Water, Fluids, and Electrolytes – The Science of Hydration

dehydration and sports performance

  • 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


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

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.


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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