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 and eventually circulatory collapse and heat stroke. 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

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. The normal body stores of carbohydrate in a typical athlete are:

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.

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 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 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.