Once you start running, all that carbohydrate you've been eating is broken down in your digestive system to extract glucose and glycogen. Glucose will supply immediate energy needs and will combine with oxygen to be burned pretty much as soon as it arrives in your system. Glycogen, however, is stored in muscles and in the liver to be converted into glucose and then ATP (adenosine triphosphate) as required in the future. Around two thirds of your body's glycogen reserves will be stored in the muscles and one third in the liver.

Because glucose is vital for producing ATP, distance runners need to store as much glycogen as possible. Increasing your training runs will naturally increase your capacity, but 2000 kcal will be about the most you can store -- no matter how fit you are. The reason so many marathon runners hit the wall at 20 miles is because energy consumption on a distance run will be around 100 Kcal per mile. Obviously if you go faster you burn more calories mile and will hit the wall sooner. That's why it's important than doing a marathon to take on calories as you run by the by energy bars or carbohydrate gel.

Calorific value

A calorie (cal) is a unit of measurement of the energy needed to raise the temperature 1g of water by 1° C. A kilo calorie (kcal or C) is equivalent to 1000 cal. This is the measurement used in food packaging to denote the amount of energy that will be provided by a particular food. Beware of high calorie foods as they will not necessarily convert efficiently into energy for exercise. Calories obtained from fat or sugars are not as good as calories from complex carbohydrates such as pasta rice potatoes.