The glycemic index is a measure of how quickly foods raise blood sugar levels. It is used to classify carbohydrate-containing foods according to their effect on blood glucose levels, with high-GI foods raising blood sugar levels rapidly and low-GI foods releasing energy more slowly. The glycemic index can help people with diabetes or those trying to manage their blood sugar levels make healthier food choices by selecting lower GI options that are less likely to cause a rapid rise in blood sugar levels.