The Glycemic Index (GI) refers to the effect carbohydrates have on blood glucose. If the carbohydrate increases blood glucose by a large amount, it is considered a high glycemic index food.If, on the other hand, it increases blood glucose a small amount, it is a low glycemic index food.A carbohydrate's GI is determined by its rate of digestion and absorption in the body.For example, foods that have a high degree of soluble fiber, such as whole grains, have lower GI values.f the carbohydrate increases blood glucose by a large amount, it is considered a high glycemic index food. If, on the other hand, it increases blood glucose a small amount, it is a low glycemic index food.