As long as you consume enough carbs during the race, even skipping breakfast altogether won’t slow you down.
There is some evidence that a low- or moderate-glycemic index pre-exercise meal (that is, a meal whose carbohydrate content is absorbed relatively slowly) enhances endurance performance compared to a high-GI meal. But the glycemic index of a pre-exercise meal might not matter if carbohydrate is consumed during exercise.
Researchers from the Chinese University of Hong Kong recently tested this idea in a new study published in the International Journal of Sports Nutrition and Exercise Metabolism. Nine men ran a 21 km modified time trial (in which they ran the first 5 km at 70 percent VO2max and the last 16 km as fast as possible) on the treadmill on three separate occasions. In random order, they consumed a high GI meal before one trial, a low GI meal before a second trial, and a placebo meal with very little energy value before a third trial. Throughout each trial, the subjects consumed a 6.6-percent carbohydrate sports drink at 2.5 km intervals. The researchers observed no difference in performance among the three trials. Even when the subjects took in virtually no calories for breakfast, use of a sports drink during running kept their performance from flagging. So when a sports drink is consumed during a half-marathon, not only does the glycemic index of the pre-exercise meal not matter, but it doesn’t even matter if you skip breakfast altogether.
Although it may surprise some, this finding is consistent with the results of past research. In fact, even carbohydrate loading has been shown to have no effect on performance if carbohydrate is consumed during a race.