Fluid intake and adequate hydration are essential and more importantly critical during prolonged training sessions and competition events. Fluid intake helps to maintain hydration, body temperature (thermoregulations), and plasma volume. For events lasting longer than 1 h, athletes should consume fluids containing carbohydrates and electrolytes rather than water alone. Reduction in body water, availability of carbohydrates, and an inadequate electrolyte balance during prolonged exercise events will hamper performance and may lead to serious medical disorders such as heat exhaustion or heat stroke. A 1% reduction in body weight due to water loss may evoke undue stress on the cardiovascular system accompanied by increases in heart rate and inadequate heat transfer to the skin and the environment, increase plasma osmolality, decrease plasma volume, and affect the intracellular and extracellular electrolyte balance (1).
Body fluid is contained mainly in the extracellular and intracellular fluid compartments. The extracellular fluid is subdivided into interstitial fluid and blood plasma. In humans, body water constitutes approximately 60% of body weight with a few minor differences depending upon numerous factors such as sex, age, training status, and percent body fat. Approximately 40% of the body water is housed in the intracellular fluid. On average, blood volume in adults accounts for approximately 7% of body weight, or approximately 5 L. Sixty percent of blood volume is in the plasma; 40% is in the red blood cells. When these levels are challenged during training and competition, they will singly or collectively reduce performance and may cause serious injury, including death. Water loss occurs through respiration, sweat, feces, and urine. During prolonged performance, most water is lost in sweat, especially during high environmental temperatures. Approximately 580 kcal are lost for every liter of sweat that is evaporated (2). Loss of body fluid can be determined by changes in body weight resulting from exercise; each kilogram of body weight loss accounts for approximately 1 L of fluid loss. Sports drinks with appropriate and adequate concentration of electrolytes and carbohydrates promote maintenance of homeostasis, prevent injuries, and maintain optimal performance (3). Water balance in the body is regulated by various means. Changes in osmotic pressure or circulating blood volume stimulate the osmoreceptors in the hypothalamus and baroreceptors in the heart and blood vessels. The rennin-angiotensin-aldosterone system regulates sodium retention. Vasopressin (antidiuretic hormone or ADH) regulates water retention in kidneys and assists with thermoregulation in hypohydrated subjects. Atrial natriuretic peptide (ANP) secreted by the heart participates in water balance regulation but only minimally during cold exposure at 10°-12°C. Exposure to −20°C environment while wearing warm clothing elicits a two-fold increase in ANP; however, this is inhibited by a 3% level of dehydration (4).
MECHANISM OF HYDRATION AND DEHYDRATION
Regulation of fluid balance is a remarkably complex process. Water is lost from the body through the skin, feces, lungs, and kidneys. Water retention by the kidneys is directly controlled by vasopressin produced in the hypothalamus. Production of vasopressin is affected by hypothalamic receptors sensitive to plasma osmolarity and stretch receptors in the atria of the heart, carotid arteries, and aorta.
The kidneys actively reabsorb sodium to regulate extracellular fluid osmolarity. Such reabsorption is controlled largely by aldosterone produced by the adrenal cortex. As serum osmolarity rises, the adrenal cortex release of aldosterone is inhibited, resulting in less sodium reabsorbed and a reduction in osmolarity. The kidneys also regulate aldosterone production through the rennin-angiotensin mechanism. Receptors in the juxtaglomerular complex of the kidney tubules respond to low volume (pressure) by releasing rennin, which leads to a hormonal cascade effect resulting in production of angiotensin II, a potent vasoconstrictor, which stimulates the release of aldosterone by the adrenal cortex.
Exercise stress greatly taxes the ability of the body to regulate fluid and electrolytes, particularly as it relates to thermoregulation (5,6). Much of the current research on hydration and exercise attempts to identify the physiological effects of hypohydration on performance levels (7), methods by which euhydration may be maintained during performance (8,9), including hyperhydration prior to exercise (10,11), and rehydration strategies following exercise (12-14).
Dehydration of 2% of body mass impairs performance in prolonged continuous exercise, particularly in warm environments (5,15,16). In particular, hot and humid environments produce the greatest challenges to thermoregulation, and thus more significantly impair performance (17-19). In addition, recent studies of hypohydration levels of 2.5% and 5.0% body mass have demonstrated impairments to performance in the total production of work in acute resistance training, although peak power and force were not significantly affected (20). Other investigations have indicated that power and force may be impaired by hypohydration (21). Additional research is needed to assess the full impact of hydration on anaerobic performance, particularly as it relates to alterations in the neural capacity to activate skeletal muscle (21).
Detriments in cognition, including short-term memory, visual-motor tracking, and attention, occur when dehydration levels of 2% to 3% body weight are produced by exercise or heat (22). However, other authors report no cognitive deficits at those levels of dehydration and in some cases improved cognitive function for short-term memory and choice response tasks immediately following exercise; these improvements may be related to the increased metabolic arousal accompanying exercise (23). Both exercise and dehydration increase the ratio of serotonin to dopamine. Coupled with the effects of hyperthermia on dopamine and noradrenalin, the interactions of these catecholamines lead to fatigue and may mediate the effects of dehydration on exercise performance (24-26).
Maintaining the euhydrated state during exercise is critical to optimal performance. Fluid should be ingested during exercise at rates essentially equal to the rate of water loss. It is recommended that athletes ingest approximately 500 mL of fluid 1-2 h before performance and continue to consume cool drinks in the amounts necessary to replace sweat losses (1,8,9). The rate of water ingestion should not exceed the rate of water loss, as it might result in water retention, weight gain, and exercise-associated hyponatremia (9,27).
Hyperhydration may be induced by the oral consumption of glycerol, which induces an osmotic gradient that favors greater renal water absorption. Studies examining the effect of hyperhydration by glycerol consumption on performance are equivocal. Several studies have shown performance enhancements (2,28,29), while others have shown no difference when comparing hyperhydration by glycerol consumption with hyperhydration by water or flavored-water consumption (30-32).
FLUID REQUIREMENTS FOR SHORT AND PROLONGED TRAINING AND COMPETITION
Dehydration will adversely affect performance, especially during prolonged exercise (33). A fluid deficit of more than 2% of body weight in warm weather may adversely affect aerobic performance and decrease cognitive function. The longer the race, the more likely dehydration will occur in competitive distance runners (34). In cold weather, 3% dehydration only minimally affects aerobic exercise performance. Anaerobic performance, including strength-related activities, generally is not affected by dehydration (1).
Low blood volume, reduced thermoregulation, decreased cognitive function, reduced gastric emptying, and subsequently a decrease in athletic performance are the results of dehydration. These effects are exacerbated under hot and humid conditions. Individuals exercising in cool environments tend to endure dehydration better than those exercising in temperate or warm environments (8). Fluid requirements for athletes vary based upon length of exercise and type of sport, environmental temperature, and individual factors such as body weight, genetics, and sweat rates (1). Fluids should be taken in a cycle of hydrating several hours before exercise, drinking consistently throughout exercise, and rehydrating afterward in a deliberate fashion (35). Fluid balance should be maintained by consuming fluid to replace what has been expended. Beginning hydration several hours before training or competition (prehydration) is essential for optimal fluid absorption and urine output (1). To decrease the chances of becoming overly dehydrated, athletes should consume fluids throughout their exercise (1).
Hyperhydration also may inhibit performance. After studying eight female marathon runners, Cheuvrant and Haymes (36) suggested that replacing 100% of body weight lost may overestimate fluid replacement needs by failing to consider that body weight loss (dehydration) is not synonymous with sweat loss. Runners who replaced 50% to 80% of their fluid deficits could avoid adverse results (36). Several factors affect the amount of fluid that should be ingested, including the length and demands of exercise, weather conditions, genetics, and level of training. Body weight should be monitored throughout training and competition to prevent excessive fluid loss and determine the amounts of fluid to be ingested in different environmental conditions (1).
Body weight should not increase (or decrease significantly) during exercise (8). For individuals exercising less than 90 min, a weight loss of 1% to 2% of total body weight should have little effect on performance (1,37,38,39). Conversely, a loss of more than 2% of total body weight does detrimentally affect sport performance in individuals exercising more than 90 min (5,40).
In a study of the effects of dehydration on short exercise training, seven women performed two graded, submaximal, exercise tests of 45 min each. During the first test they were euhydrated, and during the second test they were dehydrated. Those who were dehydrated had significantly shorter exercise tests than those who were euhydrated (41). Coyle (8) studied eight male cyclists in varying states of hydration who participated in 30-min cycling sessions in the heat and cold. The stroke volume of euhydrated individuals did not change while dehydrated individuals exhibited decreased stroke volume due to lower blood volume and increased heart rate. In a study of eight males who participated in two passive heat exposure sessions and subsequent Wingate anaerobic tests at various intervals, Cheuvront et al. (36) reported that moderate dehydration and moderate hyperthermia affect anaerobic exercise performance in a temperate environment.
Insufficient consumption of fluids by athletes during exercise may be caused by lack of the thirst sensation (33). Studies on ad libitum intake of fluid during exercise consistently show that athletes only replace one half to two thirds of their needs (8). During exercise of longer than 4 h, extra attention should be expended to decrease the chances of excessive hydration or hyponatremia (1). Although hyponatremia may occur with moderate intakes of fluids, it generally occurs with the ingestion of large amounts of fluid (42). In a study of ultra-marathoners, individuals who finished the race were more likely to maintain their body weight and meet their energy requirements than those who did not. During running events lasting more than 24 h in hot and humid conditions, maintenance of body weight despite large exercise energy expenditures is consistent with fluid overload (43).
ENVIRONMENTAL FACTORS AND FLUID BALANCE
The stresses of intense exercise training and competition are augmented by environmental conditions, such as extreme heat or cold temperatures and altitude. Fortunately, with proper hydration and repeated exposure to environmental stress, the human body is able to adapt to such environmental challenges through the process of acclimatization (44,45). Proper hydration is essential for preserving fluid balance when performing exercise in extreme heat or cold and at altitude (12,46-48). The maintenance of fluid balance depends upon matching fluid consumption, glycerol ingestion, and several key electrolytes with sweat loss (49). This can be a challenge because thirst is a relatively poor indicator of fluid status and generally is not perceived until an individual is 2% dehydated (50). Thus it generally is accepted that dehydration is inevitable during heavy exercise training or competition, especially in the heat (1). The loss of water and electrolytes through sweat is accelerated during exercise as a result of increased metabolic heat production and is influenced by environmental conditions, sweat rate, clothing, exercise intensity, level of fitness, and acclimatization (51-53).
Sweating is the principal mechanism for preventing excessive increases in body temperature (hyperthermia) and is accomplished primarily through evaporation of heat, accounting for approximately 80% of the total heat loss during exercise (54). Individuals engaged in heavy physical exercise in hot environments may lose 1.5-3 L of sweat per hour or approximately 2.5%-8% of body weight per hour, resulting in dehydration and impaired exercise performance (51,55-57) (Fig. 1).
Excessive sweat loss and low fluid intake results in a decreased blood volume and reduced skin blood flow, which eventually decreases sweat output, impairing the ability to dissipate heat and increases cardiovascular strain and the risk of heat illness (58). The extent that environmental heat stress increases the risk of a heat-related illness depends upon air temperature, relative humidity, wind velocity, and radiation. The wet bulb globe temperature (WBGT) is the most useful measure of thermal stress and is determined by the equation WBGT = 0.7 Twb + 0.2 Tbg + 0.1 Tdb, where Twb is the temperature (°C) of a wet bulb thermometer as a measure of relative humidity, Tdb is the ambient temperature of a dry bulb thermometer, and Tbg is the temperature of a black globe thermometer as a measure of solar radiation.
This index clearly highlights the major impact that environmental humidity (Twb) and sweat evaporation play in dissipation of body heat (45). The more nonevaporated sweat loss that occurs, the greater the increase in core body temperature, leading to a further increase in sweat response and ultimately a significant loss of body fluid (51). During low levels of sweat loss (dehydration of <3%), water loss occurs primarily from the extracellular space; however, as an individual experiences greater sweat loss (>3% body weight loss) and/or performs exercise in a hot environment, more of the fluid losses (up to 50%) come from the intracellular fluid matrix (59). It has been postulated that the increased intracellular water loss during exercise may be caused by glycogenolysis within the muscle cell (60).
An important consideration is the time frame in which fluid is consumed following exercise-induced hypohydration, especially if subsequent exercise or training bouts are warranted. Data suggest that immediately after exercise, individuals will rehydrate approximately 60% of the fluid lost, although complete fluid and electrolyte restoration is a relatively slow process (>3 h) and requires up to 150% of fluid losses and plenty of electrolytes (61). Ingesting large volumes of fluid decreases arginine vasopressin before plasma volume and osmolality have been restored, causing increased urine output (62). Other factors to consider during the rehydration period are gastric fluid volume and osmolality of consumed beverages (63). Recently, investigations have examined the effect of glycerol ingestion and fluids of varying tonicities (0.9% vs 0.45% NaCl) during the rehydration period after exercise-induced dehydration (~4% of body weight) and before exercise in the heat (12,46). Rehydration with either a 0.45% or 0.9% NaCl solution resulted in similar fluid restoration, similar cardiovascular, thermoregulatory, and exercise performance responses, and were superior to no fluid ingestion (12). Glycerol ingestion during the rehydration period was found to prolong significantly subsequent exercise time to exhaustion in the heat but was not associated with specific thermoregulatory or cardiovascular advantages compared with rehydration with water alone (46).
Exercise during cold exposure results in hypohydration due to decreased fluid intake, increased fluid loss via sweating, and cold-induced diuresis (64,65); however, this does not necessarily imply a performance decrement. Water immersion further increases diuresis above the response observed during cold air exposure (66).
Exposure to high altitude drastically alters fluid balance by inducing acute diuresis, which appears to be protective against acute mountain sickness (AMS) (47,67,68). Subjects who develop severe AMS have increased fluid retention during the first 3 h of altitude exposure, whereas asymptomatic subjects experience hypoxic diuresis (47). Those with AMS have increased vasopressin, epinephrine, and norepinephrine levels, with exercise exacerbating the effects (69). Thus, Westerterp (67) suggests the initial diuresis at altitude may be an adaptive mechanism to prevent against AMS.
SPORTS DRINKS AND PERFORMANCE
Consumption of sports beverage drinks during exercise is recommended to meet carbohydrate and to replace sweat, water, and electrolyte losses (1). The majority of the literature supports both fluid and carbohydrate replacement during exercise. During prolonged exercise, replacement of Na+ and K+ are essential to maintain plasma volume and hydration (70). The amount and type of sports drink ingested during exercise may impact performance (71). Different exercise tasks (metabolic requirements, duration, clothing, equipment), weather conditions, and other factors such as genetic predisposition, heat acclimatization, and training status influence sweating rate and electrolyte concentrations and determine fluid needs (1). Carbohydrate and electrolyte content, palatability, color, odor, taste, temperature, and texture of a sports drink can increase fluid consumption before, during, and after exercise (1,33). Sports drinks may improve performance by increasing blood glucose levels, improving carbohydrate oxidation, and reducing sense of fatigue (1). Sports drinks containing a low concentration (4%-8%) of carbohydrate ingested at a rate consistent with sweat loss may support fluid, energy, and sodium requirements (1,35) (Table). Carbohydrate concentrations higher than 8% may delay gastric-emptying and should be avoided (1). According to Bernadot (33), athletes should ingest 4 to 8 oz of a 6% to 7% carbohydrate solution every 10 to 20 min.
Increasing plasma volume can positively affect performance. Sodium in sports drinks may help achieve this by improving glucose and water absorption in the small intestine. Sodium is important in rehydration, especially during exercise in the heat (72). Galloway and Maughan (17) studied six healthy males who cycled to exhaustion while ingesting either no drink, a 15% carbohydrate-electrolyte drink, or a 2% carbohydrate-electrolyte drink. Consumption of the 2% carbohydrate-electrolyte drink lead to a lower serum osmolality and reduced plasma volume deficits. However, there were no differences in thermoregulatory or cardiorespiratory responses among the sessions. Mitchell et al. (73) found that greater rehydration may be achieved by consumption of a larger volume of fluid independent of Na+ content. Potassium is important in rehydration after exercise due to the increased retention of fluid in the intracellular space (72). Numerous recent studies (46,71-76) confirm that during endurance events, consumption of glucose-electrolyte solutions improves performance greater than water alone, and the addition of glycerol or magnesium to the sports drink has little effect upon fluid-regulating factors during rehydration or exercise. Carbohydrate added to sports drinks does not facilitate rehydration, but may slightly improve the intestinal uptake of sodium and water (72). In a study of triathletes, it was found that carbohydrate ingestion reduces hormonal and immune response to stress and diminished the detrimental effects on various tissues (77).
PHYSIOLOGICAL FATIGUE AND FLUID STATUS
Several factors including fluid, fuel substrate, and electrolyte depletion have been implicated in the reduction of exercise performance. Athletes engaged in endurance exercise training or competition must satisfy their needs for fluids, energy, and electrolytes to prevent fatigue and obtain optimal performance. Dehydration levels as little as 2% of body weight decrease performance and losses of greater than 5% result in significant decrements in work capacity and increased fatigue (Fig. 2).
Dehydration (~3% of body weight) has minimal effects upon muscular strength, anaerobic performance, or aerobic exercise performance in cold environments (36). In temperate or hot conditions, dehydration accelerates the onset of fatigue and increases the perception of effort compared with a well-hydrated state (58,78). Other physiologic factors may impair aerobic performance, including increased core temperature, cardiovascular strain, muscle glycogen utilization, and altered metabolic and central nervous system function (1,58). Thus, preservation of fluid balance through adequate fluid, energy, and electrolyte provisions during exercise likely will maintain performance and slow the progression of fatigue (9).
Consumption of carbohydrates during prolonged exercise (2-6 h) maintains blood glucose and delays fatigue by sparing muscle glycogen stores and preserving blood volume (79). Recent investigations suggest that consumption of lactate and fructose in energy-electrolyte hydration beverages improves performance and delays fatigue greater than glucose-electrolyte beverages via increased substrate oxidation and enhanced buffering capacity (80). Several strategies effective in enhancing performance and delaying fatigue in the heat include fluid ingestion, pre-cooling (81), and acclimatization (18). Of these, prevention of dehydration plays the most significant role in maintaining performance and delaying fatigue. These findings were confirmed in a study of well-conditioned athletes who performed a 3-h endurance ride in a moderate (~21°C) environment at 60% maximal oxygen uptake (82). Fluid ingestion (water with 0.5 mEq·L−1 Na+, 0.17 mEq·L−1 K+, 1.16 mEq·L−1 HCO3−) prevented the onset of neuromuscular fatigue as measured by mean power frequency and rate of force development.
Physiologic stresses during exercise training and competition may impair performance. Factors such as dehydration, thermoregulation, fluid balance, rehydration, electrolyte changes, plasma volume, and cardiovascular challenges accompany most physical activities, especially during prolonged endurance exercise and competition. Dehydration impairs performance, especially if the exercise is performed in a hot environment. Thus it is recommended that all individuals who exercise, train, and compete attempt to replace fluids and electrolytes that have been lost during exercise through excessive sweating. Adequate and proper hydration is not only a physiological necessity but also improves performance and reduces risks of medical complications from fluid losses.
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