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 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, which is essential for an athlete. For most people, foods with a low value are the best choice in keeping blood sugar levels consistent over long periods of time.
Foods with a high rating are those that are digested easily, but they leave the eater feeling hungry and lethargic a short time later. Examples include white bread, pasta, and white rice. These are the same foods that many nutritionists often refer to as "bad carbs."
Food with a low reading fill the eater up, but raise blood sugar slowly and steadily. This gives the eater a longer feeling of being full and more continuous energy. These foods also contain carbohydrates, but they 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 value 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. By better understanding how carbohydrates are broken down by the body, however, nutritionists have been able to design diets for diabetics that focus on those foods with a lower glycemic value.