Hitting the wall
Causes
The cause is that the muscles withdraw the required energy as glucose the blood. So that the blood glucose level does not drop, glycogen in muscle and liver is broken down to glucose. Without food intake, the liver can keep blood sugar levels only for a certain time constant and it comes to bonking. After the endurance athlete can win the energy only by burning the stored fat in fat cells, which has for the organism a significantly increased load and including a substantially increased oxygen demand result.