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Water - the antidote to heatstroke and hyperthermia

Kevin Fulthorpe explains why it is vital to maintain the body's fluid level when exercising in both hot and cold climates.

The importance of adequate fluid intake should not be underestimated, especially when working in hot, humid conditions. Anyone undertaking exercise in these conditions should be aware of the importance of hydration and re-hydration. Because water is so essential, anyone exercising should know to drink appropriate amounts of water throughout the day from an office water dispenser or one of the many other filtration systems.

Bear these facts in mind:

  • Over half the weight of an adult man is made up of water
  • Water is continuously being lost from the body through urine, faeces, skin perspiration and breathing
  • In normal conditions a person can lose about one and a half pints per day, this can be easily accommodated through a regular diet
  • Hot climates add further to the problems of dehydration with a person losing up to 1 pint of sweat per hour


During prolonged exercise using a large muscle mass with high-intensity, i.e. speed and force of contraction, a large amount of heat is produced which must be dissipated to maintain thermal equilibrium and optimal physical performance. That is to say, an increase in your work rate running, walking etc., causes an increase in body temperature to counteract this increase, the body will secrete fluid (perspiration) which will then lie on the skin to aid cooling.

The evaporation of perspiration provides the most efficient manner to dissipate excess heat in a person, sweating allows the greatest heat dissipation during exercise in the warm environment. It can result in large fluid losses over a relatively short period; as a consequence, the fluid requirement is based upon sweat losses during exercise.

One of the problems of inadequate fluid intake is dehydration.

Mild dehydration will impair exercise capacity, difficulty in concentrating, and breakdown of the simplest of skills as a result of fatigue preventing the person from achieving optimum performance. Severe dehydration is potentially fatal. An increase in workloads in a dehydrated state leads to a rapid rise in body temperature and the onset of heat illness.

Adequate fluid intake should be taken before, during and after any activity, which will increase body temperature. What constitutes "adequate intake" and the type of fluid taken in not easy but will depend on intensity and duration of activity coupled with the ambient climatic conditions and the physiology and biochemistry of the individual should be taken into consideration.

The fluid loss during exercise is linked to the need to maintain body temperature within narrow limits. Body temperature must be maintained within only a few degrees of the normal resting value of about 37ºC.

During fluid loss, water is not the only worry, certain minerals are also lost, these "electrolytes" as they are called, i.e. sodium, potassium, magnesium, etc. are crucial in keeping the equilibrium of the body fluid.

Excessive intake of fluids with low sodium content has been reported to induce hyponatremia (low blood sodium levels) during a fluid loss of long duration. Ingestion of plain water in the post-exercise period also results in a rapid fall in the plasma sodium concentration and plasma osmolity (diffusion of fluid through a porous partition into another fluid). These changes reduce the stimuli to drink (thirst) and of stimulating urine output, both of which delay the re-hydration process. Other fluids which affect the re-hydration process are coffee, tea and of course alcoholic drinks, all of which reduce the process of re-hydration.

During work rates that would cause fluid loss, it is recommended that a carbohydrate solution is taken as frequently as practicable, i.e. 150 to 250ml every 20 minutes. In hot climates, a drink with low concentration glucose or glucose polymer solution of 2.5 to 8% should maximise the gastric emptying. The addition of 10 to 20m mols1-1 sodium should allow optimum absorption of both carbohydrate and water. During recovery, the carbohydrate and electrolyte content should be increased (5 to 15% glucose or glucose polymer 30 to 40m mol 1-1 sodium).

The rate at which the stomach empties (gastric emptying) into the small intestine is influenced by different factors, including the volume and caloric content of beverages. Recent research found that the gastric emptying rate of beverages containing 6 to 8% carbohydrate was similar to water when at rest and during exercise. More recent research indicates that 6% carbohydrate-electrolyte beverage is absorbed faster than water.

Given that we need to replace fluids lost in sweat to maintain the body at a safe temperature, what do these conclusions mean?

They indicate that both water and sports drinks can replace lost fluids, but sports drinks will do so more rapidly. There are additional benefits provided by sports drinks that water cannot match, soft drinks tend to be too sweet and carbonated to drink in large gulps, whereas water tends to shut down the desire to drink before re-hydration has been completed. By contrast, the sodium in sports drinks helps maintain the desire to drink, thereby ensuring more fluids are consumed. The amount and type of carbohydrate in sports drinks have been a subject of debate which has not been resolved. But we know that carbohydrate-electrolyte beverages enter the bloodstream faster than water and deliver a little extra energy for working muscles.

The effect of extreme heat

In extremely hot conditions, the body's heat-loss mechanisms may fail. When atmospheric temperature equals the body temperature, it becomes impossible for the body to lose heat from radiation. If there is also high humidity, sweat does not evaporate well. In these circumstances, particularly during strenuous exercise, heatstroke may develop.

The body temperature

To keep the body temperature within a safe range of 36 to 38 C (97.8 to 100.4 F) the body must maintain a constant balance between heat gain and heat loss. The balance is regulated by a "thermostat" deep within the base of the brain. The body's steady heat gain produced by the conversion of food to energy (the metabolism) and by muscular activity, must in normal conditions be offset by continuous heat loss. Some methods of heat loss are passive - for example, the natural tendency of body heat to be lost to cool the surrounding air. Others are active - notably changes that occur within the circulatory system and at the skin. In hot conditions, blood vessels dilate so that more blood heat may be lost by radiation from the skin. This process is reversed when heat must be conserved.

In hot conditions, the body reacts to lose heat:

  • The blood vessels in or near the skin dilate to lose blood heat
  • Sweat glands become active
  • Heat is lost as the sweat evaporates in cooler air
  • The rate and depth of breathing will increase - warm air is expelled and cool air drawn in to replace it, cooling the blood in the vessels of the lungs.

Heat exhaustion - dehydration

Dehydration usually develops gradually and is caused by the loss of salt and water from the body through excessive sweating. It is more common in persons who are unaccustomed to working or exercising in a hot, humid environment, and in those who are unwell, especially those with diarrhoea and vomiting.

As the dehydration develops, there may be;

  • Headache, dizziness and confusion
  • Loss of appetite and nausea
  • Sweating with pale, clammy skin
  • Cramps in the limbs and abdominals
  • Rapid, weakening pulse and breathing


This condition often occurs suddenly and can cause unconsciousness in minutes. There may be a warning period when the casualty feels uneasy and unwell. Heatstroke is caused by a failure of the "thermostat" in the brain due either to prolonged exposure to very hot surroundings or illness involving a very high fever (such as malaria). The body rapidly becomes dangerously overheated.

As heatstroke develops, there may be:

  • Headache, dizziness and discomfort
  • Restlessness and confusion
  • Hot, flushed and dry skin
  • A rapid deterioration in the level of response
  • A full bounding pulse
  • Body temperature above 40 C (104 F)

Water for health

How much a person drinks will depend on the individual and activities undertaken. Individual needs vary, but as a rough guide allow 1¾ pints (1 litre) of water per one hour of exercise. In warmer conditions, drink more. Tea and coffee cannot act as a substitute for fluid replacement - the caffeine in these drinks will act as a diuretic making a person urinate more frequently and then, in turn, increases the danger of dehydration.


As dehydration reaches extreme levels, particularly in hot environments, impaired handling of body temperature rises can lead to heat stress. Dehydration will lead to discomfort as well as impairment of exercise performance and is also a major health threat. In hot conditions, 1 to 2 litres of sweat per hour can occur, and as little as a 1 to 2% loss of body weight can significantly impair exercise tolerance and stamina - as well as reduce comfort and skill levels.

Decreases in various exercise parameters are proportional to the degree of dehydration of the athlete, and for exercise, performance to be at its best the athlete should drink fluids during exercise to keep pace with sweat losses or as near as possible.

The effect of fluid loss on the body is as follows:

% body weight lost as sweat Physiological Effect
2% Impaired performance
4% Capacity for muscular work declines
5% Heat exhaustion
7% Hallucinations
10% Circulatory collapse and heat stroke

Prolonged high-intensity-exercise will gradually exhaust muscle and liver carbohydrate (glycogen) stores, particularly if the athlete begins the event with depleted glycogen levels. Low fuel supplies will result in fatigue as with hitting the wall in the marathon where depletion of glycogen results in low blood glucose. These effects can be delayed or reduced by consuming carbohydrates during exercise, thereby maintaining blood glucose levels and providing additional fuel to the muscles. 30 to 60g of carbohydrate per hour during prolonged exercise can be effective in extending endurance performance with muscle needs reaching 60g/hr during high-intensity activities lasting two to three hours.

General rule - sports or training programs longer than 90 minutes of continuous high-intensity activity may benefit from carbohydrate intake during activity.

Time should be given for the absorption of carbohydrate into the bloodstream before the onset of fatigue occurs. After most sessions, some form of dehydration will have occurred, as will the depletion of glycogen stores. Re-hydration and refuelling are, therefore, a very important part of the recovery process, immediate intake of fluid and carbohydrate is a high priority. Whilst athletes understand the importance of fluid and carbohydrate intake during events such as marathons or triathlons few athletes refuel or re-hydrate during training sessions.

Research suggests sports drinks provide a convenient way of addressing special nutritional needs. Sports drinks should contain 5 to 8% carbohydrate (50 to 80g of carbohydrate per litre). Drinks of this concentration have been shown to produce a rapid supply of fluid and carbohydrate. Carbohydrate types include glucose, glucose polymers, sucrose and fructose and a mixture of these will achieve a palatable drink that is rapidly absorbed to allow refuelling during and after exercise.

In general, it is believed that sodium does not need to be replaced during exercise unless ultra-endurance events of more than four hours HIA (High-Intensity Activity) is undertaken. However, the presence of a small amount of sodium in a drink increases the rate of intestinal absorption of carbohydrate and fluid. A dilute sodium drink may help to increase the rate of re-hydration both during and after exercise compared to water alone.

A guide to sports drinks

Commercial sports drinks fall under three main headings - hypotonic, isotonic and hypertonic. These refer to the concentration of the drink compared with the balance of the body's natural fluids - this will influence the absorption rates of the fluid.

Hypotonic - These are less concentrated than the body's fluids and will be absorbed quicker than water. They will help with rapid re-hydration during long exercise sessions and immediately afterwards.

Isotonic -These are also absorbed quickly and are more in balance with the body's natural fluids ideal for the rapid re-hydration following exercise.

Hypertonic - These are more concentrated than body fluids and are absorbed slowly; therefore, these are not ideal for re-hydration because of their high carbohydrate content. They are suitable for replenishing energy stores to aid recovery, and these drinks should be taken with water, isotonic or hypotonic drinks.

Homemade sports drinks

Sports drinks can be expensive. It is possible to make a homemade version which in most cases is just as good.

Hypotonic drink - for before, during and after exercise

  • 4 fl oz (12 ml) orange squash
  • 1 ¾ pints (1 litre) drinking water
  • Small pinch of salt

Shake ingredients until well mixed and then chill until needed

Isotonic drink - for before, during and after exercise

  • 2 oz (50 g) granulated sugar or glucose
  • 1¾ pints (1 litre) drinking water
  • Small pinch of salt

Warm 4 tablespoons of the water and mix it with the salt and glucose or sucrose, add the rest of the water and chill.

Hypertonic drink - for restoring energy after exercise

  • 1 pint (570 ml) unsweetened orange or apple juice
  • small pinch of salt

Shake ingredients until well mixed and then chill until needed.

Salt loss

Water replacement alone will not compensate for the loss of electrolytes (i.e. sodium and potassium) in the sweat. For each litre of sweat lost, 1.5 g of salt is lost as well. Exercising over 8 hours may equate to a loss of 12 g salt. Salt tablets are not recommended as they are slow to dissolve. While in the stomach, the high salt concentration encourages movements of water into the digestive tract via osmosis. Whilst dissolving, they take needed water from the bloodstream and can cause stomach cramps, weakness and high blood pressure.

Working in cold conditions

Many people associate re-hydration and fluid loss with warm climatic conditions, but dehydration can and does occur during cold conditions. Working or exercising in cold conditions will alter your fluid, and nutritional requirements and your body will generate 8 to 12 times the amount of heat compared to what it will produce whilst resting.

The body's response to the cold is vasoconstriction (narrowing blood vessels) to the arms, legs, hands and feet to reduce the amount of blood carrying heat to the skin surface. This is the body's way of protecting the vital organs. It will divert body heat to ensure core temperature is kept within the optimum range for metabolism (the process of chemical changes by which energy is provided for the maintenance of life) 36 C to 42 C.

Shivering is the body's response to a drop in core temperature; this involves the involuntary contraction of large muscles and is a very effective way of generating heat. Violent shivering can produce at least three times more heat compared with someone at rest. This, in turn, requires more fuel in the form of carbohydrate for heat generation.

During cold weather, we tend to pile on more clothes as this will help to raise the body's temperature. This, in turn, creates weights for the wearer to carry, which will require a small amount of extra energy. This is known as the "hobbling effect" which applies to skiers, hill walkers and mountaineers. This amounts to no more than a requirement of a mere 100 Calories per day. Calorific requirements increase when the body is exposed to a cold climate. An increase in energy needs is determined by the type of activity undertaken in cold conditions, e.g. walking through deep snow, hauling a sledge, digging snow holes and skiing will require extra energy needs.

For extended periods of outdoor exertion, dress in layers. Layers of clothing provide an insulating barrier of air and can be peeled off as your temperature rises and put back on when it falls. Wool is one of the best fabrics to wear for under and outer garments.

A 12 stone man walking a 2 mph on a level hard surface would use around 200 Calories per hour increasing to 800 Calories per hour in the deep soft snow. Downhill skiing at moderate speed would require from 650 to 950 Calories per hour. When exercising at these levels of energy expenditure generating heat is not a problem, although fluid loss through sweating can be significant.

When exercising in cold conditions, some people have higher calorie needs. Scientists have found that this is due to several factors including climatic conditions, amount and type of clothing and even the weight of the shoes.

As in warm conditions, your fluid replacement needs are crucial to your performance while training, during warm conditions you lose a tremendous amount of fluids through sweating, in cold conditions you lose more fluids while breathing cold air which must be warmed and moistened in your throat and lungs.

When active under these cold and dry conditions, the amount of moisture lost through respiration increases significantly and must be replaced. As you exhale, you lose lots of water as the humidity content of cold air is much less than warm air. It is essential to keep up with this loss (as mentioned with a fluid loss in hot conditions) with frequent fluid replacement during the day and while exercising if possible. You will also find the need to urinate more in cold conditions, also adding to dehydration. This is known as the phenomena of cold dieresis.

Dehydration whether in the winter or summer will result in fatigue which will affect your ability to perform at your highest level - a decrease in your blood volume caused by sweating and water loss from breathing means less blood flow to your skin and extremities. This will lead to rapid cooling of your body and an increase in susceptibility to hypothermia or frostbite.

Another problem could come from the temperature of the fluid. A litre of water heated to 60 C would provide about 18 Calories of heat to a person. In contrast, a litre of water near freezing would absorb about 35 Calories of energy to heat it up to your normal body temperature. From this, you can see drinking warm water will add very little energy to your body while drinking cold water in these conditions will deplete your body of calories.

Coldwater has a thirst-quenching aspect which appeals to most people, with adequate heat production, which occurs during exercise, the intake of cold water will not harm, however, if you are feeling chilled, it would be advantageous to avoid a further calorific drain, but if the choice is between cold water and no water, drink cold water. You can also consume energy drinks with the same consistency of carbohydrate that you might use in hot conditions.

Avoid drinks containing caffeine and alcohol. These will speed up dehydration by promoting urine production. Alcohol also decreases glucose output by the liver in the cold and speeds heat loss by dilating your skin's blood vessels.

Effects of clothing

Because perspiration is a major problem during exercise in the cold, you would be smart to purchase a set of synthetic (polypropylene) undergarments. This amazing fabric absorbs perspiration away from the skin so evaporative cooling won't strip the body of heat. Next, wear a wool shirt or sweater for warmth. A wind and rainproof top should be all the additional clothing you need during exercise. Invest in a "breathable" rain top. Synthetic fabrics have several advantages over goose down. They are less expensive, easier to care for, do not mat and have insulating qualities when wet.


As with training or working in warm climatic conditions, problems can occur at the other end of the scale and can have potentially fatal consequences. When the body loses heat faster than it can be produced, you are at risk from hypothermia. Exercise or working for long periods leads to progressive muscular fatigue.

In an attempt to preserve heat, the body will slow down the blood flow to the extremities (vasoconstriction) to maintain the temperature of the vital organs. The body will also react by shivering to increase body temperature - this as mentioned earlier will deplete the glycogen levels within the liver and muscles causing neuromuscular impairment, and the exhaustion of energy stores will lead to virtual termination of the activity.

With further exposure and loss of body heat, the cold will reach the brain, at this stage judgement and the ability to reason diminishes, speech becomes slow and slurred, loss of control of limbs and walking becomes clumsy. At this stage, all you want to do is lie down and rest. DON'T!!

You are now hypothermic; your core temperature is falling, and without medical attention, you will fall into unconsciousness and die.

Most cases of hypothermia develop in temperatures above 0 C (32 F). Cold water, wind chill and fatigue combine to set the stage for hypothermia. It is important to stay dry, but should you become wet dry off as soon as possible. Wind chill will refrigerate wet clothing, and the body will then have to work harder to keep warm using up much-needed energy hastening the latter stages of hypothermia.

When exercising or working in cold conditions take off layers of clothing before you start to raise body temperature and perspire, then put them back on as you begin to cool. Eat and rest often to maintain your energy level. Remember, prevention is the best form of cure. Always prepare for any eventuality. You cannot play around with your or anyone else's life.

The weather can be a good friend but also a bad enemy, and once the symptoms of hypothermia become apparent, you need to get to a hospital as quickly as possible. The victim's heart may fibrillate during re-warming, and emergency resuscitation equipment may be needed.

If transport is unavailable or if the case is not severe then:

  • Remove victim from wind and rain
  • Remove wet clothing
  • Provide warm drinks, NO ALCOHOL
  • Provide dry clothing and a warm, dry sleeping bag for mildly impaired victims
  • Do not allow the victim to fall into a state of unconsciousness; try to keep them awake, leave them stripped and put them in a sleeping bag with another person. This will provide warmth for the victim
  • If possible build a fire
  • Monitor heart rate, great care should be taken when attempting CPR with hypothermic victims.

CPR & Hypothermia

When a person is in severe hypothermia, they may demonstrate all the accepted clinical signs of death:

  • Cold
  • Blue skin
  • Fixed and dilated pupils
  • No discernible pulse
  • No discernible breathing
  • Comatose & unresponsive to any stimuli
  • Rigid muscles

But they still may be alive in a "metabolic icebox" and can be revived. Your job as a rescuer is to re-warm the person and do CPR if indicated. A hypothermia victim is never cold and dead only warm and dead. During severe hypothermia, the heart is hyperexcitable and mechanical stimulation (such as CPR, moving them or After-drop) may result in fibrillation leading to death. As a result, CPR may be contraindicated for some hypothermia situations:

  1. Make sure you do a complete assessment of heart rate before beginning CPR. Remember, the heart rate may be 2-3/minute, and the breathing rate 1/30 seconds. Instituting cardiac compressions, at this point, may lead to life-threatening arrhythmias. Check the carotid pulse for a longer time (up to a minute) to ascertain if there is some slow heartbeat. Also, even though the heart is beating very slowly, it is filling and distributing blood fairly effectively. External cardiac compressions only are 20-30% effective. Thus, with its severely decreased demands, the body may be able to satisfy its circulatory needs with only 2-3 beats per minute. Be sure the pulse is absent before beginning CPR. You will need to continue to do CPR as you re-warm the person.
  2. Ventilation may have stopped, but respiration may continue - the oxygen demands for the body have been so diminished with hypothermia that the body may be able to survive for some time using only the oxygen that is already in the body. If ventilation has stopped, artificial ventilation may be started to increase available oxygen. Also, blowing warm air into the person's lungs may assist in internal re-warming.

CPR Procedures

Curtis (2004)[1] suggests:

  • Check radial pulse, between 91.4 and 86-degrees F this pulse disappears
  • Check for a carotid pulse - wait at least a full minute to check for a very slow heartbeat
  • If pulse but not breathing or slow breathing, give rescue breathing (also adds heat)
  • If no discernible heartbeat, then begin CPR and be prepared to continue - persons with hypothermia have been given CPR for up to 3.5 hours and have recovered with no neurological damage
  • Begin active re-warming


Alcohol in moderation can have some health benefits. Some beers offer a significant amount of B12. Red wine can be a good source of iron and contains health protective phenolic compounds that may reduce the risk of heart disease and keep cholesterol down.

But we must remember that alcohol is the most abused drug in this country and many others, it is a depressant and in no way will enhance the performance of any athlete, quite the contrary. Alcohol is absorbed into the system directly from the stomach, and it is in the bloodstream within 5 minutes.

The dehydration process of alcohol is well known and can be a significant problem for many athletes. The body sees alcohol as a poison and will act to remove it from the system. It will use the body's fluids to flush it through the system and out. This will lead to a decrease in the body's fluids and will aid the onset of dehydration. Water loss of more than 3% of body weight can result in a 20% drop in a work capacity.


My eight tips to maintaining appropriate fluid levels:

  1. Find fluid losses by measuring body weight before and after exercise
  2. Consume enough beverage to limit bodyweight decrease to 1%
  3. Do not wait until you become thirsty; start drinking before, during and after workouts. Try to anticipate sweat loss
  4. Dehydration (body weight loss 3%) can cause gastro-intestinal distress particularly when a beverage is ingested for the first time after this level of dehydration is reached
  5. Larger people tend to sweat more than thinner people; therefore they will need to replace more fluid and earlier than those of less ample proportions
  6. Cold drinks (5 C) are ingested quicker than warmer drinks
  7. Avoid coffee, tea and alcohol; these will aid dehydration
  8. Drink about 200 to 250 ml fluid every 20 minutes

Article Reference

This article first appeared in:

  • FULTHORPE, K. (2005) Water - the antidote to heat stroke and hyperthermia. Brian Mackenzie's Successful Coaching, (ISSN 1745-7513/ 28 / December 2005-January 2006), p. 4-8


  1. CURTIS, R. (2004) Outdoor Action Guide to Hypothermia and Cold Weather Injuries. Outdoor Action Program, Princeton University

Page Reference

If you quote information from this page in your work, then the reference for this page is:

  • FULTHORPE, K. (2005) Water - the antidote to heatstroke and hyperthermia [WWW] Available from: [Accessed

About the Author

Kevin Fulthorpe is a Sports Science lecturer at Barry College in Wales and a Coach Tutor with Sports Coach UK.