The Glycemic Index: High GI vs Low GI. The glycemic index classifies carbs based on how quickly and how high they raise blood sugar levels when compared to pure glucose (sugar), although white bread is now used as the reference food in its place.ome common foods with a high GI rating include white bread, white rice, corn flakes, crackers and most sugary and/or highly processed snack foods. A carbohydrate with a low glycemic index (low GI) breaks down more slowly therefore causing a much slower and more gradual release of glucose into the bloodstream.