What is glycemic index?
Glycemic index (GI) is a measure of how carbohydrate-containing foods affect blood glucose levels. All foods that contain carbohydrates, such as starchy vegetables (potatoes, corn), desserts, fruits, bread, pasta, and rice, can be tested for how they affect blood sugar levels after being eaten.
How is GI determined?
Glycemic index is assessed by having one or more people eat a specific amount of a single food (usually 50 grams of digestible carbohydrate [total carbohydrate minus fiber]) and then measuring the change in blood sugar levels compared to the levels achieved after they have eaten a control food such as white bread. The average change in blood sugar levels over a set period of time relative to the levels after consumption of the control food is the food’s glycemic index.
What factors affect the GI of a food?
All carbohydrate-containing foods eventually raise or maintain blood sugar levels to some extent. High-fat and high-fiber foods typically have a lower glycemic index because one must consume much larger amounts to ingest 50 grams of digestible carbohydrate. Both the fat and the fiber in these foods can slow the rate of gastric emptying and therefore reduce the rate of delivery of carbohydrate to the small intestine, where it is digested and glucose is absorbed. Fat actually inhibits (delays) gastric emptying, and many (but not all) fibers simply increase the viscosity or increase the mass to slow the emptying rate.
How is GI used?
Glycemic index is often used as a scientific research tool. It may be difficult to use GI as a measure on which to base dietary recommendations for the general population, because of its wide variability depending on the ripeness of a food, the degree to which a food is cooked, as well as other factors.
Are there any benefits to eating based on glycemic index?
Ranking and eating foods according to their glycemic index has a number of problems. First, a person’s blood sugar response to eating a food can vary from day to day and also according to how the food was prepared. Even something as simple as ripeness in the case of bananas can affect the glycemic index (the riper the banana, the higher its glycemic index). Second, once a food is combined with other foods (such as cereal and milk or peanut butter and bread), the glycemic index of the meal will be very different from that of either food alone. Finally, the amount of food eaten to measure the glycemic index is often different from the amount of food eaten in a typical serving. Creating a diet based solely on glycemic index can result in an eating plan that may exclude some nutrient-rich foods, in addition to being less palatable overall.
