Glycemic index foods are best before exercise, while glycemic index foods are best immediately after exercise.

Low glycemic index foods are useful before exercise and high glycemic index foods are good after exercise.
The glycemic index of a food is a relative measure of how fast that food is broken down. A low glycemic index means slow break down, and a high glycemic index indicates fast metabolism of the food, with glucose having the highest glycemic index of 100.
Low glycemic index foods before exercise help to provide a steady source of energy during the exercise session, and high glycemic index foods quickly replenish the energy that was lost during exercise.