Introduction

Depletion of the body's carbohydrate stores and dehydration are two factors that will limit prolonged exercise.

Dehydration

Sweating is the way in which the body maintains it's core temperature at 37 degrees centigrade. This results in the loss of body fluid and electrolytes (minerals such as chloride, calcium, phosphate, magnesium, sodium and potassium) and if unchecked will lead to dehydration eventually circulatory collapse and heat stroke. The effect of fluid loss on the body is as follows:

Electrolytes
Electrolytes serve three general functions in the body:
. many are essential minerals
. they control osmosis of water between body compartments

. 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:

. Sodium
. Potassium
 Calcium
. Magnesium
. Chloride
. Bicarbonate
. Phosphate
. Sulphate

Carbohydrate

Carbohydrate 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. 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. The body's carbohydrate stores are gradually depleted and if not checked will result in reduced performance. 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.

Fluid absorption

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 stomach

. 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 ftom 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 ftom the intestine and encourage fluid retention.

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 carbohydrates or electrolytes.

Sports Drinks

There are three types of Sports drink all of which contain various levels of fluid, electrolytes and carbohydrate.

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 28 0-33 OmOsm/kg. Drinks with an osmolality of270-330mOsm/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 replace losses.

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 nonnally 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.

Want to make your own?

Isotonic - 200ml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1 g).

Mix all the ingredients together and keep chilled

Hypotonic - lOOml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1 g). Mix all the ingredients together and keep chilled.

Hypertonic - 400ml of orange squash (concentrated orange), 1 litre of water and a pinch of salt (1 g). Mix all the ingredients together and keep chilled.