The glycemic index is a ranking of carbohydrates based on their immediate effect on blood sugar levels. It compares foods gram for gram of carbohydrate. The glycemic index ranks carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating. It doesn't tell how much of that carbohydrate is in a serving of a particular food. A low Glycemic Index diet is not necessarily a low carb diet.