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:

1. the speed at which it is emptied from the stomach
2. 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.

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 280-330mOsm/kg. Drinks with an osmolality of 270-330mOsm/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 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.

Want to make your own?
Isotonic - 200ml of orange squash (concentrated orange), 1 liter of water and a pinch of salt (1g). Mix all the ingredients together and keep chilled.

Hypotonic - 100ml of orange squash (concentrated orange), 1 liter of water and a pinch of salt (1g). Mix all the ingredients together and keep chilled.

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

Dental Health
Sports drinks commonly contain citric acid. All acids have an erosive potential but the method of drinking will influence whether or not those acids affect 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]

Article by Brian Mackenzie
UK Athletics Senior Coach (UKA 4)
and president of Sports Coach.