Emmilou, its all to do with Glycemic Index. A simple sugar like..errr....sugar is absorbed super fast from the gut so it causes a spike in blood sugar. This is all detemined by nutritionists working away busily in their labs.
Anyway, this spike in blood sugar is a bit like an alarm bell and it shakes the pancreas awake and makes it produce insulin. The idea of the diet is smooth out the spikes by using foods that only release their sugar slowly. These are called Low Glycemic foods. There is a scale where sugar is 100 and everything is related to that. Mashed potatoes would register 85. Asparagus would register 15. Generally foods with a GI of less than 50 release their sugar slowly so you don't get blood sugar spikes.
The problem with continually squeezing the pancreas to make insulin is that it makes the fat cells store fat. Eventually the fat cells get fed up responding to the insulin and become resistant. So the sugar stays washing around in your blood i.e. you become diabetic.
An athlete needs to balance eating against task.
This is ripped from the BUPA website:
Many experts recommend a low glycaemic index meal one to two hours before an exercise session.5 It is thought that the slower, more consistent, release of energy will improve performance by delaying fatigue. It has also been found that low GI meals produce higher blood sugar and fatty acid levels during the latter stages of exercise.6,7 For endurance sports, this is very advantageous.
During exercise, when blood sugar levels need to be maintained, one way to achieve this is to drink an isotonic sports drink. This will provide glucose, which is quickly absorbed, fluid and also sodium.
After exercise, however, when muscles' stores of sugar (glycogen) need to be quickly restored, a high GI food is a good choice. This is because the simple sugars contained in a high GI food are rapidly absorbed into the blood stream. In turn, this rapid absorption stimulates insulin release and that results in increased uptake by the muscle cells of glucose and their manufacture of glycogen.
At the moment, I am trying to lose a stone in weight so I am avoiding all high GI foods, even after exercise.