The glycemic index (GI) is a method of measuring blood sugar levels with respect to carbohydrate consumption. It not only measures how much the blood sugar level increases, but also how quickly. Foods containing carbohydrates are measured on a scale of 1 to 100, giving each food its own glycemic index value.

If someone needs to raise his or her blood sugar quickly, such as an athlete running a sprinting race, he or she would want to consume foods with a high glycemic index. These foods provide a quick burst of energy, essential for an athlete. For most people, foods with a low glycemic index are the best choice in keeping blood sugar levels consistent over long periods of time.

Foods with a high glycemic index rating are those that are digested quickly, giving the person who eats them a fast burst of energy. These foods leave the eater feeling hungry and lethargic a short time later, however. Foods with a high glycemic index include white bread, pasta, and white rice. These are the same foods that many nutritionists refer to as "bad carbs."

Food with a low glycemic index reading fill you up, but raise blood sugar slowly and steadily. This gives the eater a longer feeling of being full, and more continuous energy. These are also foods containing carbohydrates, but are considered "good carbs" such as those found in many fruits, vegetables, and whole grains.

The glycemic index is an important tool used in treating people with diabetes and in weight loss programs. A food with a low glycemic index makes a person feel full longer and stay satisfied longer, so he or she is less likely to overeat. For the diabetic, the traditional way of controlling blood sugar was through counting carbohydrates. Since the inception of the glycemic index, endocrinologists have determined that not all carbs are the same. Those with low glycemic indexes are the best choice for diabetics.