Marathon runners are whopping energy consumers

Runners completing this weekend's London Marathon in three hours will use as much energy as a 1kW electric heater consumes in an hour, says the Royal Society of Chemistry.

Muscles need energy and the body stores this as the natural chemical glucose in a tightly packed molecular form called glycogen - and ready for action. 

"There can be a kilogram of glycogen waiting to be used and this is capable of yielding 4000 kilocalories of energy, and all of this will be needed during the marathon," said the RSC's Dr John Emsley. 

"Although a runner generates a lot of heat, the body stops itself from overheating by evaporating about two litres of sweat from the skin.

 "Along with the sweat, the body also loses electrolytes, namely up to four grams of sodium and about half a gram of potassium. These are also vital components if muscles are to work at maximum efficiency." 

A runner's main requirement is for water, but taking in more glucose, sodium, and potassium will help as well. A sports drink is designed to replenish these. 

However, it must not contain too much glucose because that delays absorption of the water.. If there is too much glucose in the drink then this is counter productive because it first has to be diluted in the stomach by taking water from the body before it can be absorbed.

Dr Emsley added: "Not surprisingly, when an athlete consumes a sports drink it boosts his or her performance more than if they simply refreshed themselves with water. 

"The Japanese introduced such drinks and in the 1980s they came up with the not inappropriately named Pocari Sweat whose sales were eventually to exceed 200 million bottles a year."

 The US has its own version called Gatorade, named after the University of Florida football team, the Gators (from alligator), who helped test it and show that it was beneficial. In the UK we have Lucozade Sport. This contains the right balance of 64 grams of glucose, 1 gram of sodium (equivalent to 2.5 grams of salt), and 100 milligrams of potassium per litre. 

"So what should a marathon runner drink during their long slog? First and foremost they need water, but if you are there as a fun event and are starting to flag, then a cool draught of a sports drink might just help you to reach the finishing line", he said.