The glycemic index was created as a way to characterize the effect of carbohydrate-rich foods on blood glucose levels. It is calculated by measuring the effect of 50 grams of carbohydrate over 2 hours in 10 healthy people after overnight fasting. The index is calculated as relative measurement to the effect of 50 grams of pure glucose itself.
An index of <100 means that the food has less effect compared to glucose, whereas an index >100 means it is has a greater effect.
Barnard ND. Nutrition guide for clinicians. 2nd ed. Washington: Physicians Committee for Responsible Medicine; 2009.