The Glycemic Index (also glycaemic index, GI) is a ranking system for carbohydrates based on their immediate effect on blood glucose levels. It compares carbohydrates gram for gram in individual foods, providing a numerical, evidence-based index of postprandial (post-meal) glycemia. The concept was invented by Dr. David J. Jenkins and colleagues in 1981 at the University of Toronto.
Carbohydrates that break down rapidly during digestion have the highest glycemic indices. An increased blood glucose response occurs very quickly. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have a low glycemic index. A lower glycemic index suggests slower rates of digestion and absorption of the sugars and starches in the foods and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. Additionally, a lower glycemic response equates to a lower insulin demand, better long-term blood glucose control and a reduction in blood lipids.
Glycemic index values for different foods are calculated by comparing measurements of their effect on blood glucose with an equal carbohydrate portion of a reference food. The current scientific validated methods use glucose as the reference food. Glucose has a glycemic index value of 100. This has the advantages in that it is universal and it results in maximum GI values of approximately 100.