Glycemic Index

In the past, carbohydrates were classified as simple or complex based on the number of simple sugars in the molecule. Carbohydrates composed of one or two simple sugars like fructose or sucrose (table sugar; a disaccharide composed of one molecule of glucose and one molecule of fructose) were labeled simple, while starchy foods were labeled complex because starch is composed of long chains of the simple sugar, glucose. Advice to eat less simple and more complex carbohydrates (i.e., polysaccharides) was based on the assumption that consuming starchy foods would lead to smaller increases in blood glucose than sugary foods (1). This assumption turned out to be too simplistic since the blood glucose (glycemic) response to "complex" carbohydrates has been found to vary considerably. A more accurate indicator of the relative glycemic response to dietary carbohydrates should be glycemic load, which incorporates the relative quality and quantity of carbohydrates in the diet.

Measuring the Glycemic Index of Foods

To determine the glycemic index of a food, volunteers are typically given a test food that provides 50 grams of carbohydrate and a control food (white bread or pure glucose) that provides the same amount of carbohydrate on different days(2). Blood samples for the determination of glucose are taken prior to eating and at regular intervals after eating over the next several hours. The changes in blood glucose over time are plotted as a curve. The glycemic index is calculated as the area under the glucose curve after the test food is eaten, divided by the corresponding area after the control food is eaten. The value is multiplied by 100 to represent a percentage of the control food. For example, a baked potato has a glycemic index of 76 relative to glucose and 108 relative to white bread, which means that the blood glucose response to the carbohydrate in a baked potato is 76% of the blood glucose response to the same amount of carbohydrate in pure glucose and 108% of the blood glucose response to the same amount of carbohydrate in white bread (3). In contrast, cooked brown rice has a glycemic index of 55 relative to glucose and 79 relative to white bread (4). In the traditional system of classifying carbohydrates, both brown rice and potato would be classified as complex carbohydrates despite the difference in their effects on blood glucose levels.

Physiological Responses to High- versus Low-Glycemic Index Foods

By definition, the consumption of high-glycemic index foods results in higher and more rapid increases in blood glucose levels than the consumption of low-glycemic index foods. Rapid increases in blood glucose are potent signals to the beta-cells of the pancreas to increase insulin secretion (2). Over the next few hours, the high insulin levels induced by consumption of high-glycemic index foods may cause a sharp decrease in blood glucose levels (hypoglycemia). In contrast, the consumption of low-glycemic index foods results in lower but more sustained increases in blood glucose and lower insulin demands on pancreatic beta-cells (5).

Glycemic Load

The glycemic index compares the potential of foods containing the same amount of carbohydrate to raise blood glucose. However, the amount of carbohydrate consumed also affects blood glucose levels and insulin responses. The glycemic load of a food is calculated by multiplying the glycemic index by the amount of carbohydrate in grams provided by a food and dividing the total by 100 (1). Dietary glycemic load is the sum of the glycemic loads for all foods consumed in the diet. The concept of glycemic load was developed by scientists to simultaneously describe the quality (glycemic index) and quantity of carbohydrate in a meal or diet.