A glycemic index (GI) 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. The glycemic index itself is not a diet plan but one of various tools — such as calorie counting or carbohydrate counting — for guiding food choices.

The term “glycemic index diet” usually refers to a specific diet plan that uses the index as the primary or only guide for meal planning.
Many popular commercial diets, diet books and diet websites are based on the glycemic index, including the Zone Diet, Sugar Busters, and the Slow-Carb Diet.