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

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. 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 1 and a half pints per day, this can be easy accommodated through a normal 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 relatively large muscle mass with high intensity i.e. speed and force of contraction, a large amount of heat is produces 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 and can result in large fluid losses over a relatively short period of time, as a consequence fluid requirements are based upon sweat losses during exercise.

One of the problems of an 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 achieving optimum performance. Severe dehydration is potentially fatal. An increase in work loads in a dehydrated state leads to 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 is 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.

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 37șC. 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 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 in 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 have an effect on the re-hydration process are coffee, tea and of course alcoholic drinks all of which reduce the process of re-hydration.

During work rates which would cause fluid loss it is recommended that a carbohydrate solution be taken as frequently as practicable, i.e. 150 to 250 ml every 20 minutes. In hot climates a drink with a low concentration glucose or glucose polymer solution 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 in order to maintain the body at a safe temperature what do these conclusions mean?

Simply that they indicate that both water and sports drinks can replace lost fluids, but the sports drinks will do so more rapidly. There are additional benefits provided by sports drinks which 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 the sports drinks has 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.

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 by radiation. If there is also high humidity sweat does not evaporate well. In these circumstances, particularly during strenuous exercise, heatstroke may develop.

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 surrounding air. Others are active - notably changes that occur within the circulatory system and at the skin. In hot conditions, blood vessels dilate in order 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 in order 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.


Dehydration usually develops gradually, and is caused by 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 failure of the "thermostat" in the brain due either to prolonged exposure to the 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)

How much a person drinks will depend on the individual and activities undertaken. Individual needs vary, but as a rough guide allow 13ž4 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 increasing 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.

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 a 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 programmes 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 blood stream 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 is therefore a very important part of the recovery process, immediate intake of fluid and carbohydrate is 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 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.


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 absorbs 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 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.

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 3ž4 pints (1 litre) drinking water
Small pinch of salt
Shake ingredients until well mixed and the chill until needed

Isotonic drink - for before, during and after exercise
2 oz (50 g) granulated sugar or glucose
13ž4 pints (1 litre) drinking water
Small pinch or 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 or salt
Shake ingredients until well mixed and the 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, approximately 1.5 g of salt is lost as well. Exercising over a period of 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.

Immersion in iced water is the most effective first-line treatment for exercise-related heat stroke, according to new UK research (‘Cooling methods used in the treatment of exertional heat illness’, British Journal of Sports Medicine 2005; 39:503-507).

Exertional heat illness – or heat stroke – typically affects young athletes or military personnel who are pushed to their physical limits and become dangerously ill as a result of inability to dissipate the heat produced by exercise. Unlike environmental heat illness, which is associated with high external temperatures, the exertional variety can occur any time anywhere, since it is a reflection of intrinsic heat production rather than climate. While there are known risk factors, including dehydration, illness, lack of sleep, alcohol ingestion, over-dressing and poor cardiovascular fitness, these are not always present, and the reason why one person is affected rather than another is not always understood.

Exertional heat stroke is life threatening, and the evidence suggests that a rapid reduction of core body temperature is the key to survival. What is less clear, though, is which is the best method of body cooling.

Dr Jason Smith of Derriford Hospital in Plymouth, UK, set out to answer this question with an examination and appraisal of the available literature on the subject, stretching back as far as 1966. In the end, 17 papers were included in his analysis, focusing on one or more of the following cooling techniques.

Body immersion in iced water.

Evaporative cooling – spraying water over the patient and using fans to facilitate evaporation and convection.

Immersing hands and forearms in cold water.

Use of ice or cold packs in the neck, groin and armpits.

Invasive methods – iced gastric, bladder or peritoneal lavage.

Chemically assisted cooling with a drug called dantrolene, which reduces the rate of muscle contraction by blocking calcium release.

Dr Smith concludes from his review that, after an initial assessment of airway, breathing and circulation, and measurement of rectal temperature, ‘it would appear that immersion in iced water is the most effective method of whole body cooling and should be used where possible’.

He acknowledges, however, that this treatment may not always be practical from a logistic or clinical perspective and may be dangerous in patients with reduced consciousness in the absence of intensive care facilities. If immersion is unavailable or inappropriate, a combination of other techniques may be used to facilitate rapid cooling, although there is no evidence to support the use of dantrolene in these circumstances.


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