If you've been mournfully steering away from some of your favorite fruits for fear that their glycemic index (GI) is "too high," well I have some GREAT news for you! Those bananas, watermelon and grapes are your friends, not your enemies. They are fiber and mineral dense and their water-rich aspect is vital for not only endurance athletes but for anyone who takes on a solid exercise regimen. The bigger picture is the glycemic LOAD of these foods, and that's where the good news comes in.
The glycemic index is a scale that rates how much certain foods raise blood sugar levels compared with glucose (or sometimes white bread is used, which has a glycemic index of 100). Various foods are compared against this index, which uses a standard quantity, i.e. "x" grams, to do the comparison. So, 50 grams of carrots are compared to 50 grams of watermelon and 50 grams of kidney beans, etc. etc.
Do you see the problem here? The GI doesn't take into account portion size, so using it to determine which fruits to eat and which to avoid is absurd. For example, by this measure, the glycemic index of a gooey candy bar is lower than the GI of the same amount of watermelon but the concentration of the "sugars" in each of these things is dramatically different! According to Loren Cordain in his groundbreaking book, The Paleo Diet for Athletes, you would have to eat 1.5 POUNDS of watermelon vs. 3 oz. of that candy bar for an accurate comparison. The glycemic load of a serving of rice cakes is over 5 times that of your bananas; and nearly 10 times higher than those grapes. Clearly, portion size AND LOAD are king when it comes to blood glucose and insulin response.
Don't let the GI numbers scare you from the health-giving benefits of fresh fruits from around the world. They can power-up your performance in your sport... and in your life!