Ranking and eating foods according to their glycemic index has a number of problems. First, a person’s blood glucose (sugar) response to eating a food can vary from day to day and also according to how the food was prepared and which foods were eaten in the same meal. Even something as simple as ripeness in the case of bananas can affect the glycemic index (the riper the banana, the higher its glycemic index). Second, once a food is combined with other foods (such as cereal and milk or peanut butter and bread), the glycemic index of the meal will be very different from that of any food in that meal alone. Finally, the amount of food eaten to give a measure of the glycemic index is often different from the amount of food a person may eat in a typical serving. Creating a diet based solely on glycemic index can result in an eating plan that may exclude some nutrient-rich foods, in addition to being less palatable (agreeable in taste) overall.

Additional Resource: 

http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/carbohydrates/