The Glycemic Index (GI) is a measure of the effects of carbohydrates on blood glucose levels. Carbohydrates that break down quickly during digestion, releasing glucose rapidly into the bloodstream (like those found in white bread), have a “high GI” (70 or higher); carbohydrates that break down slowly, releasing glucose gradually into the bloodstream (like those in whole grains and legumes), have a “low GI” (55 or lower).
The Glycemic Load (GL) is a ranking system for the glycemic impact of foods, based on their carbohydrate content, portion size, and Glycemic Index. Low = 1 to 10; Medium = 11-19; High = 20 or higher.
As explained in the book The New Glucose Revolution for Diabetes (Marlowe, 2007), the GL was developed by Harvard researchers, who posited that eating a small amount of a high-GI food would have the same effect on blood sugar as would eating large amounts of a low-GI food. Another issue with looking only at the GI of a food is that it’s tied to the number of grams of carbohydrates in that food and, obviously, that number varies by large amounts. Watermelon is a good illustration of this problem. Watermelon’s GI is high, 72. The GI, however, is based not on a normal portion, but on 50g of carbohydrates — whatever the food. To get 50g of watermelon carbs, you’d have to eat almost 5 cups. GL combines both the quality and the quantity of the actual carbohydrates consumed — and provides one “number.” The GL of one cup of watermelon is about 9, which is low.