What is the glycemic index and how might it impact your training and competing? To summarize:
There is insufficient evidence to support the concept that athletes will benefit from eating low GI carbohydrate meals prior to prolonged exercise, if they use carbohydrate supplements during the ride. They should let practical issues and individual experience guide their choice of the pre-event meal.
A limited number of individuals may benefit from a low GI pre-event meal. Those athletes that show an exaggerated (and negative) response to eating carbohydrates in the hour immediately before exercise (approximately 5% of the population that experience rebound hypoglycemia or blood sugar drop) may benefit from focusing on low GI foods. And during unusual endurance sessions, such as open water swimming, where practical difficulties prevent the athlete from consuming carbohydrate supplements during the session, the pre-event meal may have greater bearing with the slower absorption and release of glucose from a low GI carbohydrate meal theoretically sustaining slower but prolonged blood glucose level and thus enhance performance.
Athletes involved in events lasting more than 2 hours should focus on maintaining adequate carbohydrate supplementation during the event. Which carbohydrate drink or food they choose should be determined by their previous experiences, the logistics of the event, gastrointestinal tolerance, and the requirements for fluid replacement. A glucose-based sports drink with a moderate to high GI would make the most sense as it would quickly provide carbohydrate energy to the muscles.
Moderate and high GI carbohydrate foods are the logical choices for glycogen repletion after exercise.
Other characteristics (tasty, portable, cheap, easy to prepare and unlikely to cause stomach upsets) may outweigh the GI in making sport supplement choices. These will be specific to the individual and the exercise situation.
Those using exercise to lose weight will benefit from using low GI foods for their daily dietary carbohydrates. Likewise, those who may have a borderline blood sugar and thus be considered as pre diabetics.
More information at Cycling Performance Tips
and in this paper:
Demystifying the Glycemic Index: Implications for Practice (pdf)