As well as losing body mass, the rise in the body's temperature makes it less efficient.

Elite runner Butler says: "You do get progressively warmer and the key is to start off at the temperature that you will be able to maintain during the race.

"It is important early in the race to make sure you are taking on the right kind of fluids and fluids that will absorb well."

Even though London is regarded as a flat course, the values in the graph are for ambient air temperatures so dehydration and body temperatures will rise even further as the day heats up.