Purpose. The purpose of a glycemic index (GI) diet is to eat carbohydrate-containing foods that are less likely to cause large increases in blood sugar levels. The diet is a means to lose weight and prevent chronic diseases related to obesity such as diabetes and cardiovascular disease. glycemic index diet is an eating plan based on how foods affect your blood sugar level. The glycemic index is a system of assigning a number to carbohydrate-containing foods according to how much each food increases blood sugar.