Vol. 3, # 39
October 14, 2006
Q:  How important is hydration in athletics?- Layperson
A:  Endurance athletes, especially those who train in hot and humid weather conditions, constantly risk dehydration. The risk becomes greater the longer an athlete is working, or when athletes train or compete more than once in a day.
  • If exercisers lose too much fluid in sweat without replacing what they've lost in both fluids and electrolytes (like sodium and potassium), they risk becoming dehydrated. Endurance athletes can use sports drinks, like Gatorade®, to help ward off dehydration and muscle cramps by replacing fluid or electrolytes. 
  • Dehydration can diminish energy and impair performance. Even a 2-percent loss of body weight through sweat (i.e., 3 pounds for a 150-pound runner)1,2 can put athletes at a disadvantage. When the difference between top finishers in an endurance event is only a few seconds or less, athletes can't afford to lose time due to dehydration.

The Right Way to Hydrate

Because endurance events last longer than most sports, endurance athletes also run a higher risk of overhydrating, so hydration plans for these athletes should be considered carefully.

  • Remember fluids throughout the day. It's important for endurance athletes to come to workouts and competitions hydrated. To do this, athletes can start out the day by grabbing a sports drink, then using fountains, coolers, and other beverages as triggers for drinking throughout the day.
  • Hydrate 2 to 3 hours before training and competitions. Aim for at least 16 ounces (2 cups) of fluid at this time and an additional 8 ounces (1 cup) 10 to 20 minutes prior to getting on the field.
  • Drink to replace sweat; don't overdrink. Endurance athletes, especially inexperienced marathoners who tend to run slowly and stop for more fluid breaks, risk overhydrating, which can lead to a dangerous condition called "hyponatremia". Hyponatremia occurs when an athlete takes in too much fluid and the sodium level in blood drops too low. All endurance athletes should know how much fluid their body requires and use their sweat rate as a guide (see sweat rate chart below).
Know your sweat rate and how to replace it. To determine how much to drink, it's important for athletes to know their sweat rates. It can vary based on the individual, weather, and intensity of exercise, so athletes should measure:

How much weight they lose during exercise (in fluid ounces)
How much fluid they consume during exercise (in fluid ounces)
The amount they SHOULD drink to replace sweat losses



Fluid Replacers

  • Examples: Water, Gatorade, 10K, Quickick, Max
  • These drinks are absorbed as quickly as water and typically are used for activities lasting less than 2 hours.
Carbohydrate Loaders
  • Examples: Gaterlode, Exceed High, Carboplex
  • These drinks replace more muscle glycogen to enhance greater endurance.
  • They should be used after ultra-endurance events to increase muscle glycogen resynthesis after exercise.
Nutrition Supplements
  • Examples: Gatorpro, Exceed Sports, Ultra Energy
  • These supplements are fortified with vitamins and minerals and they help athletes maintain a balanced diet.
  • They can be used as a meal replacement supplement for athletes who wish to skip a high fat meal, or as extra calories for athletes who wish to gain weight.
  • Drinks with Carbohydrate (CHO) concentrations of greater than eight percent should be avoided.
  • Fruit juices, CHO gels, sodas, and sports drinks that have a CHO greater than six to eight percent are not recommended during exercise as sole beverages.
  • Beverages containing caffeine, alcohol, and carbonation are not to be used because of the high risk of dehydration associated with excess urine production, or decreased voluntary fluid intake.
  • Drink according to a schedule based on individual fluid needs.
  • Drink before, during and after practices and games.
  • Drink 17-20 ounces of water or sports drinks with six to eight percent CHO, two to three hours before exercise.
  • Drink another 7-10 ounces of water or sport drink 10 to 20 minutes before exercise.
  • Drink early - By the time you're thirsty, you're already dehydrated.
  • In general, every 10-20 minutes drink at least 7-10 ounces of water or sports drink to maintain hydration, and remember to drink beyond your thirst.
  • Drink fluids based on the amount of sweat and urine loss.
  • Within two hours, drink enough to replace any weight loss from exercise.
  • Drink approximately 20-24 ounces of sports drink per pound of weight loss.
  • Dehydration usually occurs with a weight loss of two percent of body weight or more.
  • If exercise lasts more than 45-50 minutes or is intense, a sports drink should be provided during the session.
  • The carbohydrate concentration in the ideal fluid replacement solution should be in the range of six to eight percent CHO.
  • During events when a high rate of fluid intake is necessary to sustain hydration, sports drinks with less than seven percent CHO should be used to optimize fluid delivery. These sports drinks have a faster gastric emptying rate and thus aid in hydration.
  • Sports drinks with a CHO content of 10 percent have a slow gastric emptying rate and contribute to dehydration and should be avoided during exercise.
  • Fluids with salt (sodium chloride) are beneficial to increasing thirst and voluntary fluid intake as well as offsetting the amount of fluid lost with sweat.
  • Salt should never be added to drinks, and salt tablets should be avoided.
  • Cool beverages at temperatures between 50 to 59 degrees Fahrenheit are recommended for best results with fluid replacement. 

Dehydration leads to changes in the volume of compartments within the cranium that could put sportsmen and women at risk of brain damage after head injuries, according to a team of UK researchers (‘The effects of dehydration on brain volume – preliminary results’, International Journal of Sports Medicine 2005; 26:481-485).

In adults, the cranium (the part of the skull that encloses the brain) is a rigid bony vault of fixed size, with a constant volume that is the product of the volume of the brain, the intracranial cerebrospinal fluid (CSF) in a compartment known as the subarachnoid space, and the intra- cranial blood. The brain is suspended within the sub-arachnoid space, which surrounds it with a protective cushion of fluid. The brain itself contains fluid- filled cavities known as the cerebral ventricles, which communicate with the subarachnoid space.

The aim of this pioneering study was to investigate the relationship between dehydration and changes in the volume of the brain and the cerebral ventricles in six healthy male amateur rugby union players.

The subjects underwent magnetic resonance imaging (MRI) scans of the brain before and after a period of exercise designed to cause significant dehydration, while samples of blood and urine were taken before and afterwards to assess the degree of dehydration. One of the subjects (control) undertook a further series of MRI scans to enable the researchers to assess day-to-day fluctuations of brain and ventricular volume in a normally hydrated healthy person.

They found that the subjects lost between 2.1% and 2.6% of their body mass from sweating during the exercise. They also found a correlation between the degree of dehydration and the change in ventricular volume, with changes in the latter much larger than those seen in the normally hydrated control subject.

‘Changes in the volume of the brain, the intracranial CSF (especially the subarachnoid space) and the intracranial blood may influence the outcome of closed head injuries,’ the researchers explain. ‘After an impact to the head the brain will travel further within the cranium before it meets the skull if the subarachnoid space is enlarged than in the normally hydrated state. Consequently it will accelerate to higher velocities and this may increase the likelihood of contusion injuries after blows to the head such as those sustained in boxing, football and rugby’.

Although the researchers acknowledge that their study was too small to be definitive, they conclude that dehydration causes changes in the volume of intra-cranial compartments that may put sportsmen and women at increased risk of brain damage from contusion injury (bruising) and internal haemorrhage after head injuries.

Some sportsmen and women, eg boxers, rugby players and footballers, are especially vulnerable to serious head injuries whilst dehydrated.

Overhydration, also called water excess or water intoxication, is a condition in which the body contains too much water.

Usually, drinking large amounts of water does not cause overhydration if the pituitary gland, kidneys, liver, and heart are functioning normally. To exceed the body's ability to excrete water, an adult with normal kidney function would have to drink more than 2 gallons of water a day on a regular basis.

Overhydration is much more common among people whose kidneys do not excrete urine normally—for example, among people with a disorder of the heart, kidneys, or liver. Overhydration may also result from syndrome of inappropriate secretion of antidiuretic hormone (SIADH). In this syndrome, the pituitary gland secretes too much antidiuretic hormone, stimulating the kidneys to conserve water.

Brain cells are particularly susceptible to overhydration (as well as dehydration). When overhydration occurs slowly, brain cells have time to adapt, so few symptoms occur. When overhydration occurs quickly, confusion, seizures, or coma may develop.

Doctors try to distinguish between overhydration and excess blood volume. With overhydration and normal blood volume, the excess water usually moves into the cells, and tissue swelling (edema) does not occur. With overhydration and excess blood volume, an excess amount of sodium prevents the excess water from moving into the cells; instead, the excess water accumulates around the cells, resulting in edema in the chest, abdomen, and lower legs.


Regardless of the cause of overhydration, fluid intake usually must be restricted (but only as advised by a doctor). Drinking less than a quart of fluids a day usually results in improvement over several days. If overhydration occurs because of heart, liver, or kidney disease, restricting the intake of sodium (sodium causes the body to retain water) is also helpful.

Sometimes, doctors prescribe a diuretic to increase urine excretion. In general, diuretics are more useful when overhydration is accompanied by excess blood volume.

Physiology of water intoxication

Water itself is not toxic to the body in any amount. However, body fluids contain electrolytes (particularly sodium compounds, such as sodium chloride) in concentrations that must be held within very narrow limits. Water enters the body orally or intravenously, and leaves the body primarily in the urine and in sweat. If water enters the body more quickly than it can be removed, body fluids are diluted and a potentially dangerous shift in electrolyte balance occurs.

Most water intoxication is caused by hyponatremia, an overdilution of sodium in the blood plasma, which in turn causes an osmotic shift of water from extracellular fluid (outside of cells) to intracellular fluid (within cells). The cells swell as a result of changes in osmotic pressure and may cease to function. When this occurs in the cells of the central nervous system and brain, water intoxication is the result. Additionally, many other cells in the body may undergo cytolysis, wherein cell membranes are unable to stand abnormal osmotic pressures rupture, killing the cells. Initial symptoms typically include light-headedness, sometimes accompanied by nausea, vomiting, headache and/or malaise. Plasma sodium levels below 100 mmol/L (2.3g/L) frequently result in cerebral edema, seizures, coma, and death within a few hours of drinking the excess water. As with an alcohol poisoning, the progression from mild to severe symptoms may occur rapidly as the water continues to enter the body from the stomach or intravenously.

A person with two healthy kidneys can excrete about 1.5 litres of water per hour at maximum filtration (other studies find the limit to be as little as 0.9L/h). Consuming as little as 1.8 litres of water in a single sitting may prove fatal for a person adhering to a low-sodium diet, or 3 litres for a person on a normal diet. However, this must be modulated by potential water losses via other routes. For example, a person who is perspiring heavily may lose 1 L/h of water through perspiration alone, thereby raising the threshold for water intoxication. The problem is further complicated by the amount of electrolytes lost in urine or sweat, which is variable within a range controlled by the body's regulatory mechanisms. Water intoxication can be prevented by consuming water that is isotonic with water losses, but the exact concentration of electrolytes required is difficult to determine and evolves over time, and the greater the time period involved, the smaller the disparity that may suffice to produce electrolyte imbalance and water intoxication.

Persons at high risk of water intoxication

Certain activities and conditions put a person at elevated risk of water intoxication.


Marathon runners are susceptible to water intoxication if they drink only water while running. Although sweat is relatively hypotonic compared with body fluids, marathon runners perspire heavily for long periods, potentially causing their sodium levels to drop when they consume large amounts of fluids to quench their thirst. The replacement fluids may not contain sufficient sodium to replace what has been lost, and this puts them at high risk for water intoxication. Medical personnel at marathon events are trained to immediately suspect water intoxication when runners collapse or show signs of confusion. Properly designed electrolyte-replacement drinks and some sports drinks include electrolytes that make them roughly isotonic with sweat, which helps to prevent water intoxication.

Note that overconsumption of sodium (in drinks or food), as well as inadequate intake of water, can cause hypernatremia, a disorder that is nearly the opposite of water intoxication and equally dangerous. Improper use of salt tablets can cause hypernatremia.

Hypernatremia is an electrolyte disturbance consisting of an elevated sodium level in the blood (compare to hyponatremia, meaning a low sodium level). The most common cause of hypernatremia is not an excess of sodium, but a relative deficit of free water in the body. For this reason, hypernatremia is often synonymous with the less precise term dehydration.

Water is lost from the body by a variety of pathways, including perspiration, insensible losses from breathing, and in the feces and urine. If the amount of water taken in falls consistently below the amount of water lost, the serum sodium level will begin to rise, leading to hypernatremia. Rarely, hypernatremia can result from massive salt ingestion (such as may occur from drinking seawater).

Ordinarily, even a small rise in the serum sodium concentration above the normal range results in a strong sensation of thirst, an increase in free water intake, and correction of the abnormality. Therefore, hypernatremia most often occurs in people such as infants, those with impaired mental status, or the elderly, who may have an intact thirst mechanism but are unable to ask for or obtain water.


  • Inadequate intake of water, typically in elderly or otherwise disabled patients who are unable to take in water as their thirst dictates. This is the most common cause of hypernatremia.
  • Inappropriate excretion of water, often in the urine, which can be due to medications like diuretics or lithium or can be due to a medical condition called diabetes insipidus
  • Intake of a hypertonic fluid (a fluid with a higher concentration of solutes than the remainder of the body). This is relatively uncommon, though it can occur after a vigorous resuscitation where a patient receives a large volume of a concentrated sodium bicarbonate solution.


Clinical manifestations of hypernatremia can be subtle, consisting of lethargy, weakness, and irritability. With more severe elevations of the sodium level, seizures and coma may occur.

Severe symptoms usually require an acute elevation in the plasma sodium concentration to above 158 mEq/L (normal is typically about 135-145 mEq/L). Values above 180 mEq/L are associated with a high mortality rate, particularly in adults. This may be due, in part, to the fact that such high levels of sodium rarely occur without severe coexisting medical conditions.


The cornerstone of treatment is administration of free water to correct the relative water deficit. Water can be replaced orally or intravenously. However, overly rapid correction of hypernatremia is potentially very dangerous. The body (in particular the brain) adapts to the higher sodium concentration. Rapidly lowering the sodium concentration with free water, once this adaptation has occurred, causes water to flow into brain cells and causes them to swell. This can lead to cerebral edema, potentially resulting in seizures, permanent brain damage, or death. Therefore, significant hypernatremia should be treated carefully by a physician or other medical professional with experience in treatment of electrolyte imbalances

Overexertion and heat stress

Any activity or situation that promotes heavy sweating can lead to water intoxication when water is consumed to replace lost fluids. Persons working in extreme heat and/or humidity for long periods must take care to drink and eat in ways that help to maintain electrolyte balance. Persons using drugs such as MDMA may overexert themselves, perspire heavily, and then drink large amounts of water to rehydrate, leading to electrolyte imbalance and water intoxication. Even persons who are resting quietly in extreme heat or humidity may run the risk of water intoxication if they drink large amounts of water over short periods for rehydration.

Psychiatric conditions

Psychogenic polydipsia is the psychiatric condition in which patients feel compelled to drink large quantities of water. The condition is often a single symptom in a broader syndrome of psychiatric indications. Patients suffering from psychogenic polydipsia are at high risk of water intoxication, gaining up to 15 pounds in an hour, especially as the initial symptoms of lightheadedness and confusion may be misdiagnosed by care-takers as due to other causes.

Unusual water losses in disease

Diarrhea and vomiting can result in very large electrolyte losses, and although drinking water will replace lost water, the lost electrolytes may not be adequately replaced, which can result in water intoxication. Replacement fluids for vomiting and diarrhea should be properly balanced to make them isotonic with the fluids lost in these conditions. Special formulations exist for oral rehydration therapy in these cases.

A great many disorders can affect electrolyte balance, especially disorders of the kidneys. Diuretic therapy, mineralocorticoid deficiency, osmotic diuresis (as in the hyperglycemia of uncontrolled diabetes), and the multiple disorders associated with AIDS are other common causes of electrolyte imbalance, although they do not always produce water intoxication.

Iatrogenic water intoxication

When an unconscious person is being fed intravenously (e.g., total parenteral nutrition) or via a nasogastric tube, the fluids given must be carefully balanced in composition to match fluids and electrolytes lost. If the fluids administered are hypotonic with respect to fluids lost, electrolyte imbalance and water intoxication may result. The latter may not be immediately obvious in an unconscious patient. The electrolyte status of patients on TPN must be monitored carefully even when they are ambulatory.

Famous cases of water intoxication

In a much-publicized case of fraternity hazing, four members of the Chi Tau House at California State University, Chico pled guilty to forcing 21-year-old student Matthew Carrington to drink excessive amounts of water while performing calisthenics in a frigid basement as part of initiation rites on 2 February 2005. He collapsed and died of heart failure due to water intoxication.

Other fatalities due to water intoxication include Leah Betts, Anna Wood, 2002 Boston Marathon competitor Cynthia Lucero, and Washington, D.C. police officer James McBride. New Zealand race-walker Craig Barrett collapsed during the last kilometer of the 50 km walk in the 1998 Commonwealth Games in a non-fatal case of water intoxication.

Prevention of water intoxication

Water intoxication can be prevented if a person's intake of water and electrolytes closely matches his or her losses. The body's regulatory mechanisms provide a very generous margin of safety if the two are imbalanced, but some extreme activities (such as heavy, prolonged physical exertion), as well as disease states, can overwhelm or impair these mechanisms. Avoiding situations that provoke extreme or prolonged perspiration and/or drinking fluids that are specially balanced to replace lost electrolytes can help to prevent intoxication. Eating regularly can provide needed electrolytes if only normal water is available for rehydration.

Sports drinks are popular among athletes because they provide the necessary electrolytes to support extended exercise. They help keep the body balanced and carrying the right amount of fluids. However, not all drinks advertised as sports drinks are suitable for this purpose, and professional advice should be sought for potentially risky situations such as those described above.

Note that a person's innate sense of thirst is triggered by overall dehydration, not by changes in electrolytes. Thus, it is possible to develop water intoxication while trying to satisfy thirst, if one drinks a great deal of water over a short period. A dangerous drop in electrolytes, such as the hyponatremia that leads to water intoxication, will not have any effect on thirst.

For people suffering from dehydration due to the heavy perspiration associated with heavy exertion or heat stress, drinking water to rehydrate is much more important than avoiding water intoxication, since the former is extremely common and the latter is rare. One should never avoid drinking water under such conditions; instead, other steps should be taken to ensure that electrolytes are replaced as well, as noted above.


Overhydration occurs when the body takes in more water than it excretes and its normal sodium level is diluted. This can result in digestive problems, behavioral changes, brain damage, seizures, or coma. An adult whose heart, kidneys, and pituitary gland are functioning properly would have to drink more than two gallons of water a day to develop water intoxication. This condition is most common in patients whose kidney function is impaired and may occur when doctors, nurses, or other healthcare professionals administer greater amounts of water-producing fluids and medications than the patient's body can excrete. Overhydration is the most common electrolyte imbalance in hospitals, occurring in about 2% of all patients.

Infants seem to be at greater risk for developing overhydration. The Centers for Disease Control and Prevention has declared that babies are especially susceptible to oral overhydration during the first month of life, when the kidneys' filtering mechanism is too immature to excrete fluid as rapidly as older infants do. Breast milk or formula provide all the fluids a healthy baby needs. Water should be given slowly, sparingly, and only during extremely hot weather. Overhydration, which has been cited as a hazard of infant swimming lessons, occurs whenever a baby drinks too much water, excretes too little fluid, or consumes and retains too much water.

Causes and symptoms

Drinking too much water rarely causes overhydration when the body's systems are working normally. People with heart, kidney, or liver disease are more likely to develop overhydration because their kidneys are unable to excrete water normally. It may be necessary for people with these disorders to restrict the amount of water they drink and/or adjust the amount of salt in their diets.

Since the brain is the organ most susceptible to overhydration, a change in behavior is usually the first symptom of water intoxication. The patient may become confused, drowsy, or inattentive. Shouting and delirium are common. Other symptoms of overhydration may include blurred vision, muscle cramps and twitching, paralysis on one side of the body, poor coordination, nausea and vomiting, rapid breathing, sudden weight gain, and weakness. The patient's complexion is normal or flushed. Blood pressure is sometimes higher than normal, but elevations may not be noticed even when the degree of water intoxication is serious.

Overhydration can cause acidosis (a condition in which blood and body tissues have an abnormally high acid content), anemia, cyanosis (a condition that occurs when oxygen levels in the blood drop sharply), hemorrhage, and shock. The brain is the organ most vulnerable to the effects of overhydration. If excess fluid levels accumulate gradually, the brain may be able to adapt to them and the patient will have only a few symptoms. If the condition develops rapidly, confusion, seizures, and coma are likely to occur.

Risk factors

Chronic illness, malnutrition, a tendency to retain water, and kidney diseases and disorders increase the likelihood of becoming overhydrated. Infants and the elderly seem to be at increased risk for overhydration, as are people with certain mental disorders or alcoholism.


Before treatment can begin, a doctor must determine whether a patient's symptoms are due to overhydration, in which excess water is found within and outside cells, or excess blood volume, in which high sodium levels prevent the body from storing excess water inside the cells. Overhydration is characterized by excess water both within and around the body's cells, while excess blood volume occurs when the body has too much sodium and can't move water to reservoirs within the cells. In cases of overhydration, symptoms of fluid accumulation don't usually occur. On the other hand, in cases of excess blood volume, fluid tends to accumulate around cells in the lower legs, abdomen, and chest. Overhydration can occur alone or in conjunction with excess blood volume, and differentiating between these two conditions may be difficult.


Mild overhydration can generally be corrected by following a doctor's instructions to limit fluid intake. In more serious cases, diuretics may be prescribed to increase urination, although these drugs tend to be most effective in the treatment of excess blood volume. Identifying and treating any underlying condition (such as impaired heart or kidney function) is a priority, and fluid restrictions are a critical component of every treatment plan.

In patients with severe neurologic symptoms, fluid imbalances must be corrected without delay. A powerful diuretic and fluids to restore normal sodium concentrations are administered rapidly at first. When the patient has absorbed 50% of the therapeutic substances, blood levels are measured. Therapy is continued at a more moderate pace in order to prevent brain damage as a result of sudden changes in blood chemistry.


Mild water intoxication is usually corrected by drinking less than a quart of water a day for several days. Untreated water intoxication can be fatal, but this outcome is quite rare.

In short, maintain adequate hydration by monitoring fluctuations in weight, outward physical signs(swelling/puffiness), clothes tightness/looseness, mental status, and respirations, pulse, blood pressure, skin suppleness/rebound by the pinch test, and so on. Thirst is not always a reliable indicator of hydration, especially when not properly hydrated. Those with any organ or glandular disease should monitor or be monitored more closely. Even healthy individuals can experience life threatening situations.


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DISCLAIMER:  The information in this column, is NOT intended to diagnose and/or treat any health related issues and is provided solely for informational purposes only. Consult the appropriate healthcare professional before making any changes to your healthcare regime. Even what may seem like simple changes in the diet for example, can interact with, and alter, the efficiency of medications and/or the body's response to the medications. Many herbs and supplements exert powerful medicinal effects. Neither the author, nor the website designers, assume any responsibility for the reader's use or misuse of this information.

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