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WATER
- THE ANTIDOTE TO HEAT STROKE
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.
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EFFECTS OF EXTREME HEAT
HEATSTROKE
WATER FOR HEALTH
MAKE YOUR OWN SPORTS DRINKS
OTHER HEALTH AND FITNESS ARTICLES
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.
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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.
THE EFFECTS 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 by 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 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.
HEAT EXHAUSTION - DEHYDRATION
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
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HEATSTROKE
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)
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 134 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.
RE-HYDRATION
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.
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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 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.
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 34 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
134 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).
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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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