Research has documented that low glycemic index carbohydrates ingested before endurance running or biking improves performance when compared to high glycemic index carbohydrates (Wu CL, Williams C. 2006, Moore LJ, et al. 2010).
The research conducted by Wu C.L. and Williams C. compared a low glycemic index meal GI value 37 with a high glycemic index meal GI value 77 on two different occasions. The study was randomized and used the same participants at both occasions. Both meals were ingested 3 hours prior to the trials and contained 2 grams of carbohydrates per kg body mass.
The results showed that the low glycemic index meal resulted in a greater endurance capacity compared to the high glycemic index meal.
The studies reviewed here showed that a low glycemic index meal prior to endurance activities results in increased carbohydrate and energy availability at the end of the exercise leading to improved performance.
Low glycemic index meals are absorbed slowly and should be eaten at least 2 hours before the exercise. Maybe you would still get the benefits from a low glycemic index meal even if it is eaten 1 ½ hour before exercising, but I would not recommend eating any closer to the exercise than that.
A bean burrito would be a good choice since the beans are an excellent source of low glycemic index carbohydrates. You could also mix a little bit of pasta with beans if you don’t want a burrito. Even if the tortilla and the pasta are higher glycemic index, when combined with the beans, the meal is still low glycemic index and high in carbohydrates. You could also take some essential amino acids at the same time.
A couple of days before an endurance competition consume an extra 200 to 300 grams per day of carbohydrates, pasta is a good choice, and eat a pasta meal as the last meal the evening before the competition. Doing this will ensure saturation of glycogen storage in both muscles and the liver.
Just remember the meal especially before endurance type exercises needs to be high in carbohydrates, not only low glycemic index.
Moore LJ, Midgley AW, Thurlow S, Thomas G, Mc Naughton LR. Effect of the glycaemic index of a pre-exercise meal on metabolism and cycling time trial performance. J Sci Med Sport. 2010 Jan;13(1):182-8. Epub 2009 Feb 20.
Thomas DE, Brotherhood JR, Miller JB. Plasma glucose levels after prolonged strenuous exercise correlate inversely with glycemic response to food consumed before exercise. Int J Sport Nutr. 1994 Dec;4(4):361-73.
Wu CL, Williams C. A low glycemic index meal before exercise improves endurance running capacity in men. Int J Sport Nutr Exerc Metab. 2006 Oct;16(5):510-27.