Glycemic index, abbreviated GI, is a number that describes the rise in blood glucose (sugar) levels caused by eating a particular food. Glucose obtained from food provides energy for the body’s cells. Some evidence suggests, however, that elevated blood glucose levels may contribute to a variety of health problems, including obesity and Type 2 diabetes. Scientists at the University of Toronto developed the glycemic index in 1981.
Several characteristics of a food influence its glycemic index. These include the type of carbohydrates (sugars, starches, and fibers) it contains; the degree of cooking and processing; and the ripeness of fruits. Soluble fiber is a type of carbohydrate that cannot be digested by the body. Foods rich in soluble fiber, such as beans, tend to have lower GI values. Whole grains have lower GI values than the same grains that have been milled or processed to a greater degree.
Scientists have determined GI values for many foods through experimental studies. Highly processed foods and high-starch foods tend to have high GI values. Such foods include white bread, many cold breakfast cereals, and potatoes. Low-starch vegetables, most fruits, legumes (members of the pea family), and milk all have lower GI values.
Foods with high GI values have carbohydrates that are rapidly digested and absorbed into the bloodstream, causing a sharp rise in blood glucose levels. When blood glucose levels rise, the pancreas increases its secretion of insulin into the blood. Insulin is a hormone that is required for the movement of glucose from the bloodstream into cells of the body. The rise in insulin causes a drop in blood glucose levels, which may cause hunger. Foods with low GI values, on the other hand, have carbohydrates that are absorbed more slowly, causing smaller rises in blood glucose and thus insulin. Such foods may delay hunger for longer periods.
Health professionals may recommend a diet that emphasizes low-GI foods to promote weight loss. Many medical experts believe that a long-term diet rich in high-GI foods may increase a person’s risk of obesity and other health problems. Other scientists, however, do not think that glycemic index is a significant indicator of the healthfulness of a diet.
Determining the GI value
of a particular food involves feeding a portion of that food, typically 50 grams (1.8 ounces), to a human test subject following a period of fasting. Scientists then take blood samples from the subject over a period of two to three hours and measure the glucose levels in a laboratory. These measurements show the glycemic response, the effect of the food on the individual’s blood glucose level, over the time period. Scientists compare this to the glycemic response for an equivalent portion of a carbohydrate standard. Such a preparation, also known as a reference carbohydrate, typically consists of either pure glucose or white bread. Scientists usually test the glycemic response multiple times in several individuals to calculate a particular food’s GI value.
The glycemic index equals the ratio of the glycemic response of the test food to that of the carbohydrate standard, multiplied by 100. It is usually expressed as a whole number. Most foods have GI values between 0 and 150.
Most meals combine several foods with various GI values. Some scientists argue that the GI value of a mixed meal can be calculated as the weighted average of the GI values of its ingredients. In calculating such an average, scientists consider each ingredient’s GI value according to its proportion of the total carbohydrates in the meal. However, some critics argue that in many cases the GI value of a mixed meal is impossible to predict based on the glycemic indexes of individual foods. This is because other dietary components affect the overall GI value of a meal, including the amount of fat and protein the meal contains.
Glycemic load.
People typically consume different foods in different sized servings. A food’s GI value therefore may not accurately represent a normal serving’s impact on blood glucose levels. For this reason, scientists have developed another measure, glycemic load (GL), to account for both the type and amount of carbohydrate consumed. To calculate a serving’s GL value, scientists multiply the food’s GI value by the grams of carbohydrate in the serving, and then divide the result by 100.
GI value and health.
Epidemiologists (scientists who study illness in large groups of people) have conducted some research that associates a high-GI/GL diet with an increased risk of Type 2 diabetes, heart disease, and obesity. High-GI/GL diets also may increase the risk of certain cancers, including colorectal cancer and cancers of the prostate, breast, and pancreas. Scientists believe the harmful effects of high-GI/GL diets may result from the elevation of glucose and insulin levels in the body. However, scientists caution that more studies must be done on the relationship between glycemic index and health.
A well-balanced diet usually contains a mixture of foods with low and moderate GI values. People should seek the advice of a qualified health professional before making major changes in their diet based on glycemic index.
See also Carbohydrate ; Glucose ; Insulin .
