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Taking Measure of the Glycemic Index

Bruce Goldfarb

Is the glycemic index ready for prime time? Long used as a research tool to study diet and metabolism in animals and humans, the glycemic index (GI) is making headway in the clinical arena and becoming better known among patients struggling with obesity and diabetes.

Those who work with the GI contend that it is more meaningful and clinically relevant than calories, portion size, or counting fats or carbohydrates. "This could be a very useful tool, not only for diabetes but for other health issues as well," said David Grotto, director of nutrition at Block Center for Integrative Cancer Care (Evanston, Ill.), at the recent American Diabetes Association's Scientific Sessions in Orlando, Fla.

Rather than taking stock of calories or carbohydrates, the GI is a measure of the quality of carbohydrates based on their immediate effect on blood glucose levels—within 2 to 3 hours of a meal. Glucose itself has a GI of 100, which is used as a reference point (see table). Foods with a high GI are rapidly digested and tend to dramatically affect blood glucose levels, whereas healthful foods with a low GI release glucose slowly and gradually.

The glycemic index factors into the glycemic load (GL), which takes the amount of carbohydrates into consideration. GL is calculated by multiplying the GI by the amount of available carbohydrates in grams and dividing the total by 100. For example, carrots have an average GI of around 46 and about 4 grams of carbohydrates per serving, resulting in a low GL of 1.8.

"The preponderance of evidence suggests that glycemic index plays an important role in obesity and diabetes," says Harvard obesity authority David Ludwig, MD. "A low-glycemic index diet is perfectly consistent with other health goals, such as increasing whole foods and fiber."

Studies suggest a correlation between consumption of a high-GI diet and development of type 2 diabetes, whereas consumption of a low-GI diet improves glucose control. Grotto said that research also indicates a link between high-GI foods and cardiovascular disease and breast, endometrial, pancreatic, and colorectal cancer.

Population studies show that GI varies with gender, ethnic group, and other demographic variables. "You can consider glycemic index as a marker of lifestyle," says Angela D. Liese, of the University of South Carolina.

Although perhaps not as familiar as some other nutritional buzzwords, the GI is gaining notice among patients. Consideration of the GI features prominently in the popular South Beach diet and is central to Jennie Brand-Miller's The Glucose Revolution books pitched to overweight and diabetic readers.

The GI is a ranking of carbohydrates based on their effect on blood glucose levels within 2 to 3 hours after a meal. A variety of factors can influence the GI of a food, including ripeness, processing, cooking, and storage. Values provided here are approximations for the purpose of comparison.

Two years ago, food producers in Australia began including information about the GI value of their products under a pioneering program sponsored by the University of Sydney, Diabetes Australia, and the Juvenile Diabetes Research Foundation. Under the program, food labels of participating products list GI as calculated by accepted laboratory means, helping consumers make healthful dietary choices.

Interest in similar GI labeling programs is growing in Japan and several other countries but to date has not gained momentum in the United States.

Glycemic index "isn't affected by the amount of food you eat, so long as the carbohydrate in the food doesn't change," explained Liese at the recent American Diabetes Association's Scientific Sessions in Orlando, Fla.

However, GI can be affected by a number of factors, including the variety and ripeness of a food source, how it is processed and cooked, and storage. The GI of an apple, says Liese, can vary from 28 to 44, depending on the variety.

Because of the vagaries of GI, it is difficult for most people to calculate, particularly for mixed meals. Rather than pay attention to precise numbers, suggests Ludwig, it's important for patients to recognize foods that have a high or low GI.

In general, foods with a low GI include most fruits and vegetables, beans and legumes, and whole grains. High-GI foods also tend to be higher in fats as well.

The glycemic index "shouldn't be followed too closely," said Ludwig. "People should know whether a food is low, medium, or high, rather than worrying about specific numbers."

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Services Email this article to a friend Similar articles in this journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Goldfarb, B. Search for Related Content PubMed Articles by Goldfarb, B. Social Bookmarking                What's this?