis et al., 2001). Women have approximately 10 percent less body water than men and tend to be smaller. Smaller runners who drink the same amount of fluid as larger runners are more likely to develop a fluid overload (Venables, 2003).
Mildly lowered sodium levels in the range of 130 to135 mmol/liter are likely to be asymptomatic, where those whose sodium levels drop to 125 or below typically present with nausea, light headedness, and fatigue. More severe symptoms, which indicate cerebral edema, occur when sodium levels drop below 120 and can include confusion, headache, seizures, coma, and even death (Speedy, Noakes, & Schneider, 2001; Venables, 2003). Because some of the initial symptoms of hyponatremia are similar to dehydration or heat exhaustion, one of the greatest risk factors for an athlete that develops hyponatremia is getting a correct diagnosis. Emergency medical personnel often assume that endurance athletes are suffering from dehydration and administer even more fluids, compounding the problem (Backer, Shopes, Collins, & Barkan, 1999; Dallam, Jonas, & Miller, 2005; Hsieh, 2004; Noakes, 2000; Shopes, 1997; Speedy, Noakes, & Schneider, 2001; Venables, 2003).
Effects of hyponatremia on endurance performance were not addressed in the literature. Perhaps one reason for this is because symptoms are often delayed, sometimes by up to six hours after exercise (Shopes, 1997). A theory for this is that hypotonic fluids that are ingested during activity may remain in the intestinal lumen, eventually extruding sodium from the vascular compartment resulting in lower plasma sodium levels (Shopes, 1997).
To summarize, dehydration is most likely to occur in endurance activities lasting 60 minutes or more, is exacerbated by hot environmental conditions or restricted fluid intake, and can have a negative effect on endurance performance when approximately 2 percent of body mass is lost. The greatest risk associated with dehydration during exercise appears to be heat stroke, but the incidence of this occurring is rare. Athletes have been able to tolerate body mass losses as a result of dehydration during exercise by as much as eight percent with no health consequences.
In contrast, hyponatremia is most likely to occur in endurance activities lasting longer than four hours, is exacerbated by hot environmental conditions, NSAID medications, female gender, and over indulgence of hypotonic fluids. The literature reviewed did not report the effects of hyponatremia on exercise performance. The greatest risk factor associated with developing hyponatremia appears to be getting a fast and accurate diagnosis with appropriate treatment. An inaccurate diagnosis can allow for a rapid progression of symptoms leading to cerebral edema or death. Treating a hyponatremic patient with additional fluids under the assumption that they are dehydrated has led to fatal consequences (Ayus, Varon, & Arieff, 2000).
Based on the literature reviewed, whether an athlete should be more concerned about dehydration or hyponatremia may depend on the individual, the type of activity, and the environment in which it will take place. A female participating in an ultradistance event lasting longer than four hours may be more concerned about hyponatremia, where another individual participating in a 90 minute bout of exercise in hot and humid weather may be more concerned about dehydration. It seems prudent that the best way to avoid suffering effects of either dehydration or hyponatremia is to practice sensible fluid replacement strategies. Being overly fearful of dehydration should not lead an individual to fluid overload, and being fearful of hyponatremia should not lead an individual to avoid rehydrating themselves during prolonged exercise, especially in a hot environment.
In conclusion, following appropriate fluid replacement recommendations should help an athlete avoid becoming dehydrated or hyponatremic. The International Marathon Medical Directors Association recommends that athletes understand the risks of both dehydration and hyponatremia and drink as needed, but not to exceed 800 ml per hour. Since individuals have different sweat rates based on intensity, environment, and body weight, athletes will need to determine what works best for them (Schnirring, 2003). The American College of Sports Medicine recommends that athletes consume a balanced diet before the event, start drinking early and at regular intervals during exercise, and add an appropriate amount of carbohydrates or electrolytes to fluids for exercise lasting longer than one hour (ACSM, 1996).
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