- Understand hydration basics for clients in training settings
- Be able to assess your client’s hydration status
- Describe how different ambient temperatures affect hydration
- Know guidelines for intake of fluid before, during and after PA
Hydration
Water is the most important nutrient and it accounts for 50 to 60% of overall body mass. Lean body tissues (e.g., muscle, heart, liver, etc.) are about 72 to 75% water by mass, whereas adipose (fat) tissue is about 5% by mass. Therefore, it is crucial to emphasize the importance of regular fluid consumption throughout training and performance.
Hydration Status
Athletes constantly risk dehydration. This is particularly true for those who train in hot and humid environments. The longer and harder the athlete works, the greater the risk. Dehydration will hurt performance. In fact, it only takes a 2% loss of body weight (i.e., 3 pounds for a 150-pound person during exercise) for performance to suffer. It is important to emphasize to clients that weight loss in one practice, game, or training session is NOT recommended
Dehydration also impairs the body’s ability to lose heat. Both sweat rate and skin blood flow are lower at the same core temperature for the dehydrated state compared with the euhydrated state. Body temperature rises faster during exercise when the body is dehydrated. The reduced sweating response in the dehydrated state is probably mediated through the effects of both a fall in blood volume (hypovolemia) and elevated plasma osmolality (i.e., dissolved salt concentration) on hypothalamic neurons. As core temperature rises towards about 39.5°C (103°F), sensations of fatigue will follow. This critical temperature is reached more quickly in the dehydrated state.
Dehydration not only elevates core temperature responses but also negates the thermoregulatory advantages conferred by high aerobic fitness and heat acclimatization. Heat acclimation lowers core temperature responses when subjects are euhydrated. When dehydrated, however, similar core temperature responses were observed in both unacclimated and acclimated states. Your client’s ability to tolerate heat strain can be impaired when dehydrated, with the critical temperature for experiencing central fatigue being near 102.2°F (ambient); conditions worsen if our client is dehydrated by more than about 5% of their body mass. The rise in core temperature during exercise in the dehydrated state is associated with a greater catecholamine response, and these effects may lead to increased rates of glycogen breakdown in the exercising muscle, which, in turn, may contribute to earlier onset of fatigue in prolonged exercise. Although it is well established that hypohydration (reduced total body water) impairs endurance exercise performance, the influence of hypohydration on muscular strength, power, and very high-intensity endurance (maximal activities lasting more than 30 seconds but less than two minutes) is poorly understood because of inconsistent results.
Fluid consumption should begin during the early stages of exercise in the heat, not only to minimize the degree of dehydration but also to maximize the bioavailability of ingested fluids. Dehydration poses a serious health risk in that it increases the risk of cramps, heat exhaustion, and life-threatening heatstroke
Mechanisms of Heat Illness
Heat injury is most common during exhaustive exercise in a hot, humid environment, particularly if the person is dehydrated. These problems affect not only highly trained athletes but also less well-trained sport participants, who have less-effective thermoregulation during exercise, work less economically, use more carbohydrate for muscular work, and take longer to recover from exhausting exercise than highly trained people.
During the initial stages of exercise in a hot environment, sweating begins and the skin blood vessels dilate, causing increased heat loss from the body. But as central blood volume and pressure fall, sympathetic nervous activity increases and the skin blood vessels constrict. A more powerful constriction of the blood vessels supplying the abdominal organs leads to cellular hypoxia in the region of the gastrointestinal tract, liver, and kidneys.
Cellular hypoxia leads to the production of reactive oxygen species (ROS), including superoxide anion, hydrogen peroxide, hydroxyl radical, peroxynitrite, and nitric oxide (NO), which is a powerful blood vasodilator. In the gastrointestinal tract, this action allows the passage of bacterial toxins from the gut into the systemic blood circulation, leading to endotoxemia (blood poisoning) and a drastic fall in blood pressure (hypotension). Increased levels of NO probably contribute to the development of hypotension. The consequences for the athlete can be heat syncope (fainting) and organ injury.
Thus, ROS generation appears to increase within abdominal tissues during heat exposure. Antioxidant status is compromised within the first few hours but gradually recovers and is enhanced after 24 hours of heat exposure.
Further studies are needed about the possible benefits of antioxidant supplementation in people who regularly experience high body temperatures such as athletes who train and compete in hot, humid climates. Although it may not be frequently reported, heat-related illness contributes to significant morbidity as well as occasional mortality in athletic, elderly, children, and disabled populations.
Among U.S. high school athletes, heat illness is the third-leading cause of death. Significant risk factors for heat illness include dehydration, hot and humid climate, obesity, low physical fitness, lack of acclimatization, previous history of heatstroke, sleep deprivation, medications (especially diuretics or antidepressants), sweat gland dysfunction, and upper respiratory or gastrointestinal illness. Many of these risk factors can be addressed with education and awareness of clients at risk.
Dehydration, with fluid loss occasionally as high 10% of body weight, appears to be one of the most common risk factors for heat illness in people who exercise in the heat. Core body temperature has been shown to rise an additional 0.15 to 0.2°C for every 1% of body weight lost during exercise. Identifying athletes at risk, limiting environmental exposure, and monitoring closely for signs and symptoms are all important components of preventing heat illness. But monitoring hydration status and promoting appropriate drinking strategies may be the most important factors in preventing severe heat illness.
Effects of Fluid Intake on Exercise Performance
Oral fluid ingestion during exercise helps restore plasma volume to near pre-exercise levels and prevents the adverse effects of dehydration on muscle strength, endurance, and coordination. Elevating blood volume just before exercise by various hyperhydration strategies has been suggested to be effective in enhancing exercise performance.
Pre-exercise Hyperhydration
Because even mild dehydration has debilitating effects on exercise performance, hyperhydration (greater than normal body water content) has been hypothesized to improve thermoregulation by expanding blood volume and reducing plasma osmolality, thereby improving heat dissipation and exercise performance. Although some studies report higher sweating rates, lower core temperatures, and lower heart rates during exercise after hyperhydration, several of these studies used control conditions that represented dehydration rather than euhydration, calling results into question. But the findings generally support the notion that hyperhydration reduces the thermal and cardiovascular strain of exercise. Relatively few studies have directly investigated the effects of hyperhydration on exercise performance, but one well-controlled study reported that expansion of blood volume by 450 to 500 ml improved cycling time-trial performance by 10%.
When hyperhydration is induced in test subjects, most of the fluids consumed (over 1-3 hours) is quickly excreted. The expansion of body water and blood volume is only transient. Studies in which the blood volume was directly expanded by infusion reported decreased cardiovascular strain during exercise but yielded conflicting results on sweat loss, heat dissipation, and exercise performance.
It is now believed that greater fluid retention is possible if glycerol is added to fluids consumed before exercise. When glycerol is consumed orally, it is rapidly absorbed primarily in the small intestine. It is reported to be evenly distributed among all fluid compartments, with the exception of the cerebrospinal fluid and aqueous humour, and promotes hyperhydration by inducing an osmotic gradient.
When glycerol is consumed, the plasma glycerol concentration increases in proportion to the dose ingested, which easily exceeds the renal threshold for glycerol reabsorption, resulting in urinary glycerol excretion. Thus, without supplemental glycerol ingestion, a decrease in the osmotic gradient occurs, resulting in a loss of hyperhydration.
One study has reported a higher sweating rate and lower core temperature when subjects exercised in the heat after hyperhydrating with glycerol and water. Other studies, however, report no thermoregulatory advantage during exercise after glycerol solution–induced hyperhydration. In these studies, the volume of water consumed (500 ml) may have been too small.
Generally, however, the ingestion. of glycerol with 1 to 2 L of water seems to protect against heat stress and thus may have some health benefits for people who exercise in hot conditions. Although pre-exercise glycerol ingestion does not seem to have an affect on skin temperatures, muscle temperature, circulating catecholamine, or muscle metabolic responses to the steady state exercise, heart and core temperature were lower than with the ingestion of water alone.
Fluid Intake During Exercise
During exercise, especially in a hot environment, dehydration can be avoided only by matching fluid consumption with sweat loss.
Reaching this goal can be a challenge, since sweat rates during strenuous exercise in the heat can be around 2 to 3 liters per hours. When a client consumes even one liter of fluid, they will likely feel uncomfortable, so achieving fluid intake that matches sweat loss during exercise is often impractical.
Sweat rates vary widely among individuals under the same ambient conditions and to prescribe the specific amount that a person should drink is difficult without knowing the person’s sweat rate under the prevailing weather conditions.
Thirst is not a good indicator of body-water requirements or the degree of dehydration. In general, the sensation of thirst is not perceived until a person has lost at least 2% of body weight through sweating. As already mentioned, even this mild degree of dehydration is sufficient to impair exercise performance. Numerous studies show that ad libitum intake of water during exercise in the heat results in incomplete replacement of body- water losses.
The rules or practicalities of specific sports may limit the opportunities for drinking during competition. Because even mild dehydration and small decreases in plasma volume impair endurance exercise capacity, athletes should try to minimize the extent of dehydration by ingesting fluids during exercise. Regular water intake during prolonged exercise is effective in improving both exercise capacity and exercise performance, especially in the gym.
Fluid intake during prolonged exercise offers the opportunity to ingest some fuel as well. The addition of carbohydrate to some drinks consumed during exercise has an additional independent effect in improving exercise performance
Too much added carbohydrate in a sports drink, while providing more fuel for the working muscles, decreases the amount of water that can be absorbed. In this situation, water is drawn out of the interstitial fluid and plasma into the lumen of the small intestine by osmosis.
As long as the fluid remains hypotonic with respect to plasma, the uptake of water from the small intestine is not adversely affected. In fact, the presence of small amounts of glucose and sodium tends to cause a slight increase in the rate of water absorption compared with pure water. Sodium and other electrolytes are added to sports drinks not to replace electrolytes lost through sweating but to provide the following benefits:
- To increase palatability
- To maintain thirst (and therefore promote drinking)
- To prevent hyponatremia (low serum sodium concentration, which can occur when people ingest far more water than required)
- To increase the rate of water uptake
- To increase the retention of fluid
Replacement of the electrolytes lost in sweat can normally wait until the post-exercise recovery period. Fluid intake during strenuous exercise of less than 30 minutes offers no advantage. Gastric emptying is inhibited at high work rates, and insignificant amounts of fluid are absorbed during exercise of such short duration. For exercise lasting more than one hour or exercise in hot or humid conditions, consumption of carbohydrate–electrolyte sports drink is warranted. These drinks supply fluid together with carbohydrate that helps maintain blood glucose and high levels of carbohydrate oxidation. The electrolyte (sodium) content partly offsets salt losses in sweat, but perhaps more important, it maintains the desire to drink.
Sweat-loss rates during exercise depend on exercise intensity, duration, and environmental conditions but vary considerably among individuals. Some people may lose up to 3 L/h of sweat during strenuous activity in a warm environment and even at low ambient temperatures of about 54°F, sweat loss can exceed 1 L/h. Because the electrolyte composition of sweat is hypotonic to plasma, the replacement of water rather than electrolytes is the priority during exercise. Plasma volume can drop by up to 20% during exercise, the magnitude of this drop being related to the relative exercise intensity. Typically, at work rates equivalent to 60 to 80% of the maximal oxygen uptake, plasma volume falls acutely by about 10 to 15% because of the increased capillary hydrostatic pressure and osmotic uptake of water into active skeletal muscle tissue. Without fluid intake, particularly in a warm, humid environment, further falls in plasma volume and increases in plasma osmolarity occur because of the loss of hypotonic sweat, as exercise proceeds.
As mentioned previously, the decrease in plasma volume that accompanies dehydration may be of particular importance in influencing work capacity. Blood flow to the muscles must be maintained at a high level to supply oxygen and fuel substrates (glucose and fatty acids), but high blood flow to the skin is also necessary to convect heat to the body surface, where it can be dissipated. When the ambient temperature is high and plasma volume decreases through sweat loss during prolonged exercise, skin blood flow is likely to be compromised, allowing central venous pressure and blood flow to the working muscle to be maintained but reducing heat loss and causing body temperature to rise to dangerous levels. To prevent dehydration, water must be replaced at a faster rate. Metabolic water production increases during exercise but not enough to compensate for water loss through sweating. Oral fluid ingestion during exercise helps restore plasma volume to near preexercise level and prevents the adverse effects of dehydration on thermal and cardiovascular strain, muscle strength, endurance, and coordination.
Balancing Water Intake Each Day
Variable amounts of water are lost from the body through sweating in response to the requirement for thermoregulation, but for a sedentary person in cool conditions, evaporative loss of water through the skin amounts to only about 600 ml/day. Additional water is lost in the feces (about 100 ml/day) and urine. Normally, about 800 ml to 1,600 ml of urine is produced each day. The kidneys regulate the amount of water lost in urine; although even in severe dehydration, some urine is still produced to maintain fluid flow through the kidney tubules (nephrons) and excrete toxic nitrogenous wastes such as ammonia and urea. Urinary water loss is not usually less than 800 ml/day.
Environmental conditions affect a person’s water requirements by altering the loss occurring by the various routes. Water losses may be two to three times greater for a sedentary person living in a hot climate compared with a sedentary person living in a temperate climate. These higher rates of water loss are not caused exclusively by increased sweating; they may also be incurred by a marked increase in transcutaneous and respiratory water losses. These routes of water loss are heavily influenced by the humidity of the ambient air, which may be a more important factor than the ambient temperature. Respiratory water losses are greater when the relative humidity (RH) of the ambient air is low because air breathed out of the body is fully saturated with water vapor (RH = 100%). Although these losses are small for a sedentary individual in a moist, warm environment (about 200 ml/day), they may increase approximately twofold in low-humidity conditions (RH = 0% to 20%) and may rise up to 1,500 ml/day during periods of hard training in cold, dry air at altitude.
Water intake comes from drinks and food; some foods (especially plant material) have high water content. Water in food, in fact, makes a major contribution to total water intake. Water is also produced internally (metabolic water) from the catabolism of carbohydrate, fat, and protein. For example, in the complete oxidation of one molecule of glucose, six molecules of carbon dioxide and six molecules of water are produced. In a sedentary individual, metabolic water production amounts to about 300 ml/day, although most of this water is lost in expired gas, because oxidizing fuel in the body generates carbon dioxide, which stimulates breathing and hence increases respiratory water loss. Although an athlete increases his or her metabolic water production because of the increased rate of fuel catabolism during exercise, this increase, again, is offset by the obligatory increase in lung ventilation and respiratory evaporative water loss.
The body’s water balance is under tight regulation involving nervous and hormonal factors that respond to a number of inputs. The osmolarity of the blood plasma is maintained within tight limits around 290 mOsmol/L. A rise or fall in the plasma osmolarity is sufficient to alter kidney function from maximum water conservation to maximum water excretion. Because sodium is the major electrolyte in the extracellular fluids (accounting for 50% of plasma osmolarity), the maintenance of osmotic balance is closely coupled to the intake and excretion of sodium and water. Even small reductions in plasma osmolarity invoke a marked increase in urine output (diuresis), and this increase is normally sufficient to prevent fluid overload when large volumes of water or low-electrolyte drinks such as beer are consumed. But some cases of hyponatremia (low plasma sodium concentration) have been reported, usually in people who have ingested excessively large volumes of plain water or low-electrolyte drinks in a relatively short time.
The subjective sensation of thirst initiates the desire to drink and is therefore a key factor in the regulation of fluid intake. Although the kidneys can effectively conserve water or electrolytes by reducing the rate of loss, they cannot restore a fluid deficit. Only consumption of fluid can correct this imbalance. The sensation of thirst is mainly evoked by the detection of elevated plasma osmolarity (and to a lesser extent, by reductions in blood volume and pressure) by osmoreceptors located in the hypothalamus of the brain. The thirst sensation results in a profound desire to drink and an increase in the secretion of antidiuretic hormone (ADH) from the posterior pituitary gland, which acts on the kidneys to reduce urine excretion. Other factors that promote thirst are learned responses such as dryness of the mouth or throat, salty tastes, and feeling hot. Thirst is quickly alleviated by drinking fluid, and alleviation can occur before a significant amount of the fluid is absorbed in the gut. This effect suggests a role for sensory receptors in the mouth and stomach. Distension of the stomach wall appears to reduce the perception of thirst and may result in premature cessation of fluid ingestion. Thus, the absence of the sensation of thirst cannot be used as an indicator that fluid balance (euhydration) is established; the perception of thirst is often not present until a significant degree of dehydration has occurred.
An electrolyte imbalance commonly called water intoxication, which results from hyponatremia (low plasma sodium) caused by excessive water consumption, is occasionally reported in endurance athletes. This condition appears to be most common among slow runners in marathon and ultramarathon races and probably arises because of the loss of sodium in sweat coupled with extremely high intakes (8 L to 10 L) of water.
The symptoms of hyponatremia are similar to those of dehydration and include mental confusion, weakness, and fainting. Therefore, this condition can be misdiagnosed when it occurs in people who participate in endurance races. The usual treatment for dehydration is administration of fluid intravenously and orally. If this treatment is given to a hyponatremic individual, the consequences can be fatal. The normal plasma sodium concentration is around 140 to 144 mmol/L. Symptomatic hyponatremia can occur when the plasma sodium concentration rapidly drops to 130 mmol/L or less. The longer that it remains low, the greater the risk is of developing swelling of the brain (the clinical term is dilutional encephalopathy) and accumulating extracellular fluid in the lungs (pulmonary edema). When plasma sodium falls well below 120 mmol/L, the risk of brain seizure, coma, and death increases. In long-distance events, symptomatic hyponatremia is more likely to occur in small, lean people who run slowly, sweat less, and ingest large volumes of water or hypotonic fluids before, during, and even after the event. People with genes for cystic fibrosis tend to be more prone to salt depletion and therefore may be at higher risk for developing exercise-associated hyponatremia. Women generally sweat less than men and are thus at higher risk for developing exercise-associated hyponatremia.
Ensuring Adequate Hydration Before Exercise
An adequately hydrated state can be facilitated by higher fluid at regular intervals while your client is in training. A useful check is to have your client observe the color of their urine. It should be pale in color, although this simple test cannot be reliably used if the client or athlete is taking vitamin supplements. In particular, some of the excreted water-soluble B vitamins add a dark yellow tint to urine.
A sudden drop in body mass on any given day is likely to indicate dehydration. This is why the technique of weighing the client before and after session work can be useful. And for clients training in hot climates, the fluid intake requirement (to maintain water balance, or euhydration) increases as ambient temperature increases. In general, if daily energy expenditures increase (from training), water intake must also follow in the same increasing manner.
Ensuring Hydration During Exercise
Relying on feeling thirsty as the signal to drink is unreliable because a considerable degree of dehydration (certainly sufficient to impair athletic performance) can occur before the desire for fluid intake is evident. Ideally, athletes should consume enough fluids during activity to make body weight remain fairly constant before and after exercise. Guidelines for fluid consumption before, during, and after exercise can only be general because of the large variation in individual sweating responses. The American and Canadian Dietetic Associations recommend that approximately 500 ml of fluid be consumed two hours before exertion and another 500 ml be consumed about 15 minutes before prolonged exercise. In hot and humid environments, frequent consumption (every 15 to 20 minutes) of small volumes (120 to 180 ml) of fluid is recommended throughout exertion. Detailed recommendations on fluid replacement strategies during and following exercise have been given in the 2007 American College of Sports Medicine position on exercise and fluid replacement (available at www.acsm-msse.org). Athletes should become accustomed to consuming fluid at regular intervals (with or without thirst) during their training session. For most people exercising for 30 to 60 minutes in moderate temperatures, an appropriate beverage is cool water.
Composition of Sport Drinks During Exercise
Fluid ingestion during exercise also supplies exogenous fuel substrate (usually carbohydrate) as well as helps maintain plasma volume and prevents dehydration. But the availability of ingested fluids may be limited by the rate of gastric emptying or intestinal absorption. Gastric emptying of fluids is slowed by the addition of carbohydrate or other macronutrients that increase the osmolarity of the solution ingested. Hence, with increasing glucose concentration in the fluid ingested, the rate of fluid volume delivery to the small intestine decreases, although the rate of glucose delivery increases. Water absorption in the small intestine is by osmosis and is promoted by the coupled transport of glucose and sodium. Hence, the composition of fluids to be used during exercise depends on the relative needs to replace water and provide fuel substrate. Where rehydration is the main priority (e.g., for prolonged exercise in the heat), the solution should contain some carbohydrate as glucose or glucose polymers (20 to 60 g/L) and sodium (20 to 60 mmol/L) and should not exceed isotonicity (290 mOsmol/L). Most commercially available sports drinks contain 60 to 80 g/L of carbohydrate (predominantly as glucose, glucose polymers, or both, although some drinks may also contain fructose or sucrose) and 20 to 25 mmol/L of sodium.
To minimize the limitation imposed by the rate of gastric emptying, the osmolarity of the beverage should be minimized by providing the glucose in the form of glucose polymers, and the volume of fluid in the stomach should be kept as high as is comfortable by frequent ingestion of small amounts of fluid. Practicing drinking during training is often neglected. This practice will accustom athletes to the feeling of exercising with fluid in the stomach. It also provides the opportunity to experiment with different volumes and flavorings to determine how much fluid intake athletes can tolerate and which formulations suit them best. Measuring fluid consumption and body mass changes before and after training gives an idea of the athlete’s sweat rate under different environmental conditions. This information will help determine the athlete’s requirements for fluid intake during competition.
Rehydration after Exercise
Replacement of water and electrolytes during the post-exercise recovery period is especially important when it comes to repeat bouts of exercise when rehydration must be maximized.
If dehydration continues unaddressed, loss of both intracellular and extracellular fluid volume occurs. Loss of intracellular volume may have important implications for recovery given the emerging evidence of the role cell volume plays in regulating cell metabolism. Reduced intracellular volume reduces rates of glycogen and protein synthesis, whereas high cell volume stimulates these processes. The main factors influencing the effectiveness of post-exercise rehydration are the volume and composition of the fluid consumed.
Plain water is not the ideal rehydration beverage when rapid and complete restoration of body fluid balance is necessary and when all intake is in liquid form. Ingesting water alone causes a rapid fall in plasma sodium concentration and in plasma osmolarity. These changes reduce the stimulation to drink (thirst) and increase urine output, both of which delay the rehydration process. Plasma volume is more rapidly and completely restored if some sodium chloride (77 mmol/L, or 0.45 g/L) is added to the water consumed (Nose et al. 1988). This sodium concentration is similar to the upper limit of the sodium concentration found in sweat but is considerably higher than the sodium con- centration of many commercially available sports drinks, which usually contain 10 to 25 mmol/L. Optimal rehydration after exercise can be achieved only if the sodium lost in sweat is replaced along with the water. Studies have shown that if an adequate volume of fluid is consumed, euhydration is achieved when sodium intake is greater than sodium loss. Ingesting a beverage containing sodium not only promotes rapid fluid absorption in the small intestine but also allows the plasma sodium concentration to remain elevated during rehydration and helps maintain thirst while delaying urine production. Sodium is omnipresent in extracellular fluid. To include potassium in beverages consumed after exercise would be expected to enhance the replacement of intracellular water and therefore, promote rehydration. But this is only theory and presently little experimental evidence supports this expectation. The rehydration drink should also contain carbohydrate (glucose or glucose polymers) because the presence of some glucose also stimulates fluid absorption in the gut and improves beverage taste. After exercise, the uptake of glucose into the muscle for glycogen resynthesis should also promote intracellular rehydration.
Fluid Consumption after
Exercise
For a person undertaking regular exercise, any fluid deficit incurred during one exercise session can potentially compromise the next exercise session if adequate fluid replacement does not occur. Fluid replacement after exercise can, therefore, frequently be thought of as hydration before the next exercise bout. Until recently, athletes were generally encouraged to consume a volume of fluid equivalent to their sweat loss incurred during exercise to rehydrate adequately. In other words, they were to consume about 1 L of fluid for every kilogram lost during an exercise session. This amount is insufficient because it does not take into account the obligatory urine loss incurred after beverage consumption over a period of hours.
Summary
High rates of sweat secretion are necessary during hard exercise to limit the rise in body temperature that would otherwise occur. If the exercise is prolonged, body-temperature increase leads to progressive dehydration and loss of electrolytes. A body temperature of 36 to 38°C (96.8 to 100.4°F) is considered the normal range at rest, and it may increase to 38 to 40°C (100.4 to 104°F) during exercise. When body temperature rises toward 39.5°C (103°F), central fatigue ensues. Further increases are commonly associated with heat exhaustion and occasionally with life-threatening heatstroke, characterized by lack of consciousness after exertion and by clinical symptoms of organ damage.
Some people may lose up to 2 to 3 L/h of sweat during strenuous activity in a hot environment. Even at low ambient temperatures of about 10°C (50°F), sweat loss can exceed 1 L/h.
Because the electrolyte composition of sweat is hypotonic to plasma, the replacement of water rather than electrolytes is the priority during exercise.
Fatigue toward the end of a prolonged event may result as much from the effects of dehydration as from substrate depletion.
Exercise performance is impaired when a person becomes dehydrated by as little as 2% of body weight, and losses in excess of 5% of body weight can decrease the capacity for work by about 30%. Some evidence indicates that lower levels of dehydration can also impair performance even during relatively short-duration, intermittent exercise. Although additional research is needed to produce greater understanding of the effect of low-level dehydration on physical performance, we can generalize that when performance is at stake, being well hydrated is better than being dehydrated.
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