Glycemix Index: Pros and Cons

Carbohydrates (CHOs) are the most important energy source in the majority of human diets. In the past, they were classifi ed by their molecular size as sugar, oligosaccharides, polysaccharides, and polyols (hydrogenated CHOs). However, the goal of having a physiologic measure of the impact of carbohydrate foods on blood glucose was motivated by the desire to provide dietary advice for the control of blood glucose in diabetic persons.

The notion of adding a physiologic measure of carbohydrate quality to the chemical defi nitions was proposed in the early 1970s when the concept of carbohydrate exchange for diabetics was developed.¹ The approach was put on a more quantitative basis as a result of the development of the glycemic index (GI) by Jenkins.² Glycemic index is defi ned as the incremental area under the 2-hour blood glucose response curve (IAUC) of a fi xed amount, usually 50 g, of available CHOs of a test food expressed as a percentage of the response to the same amount of CHOs from either white bread or glucose when taken by the same subject. Carbohydrates that breakdown quickly during digestion have the highest glycemic index, and the blood glucose curve is high. Those that are slowly digested result in much lower AUC over the two hours and, it is assumed, a lower insulin response. But this relationship is linear only if pure CHOs or primarily carbohydrate foods are consumed. The glycemic load (GL) of foods, meals and diets is calculated by multiplying the GI by the amount of carbohydrate in the food.

Since 1981 many foods have been assayed to produce a quantitative number ranking the effect of foods on blood glucose immediately following their ingestion and based on their carbohydrate content. Catalogues of the GI of many foods have been published.^{3, 4} In general, processed foods made from grain fl our such as snack foods, breads and ready to eat cereals have a high GI. On the other hand, whole grains, pasta and fruits have a moderate GI. The lowest glycemic response is associated with legumes and dairy products.

The promoters of the concept claim that quantitative knowledge of the effect of a food on blood glucose allows for the selection of foods and the construction of diets that minimize fl uctuations in blood glucose and insulin, improve glucose and lipid metabolism in diabetes, lower blood triglycerides, if elevated, and benefit weight control and athletic performance. At present, however, there are polarized views on the merits of the GI and especially the need to have foods labeled with it, as has occurred in Australia, Sweden and South Africa. The assay of foods for their GI has also become a commercial business not only for the promoters of the concept but other laboratories.

The critics of the concept suggest that clinical utility is in doubt because many components of food affect glucose response, blood lipids and weight control and because insulin response to carbohydrate foods is the more important measure. Public health application is also thought to be doubtful because of public misunderstandings of the label and of monitoring and regulating the label claim because the composition of processed foods are often changed quickly by manufacturers based on the price of ingredients. The legal liability of a label claim with an unpredictable outcome has also been raised as a concern.⁵ The assay has also been claimed to be unreliable because the GI is an average of highly variable responses among test subjects6 and does not predict how an individual consuming the food will respond.⁷

At the present time, there seems to be little consensus on the application of the GI and the need to have all carbohydrate foods labeled to guide consumer choice. Indeed a recent consensus report concluded that, “As with all areas of science, a lack of data promotes controversy. Without new data it will be difficult to move beyond personal beliefs to knowledge in this important area of science.”⁷ The conflict in view arising from the application of the GI in treatment vs. prevention is also raised in the consensus report and reflected in the National Academy of Sciences summary report on the dietary reference intakes. It concludes that it is theoretically plausible “to expect a low GI diet to reduce the risk of Type II diabetes and cardiovascular disease. However, sufficient evidence needed to recommend substantial dietary changes based on GI is not available,” as was stated four years ago in a review by Pi-Sunyer.⁸ Recently, the American Diabetes Association⁹ has also provided rather ambivalent statements about the utility of the glycemic index in diabetes management, which was the original goal of the concept of 30 years ago. It identifies the methodological problems with the index and highlights the variability in response of individuals to specific carbohydrate containing foods. The report also suggests that the potential role of converting from a high to a low-glycemic index and -glycemic load diets in management of Type II diabetes is modest at best.

Despite the lack of consensus, the use of the GI has expanded over the years and for the public it seems to resonate. However the merit of having a GI for every food seems to be a complex and an unjustified cost benefit in the goal of achieving public health application. Most individuals appear to consume a moderate glycemic index diet.^{10, 11} Thus, for the majority, dietary advice for the prevention of diabetes and cardiovascular disease should focus on controlling body weight. But the role of the GI in the prevention of obesity is uncertain. A recent publication¹² of a prospective cohort study found unconvincing evidence that there is an association between the GI or GL of habitual diets and obesity.¹³ Whether or not suffi ciently large, long-term, well-powered, randomized control trials will be conducted to improve understanding of the application of GI and GL to body weight regulations remains to be seen. Furthermore, the role of dietary advice based on the GI for the control of body weight is uncertain because adding education on the GI of foods did not improve treatment outcomes in a behavioral weight loss program.

In the face of the uncertainty surrounding the benefi ts of the GI in dietary guidance, dietitians can be assured that promoting foods and eating patterns that reduce the rate of absorption of CHOs is unlikely to cause any adverse effects. Of course, a food that elicits a low glucose response may not be necessarily healthier because the food may be high in fat or low in nutrient content, but again this information is easily derived from food composition tables. As noted earlier there is suffi cient data available to provide the practitioner with practical and general guidance on the glycemic response to carbohydrate containing foods and for the construction of diets and eating patterns that will contribute to a modest improvement in glycemic control in individuals that have been habitually consuming high GI and GL diets, or have Type II diabetes.

GI/GL Ranges

GLYCEMIC INDEX
Low GI = < 55
Moderate GI = 56-69
High GI = 70+

GLYCEMIC LOAD
Low GI = < 10
Moderate GI = 11-19
High GI = 20+

Editor’s Note:
Application of the glycemic index to dietary recommendations remains controversial, as the science in this area continues to evolve. This issue of The Soy Connection presents opposing views in this area. The ADA position paper on the topic is available online at http://www.eatright.org/ada/fi les/GlycemicIndex.pdf.

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