Sweating is the way in which the body
maintains its core temperature at 37 degrees centigrade. This results in
the loss of body fluid and electrolytes (minerals such as chloride,
calcium, magnesium, sodium and potassium) and if unchecked will lead to
dehydration and eventually circulatory collapse and heat stroke. The
effect of fluid loss on the body is as follows:
% body weight lost as sweat
||Capacity for muscular
||Circulatory collapse and
Electrolytes serve three general
functions in the body:
- many are essential minerals
- they control osmosis of water between
- they help maintain the acid-base
balance required for normal cellular activities
The sweat that evaporates from the skin
contains a variety of electrolytes. The electrolyte composition of sweat
is variable but comprises of the following components:
A litre of sweat typically contains 0.02g
Calcium, 0.05g Magnesium, 1.15g Sodium, 0.23g Potassium and 1.48g
Chloride. This composition will vary from person to person.
is stored as glucose in the liver and muscles and is the most efficient
source of energy as it requires less oxygen to be burnt than either
protein or fat. The normal body stores of carbohydrate in a typical
- 70kg male athlete - Liver glycogen 90g
and muscle glycogen 400g
- 60kg female athlete - Liver glycogen
70g and muscle glycogen 300g.
During hard exercise, carbohydrate can be
depleted at a rate of 3-4 grams per minute. If this is sustained for 2
hours or more, a very large fraction of the total body carbohydrate stores
will be exhausted and if not checked will result in reduced performance.
Recovery of the muscle and liver glycogen stores after exercise will
normally require 24-48 hours for complete recovery.
During exercise there is in an increased
uptake of blood glucose by the muscles and to prevent blood glucose levels
falling the liver produces glucose from the liver stores and lactate.
Consuming carbohydrate before, during and
after exercise will help prevent blood glucose levels falling too low and
also help maintain the body's glycogen stores. Many athletes cannot
consume food before or during exercise and therefore a formulated drink
that will provide carbohydrate is required.
There are two main factors that affect
the speed at which fluid from a drink gets into the body:
- the speed at which it is emptied from
- the rate at which it is absorbed
through the walls of the small intestine
The higher the carbohydrate levels in a
drink the slower the rate of stomach emptying. Isotonic drinks with a
carbohydrate level of between 6 and 8% are emptied from the stomach at a
rate similar to water. Electrolytes, especially sodium and potassium, in a
drink will reduce urine output, enable the fluid to empty quickly from the
stomach, promote absorption from the intestine and encourage fluid
What's wrong with water?
Drinking plain water causes bloating,
suppresses thirst and thus further drinking. It stimulates urine output
and therefore is inefficiently retained. A poor choice where high fluid
intake is required. Water contains no carbohydrate or electrolytes.
Calculating personal fluid needs
During an endurance event you should
drink just enough to be sure you lose no more than 2% of pre-race weight.
This can be achieved in the following way:
- Record your naked body weight
immediately before and after a number of training sessions, along with
details of distance/duration, clothing and weather conditions
- Add the amount of fluid taken during
the session to the amount of weight lost - 1 kilogram (kg) is roughly
equivalent to 1 litre of fluid.
- After a few weeks you should begin to
see some patterns emerging and can calculate your sweat rate per hour
- Once you know what your sweat losses
are likely to be in any given set of environmental conditions, you can
plan your drinking strategy for any particular event
There are three types of Sports drink all
of which contain various levels of fluid, electrolytes and carbohydrate.
||Fluid, electrolytes and
6 to 8% carbohydrate
||Fluids, electrolytes and
a low level of carbohydrate
||High level of
The osmolality of a fluid is a measure of
the number of particles in a solution. In a drink these particles will
comprise of carbohydrate, electrolytes, sweeteners and preservatives. In
blood plasma the particles will comprise of sodium, proteins and glucose.
Blood has an osmolality of 280 to 330mOsm/kg. Drinks with an osmolality of
270 to 330mOsm/kg are said to be in balance with the body's fluid and are
called Isotonic. Hypotonic fluids have fewer particles than blood and
Hypertonic have more particles than blood.
Consuming fluids with a low osmolality,
e.g. water, results in a fall in the blood plasma osmolality and reduces
the drive to drink well before sufficient fluid has been consumed to
Which is most suitable?
Isotonic - quickly replaces fluids
lost by sweating and supplies a boost of carbohydrate. This drink is the
choice for most athletes - middle and long distance running or team
sports. Glucose is the body's preferred source of energy therefore it may
be appropriate to consume Isotonic drinks where the carbohydrate source is
glucose in a concentration of 6% to 8% - e.g. High Five, SiS Go, Boots
Isotonic, Lucozade Sport.
Hypotonic - quickly replaces
fluids lost by sweating . Suitable for athletes who need fluid without the
boost of carbohydrate - jockeys and gymnasts.
Hypertonic - used to supplement
daily carbohydrate intake normally after exercise to top up muscle
glycogen stores. In ultra distance events high levels of energy are
required and Hypertonic drinks can be taken during exercise to meet the
energy requirements. If used during exercise Hypertonic drinks need to be
used in conjunction with Isotonic drinks to replace fluids.
Isotonic - 200ml of orange squash
(concentrated orange), 1 litre of water and a pinch of salt (1g). Mix all
the ingredients together and keep chilled
Hypotonic - 100ml of orange squash
(concentrated orange), 1 litre of water and a pinch of salt (1g). Mix all
the ingredients together and keep chilled.
Hypertonic - 400ml of orange
squash (concentrated orange), 1 litre of water and a pinch of salt (1g).
Mix all the ingredients together and keep chilled.
Sports drinks commonly contain citric
acid. All acids have an erosive potential but the method of drinking will
influence whether or not those acids effect the teeth. Sports drinks
should be consumed as quickly as possible, preferably with a straw and not
be held or swished around the mouth. Retaining drinks in the mouth will
only increase the risk of erosion. Refrigerated drinks will have a reduced
erosive potential as the acid dissolution constant is temperature
dependant and cold drinks are absorbed more quickly.
Food for thought
In a trail conducted by scientists in the
city of Aberdeen it was determined that a 2% carbohydrate-electrolyte
drink provided a more effective combat to exercise fatigue in a hot
climate when compared to a 15% carbohydrate-electrolyte mixture.
[Galloway SDR & Maughan RJ, The effects of substrate and fluid provision
on ermoregulatory and metabolic responses to prolonged exercise in a hot
environment. Journal of Sports Sciences, Vol 18, No5, pp339-351]
Seven Rules of Hydration
- The rate of passage of water from your
stomach into your small intestine depends on how much fluid is actually
in your stomach. If there is lots of water there, fluid flow from
stomach to intestine is like a springtime flood; if there is little
water, the movement resembles a lightly dripping tap. Therefore, to
increase stomach-intestinal flow (and overall absorption of water) you
need to deposit a fair amount of liquid in your stomach just before you
begin your exercise. In fact, 10-12 ounces of fluid is a good start.
This will feel uncomfortable at first, so practise funelling this amount
of beverage into your "tank" several times before an actual competition.
- To sustain a rapid movement of fluid
into your small intestine during your exertions, take three to four sips
of beverage every 10 minutes if possible, or five to six swallows every
- If you are going to be exercising for
less than 60 minutes, do not worry about including carbohydrate in your
drink; plain water is fine. For more prolonged efforts, however, you
will want the carbohydrate.
- Years of research have suggested that
the correct concentration of carbohydrate in your drink is about 5 to
7%. Most commercial sports drinks fall within this range, and you can
make your own 6% drink by mixing five tablespoons of table sugar with
each litre of water that you use. A bit of sodium boosts absorption;
one-third teaspoon of salt per litre of water is about right. Although 5
to 7% carbohydrate solutions seem to work best for most individuals,
there is evidence that some endurance athletes can fare better with
higher concentrations. In research carried out at Liverpool John Moores
University, for example, cyclists who ingested a 15% maltodextrin
solution improved their endurance by 30 per cent compared to individuals
who used a 5% glucose drink. The 15% drink also drained from the stomach
as quickly as the 5% one, though many other studies have linked such
concentrated drinks with a slowdown in water movement.
- A 6% "simple sugar" drink will empty
from your stomach at about the same rate as a fancy 6% "glucose polymer"
beverage, so don't fall for the idea that the latter can boost water
absorption or enhance your performance more than the former, and don't
pay more for the glucose-polymer concoction.
- Contrary to what you've heard, cold
drinks aren't absorbed into your body more quickly than warm ones.
However, cold drinks are often more palatable than warm ones during
exercise, so if coldness helps you to drink large quantities of fluid
while you exert yourself, then keep your drinks cool.
- Swilling drinks during exercise does
NOT increase your risk of digestive-system problems. In actuality, most
gut disorders that arise during exercise are caused by dehydration, not
from taking in fluid. Dehydration induces nausea and discomfort by
reducing blood flow to the digestive system, so by all means keep
Intracellular fluid and interstitial
fluid have the same osmotic pressures under normal circumstances. The
principal cation inside the cell is K+ (Potassium) , whereas the principal
cation outside is Na+ (Sodium). When a fluid imbalance between these two
compartments occurs, it is usually caused by a change in the Na+ or K+
concentration. Sodium balance in the body normally is controlled by
aldosterone and ADH (antidiuretic hormone) . ADH regulates extracellular
fluid electrolyte concentration by adjusting the amount of water
reabsorbed into the blood by the distal convoluted tubules and collecting
tubules of the kidneys. Aldosterone regulates extracellular fluid volume
by adjusting the amount of sodium reabsorbed by the blood from the kidneys
which, in turn, directly affects the amount of water reabsorbed from the
Certain conditions, however, may result
in an eventual decrease in the sodium concentration in interstitial fluid.
For instance, during sweating the skin excretes sodium as well as water.
Coupled with replacement of fluid volume with plain water, these
conditions can quickly produce a sodium deficit. The decrease in sodium
concentration in the interstitial fluid lowers the interstitial fluid
osmotic pressure and establishes an effective water concentration gradient
between the interstitial fluid and the intracellular fluid. Water moves
from the interstitial fluid into the cells, producing two results that can
be quite serious:
- The first result, an increase in
intracellular water concentration, called overhydration, is particularly
disruptive to nerve cell function. In fact, severe overhydration, or
water intoxication, produces neurological symptoms ranging from
disoriented behavior to convulsions, coma, and even death.
- The second result of the fluid shift
is a loss of interstitial fluid volume that leads to a decrease in the
interstitial fluid hydrostatic pressure. As the interstitial hydrostatic
pressure drops, water moves out of the plasma, resulting in a loss of
blood volume that may lead to circulatory shock.