GI and its effect on athletic performance has been investigated for more than 15 years. Currently, there is no consensus on whether consuming carbohydrate of a high or low GI improves endurance performance. For now, the athlete is encouraged to follow the carbohydrate recommendations and let their individual experience determine the use of high or low glycemic index foods and supplements before, during and after exercise. The best advice is to consume "healthier" carbohydrates; i.e. whole grains and complex carbohydrates instead of added sugars. Here is a resource for implementing the GI with athletes: Glycemic Index and the Athlete.