Marathon Mania: Banking on the carbs

You could eat as much as you wanted before a marathon, but for such a long race, it's almost inevitable that you'll still run out of energy - what runners call "hitting the wall" - if you only rely on the body's natural stores.

"You will manage a dawdle or slow jog at best," says Dr Duncan MacFarlane, sports physiologist at University of Hong Kong's Institute of Human Performance. This is because the body only stores enough energy for up to two hours of medium intensity exercise.

Carbohydrates are the body's main energy source. Carbs may be complex (such as legumes, grains and starchy vegetables) or simple (such as fruits and foods made with sugar). During digestion, the body converts carbs into sugars, which enter individual cells to provide energy. Sugar is stored in the liver and muscles as glycogen - your main energy source for a marathon.

To stay energised, MacFarlane recommends refuelling on the run. "One decent bite of a sports bar every 30 minutes, several mouthfuls of a banana, or an energy gel pack every hour with water will help."

Research shows you can also guard against the mid-race bonk by carbo-loading in the days leading up to a marathon. A study by Liverpool John Moores University published in the International Journal of Sports Medicine in 2011 got 257 male and female runners at the 2009 London Marathon to keep a detailed food and training diary, which the researchers compared with the runners' finishing times. Each runner's pace was also tracked every five kilometres throughout the race.

It was found that competitors who had consumed carbs the day before the race at a quantity of more than seven grams per kilogram of body mass had significantly faster overall race speeds and maintained their running speed during the race longer than those who consumed fewer carbs. The difference was especially striking beginning at about the 29-kilometre mark, just when many runners famously "hit the wall".