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Journal of the American College of Nutrition, Vol. 23, No. 1, 1-4 (2004)
Published by the American College of Nutrition

Commentary

Glycemic Index in the Treatment of Diabetes: The Debate Continues

John L. Sievenpiper, MSc and Vladimir Vuksan, PhD

Department of Nutritional Sciences, Faculty of Medicine, University of Toronto and Clinical Nutrition and Risk Factor Modification Centre, St. Michael’s Hospital, Toronto, CANADA

Dr. Vladimir Vuksan, Clinical Nutrition and Risk Factor Modification Centre, St. Michael’s Hospital, #6 138-61 Queen Street East, Toronto, Ontario, M5C 2T2, CANADA. E-mail: v.vuksan{at}utoronto.ca

The glycemic index (GI) remains a promising yet controversial area of research. Controversy regarding the application of GI in the management of diabetes was recently reignited with the publication in Diabetes Care of a positive meta-analysis on low-glycemic index interventions in diabetes by Brand Miller and coworkers [1] and the negative editorial it received from Franz, the past co-chair of the American Diabetes Association working group on nutrition recommendations [2].

The merits and limitation of the GI concept have been debated since its introduction in 1981 by Jenkins and coworkers [3]. The debate has centered on the importance of carbohydrate quality versus quantity in medical nutrition therapy (MNT) for diabetes (Table 1). The GI can be considered as a measure of carbohydrate quality. It measures the postprandial glycemia raising potential of a single food by expressing the rise in glycemia in response to a 50 g available carbohydrate portion of that food as a percentage of the rise in glycemia in response to a 50 g available carbohydrate portion of a reference food (white bread or glucose) [3]. Proponents of giving priority to carbohydrate quality argue that GI is a robust measurement [4–7], predicts the relative glycemic response to mixed meals [5–7], is easy to implement and follow [8–10], results in consistent improvements in glycemic control when applied in people with diabetes [7,11–17], and shows consistency for a benefit of low-GI in the prevention and treatment of diabetes across the three main levels of evidence: epidemiology, clinical interventions, and basic science [13–17]. In contrast, the opponents that favor giving priority to carbohydrate quantity argue that GI is highly variable [18–20], not physiological [18,21], cannot reliably predict mixed meal responses [18–26], difficult to learn and follow [18,19,22,23], and has inconsistent effects on markers of glycemic control and other aspects of metabolic control in people with diabetes [18–20,22,23,27–29].


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  		Table 1. The Two Sides of the Glycemic Index (GI) Debate

 
These opposing positions have been reflected in the nutrition recommendations of International diabetes organizations (Table 2). The American Diabetes Association (ADA) [30] in their most recent evidence based guidelines maintained their position that carbohydrate quantity is a more important consideration than quality, dismissing the value of the GI in diabetes therapy. In contrast, most other major international diabetes organizations have interpreted the available data differently, supporting the application of the GI concept in the management of diabetes in their most recent guidelines. These organizations include the Canadian Diabetes Association (CDA) [31], Diabetes Australia (DA) [32], Diabetes UK [33], and the European Association for the Study of Diabetes (EASD) [34].


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  		Table 2. Major International Recommendations for the Nutritional Management of Diabetes

 
One of the principal points of divergence between the ADA and the other international diabetes organizations has been the clinical utility of the GI. The recent meta-analysis by Brand-Miller and coworkers [1] addresses this issue directly. It provides compelling evidence that there is a clear trend of outcome benefit. The low-glycemic index interventions decreased HbA1c by 0.43% units compared with high-glycemic index interventions [1]. Although modest, this reduction is comparable with the 0.5% unit reductions in HbA1c achieved typically with the {alpha}-glucosidase inhibitor, acarbose [35]. But in the accompanying editorial, Franz [2] questioned the clinical significance of the reductions. She argued that when the meta-analysis results are viewed in the context of conventional MNT interventions, these interventions are more effective than low-GI interventions in improving glycemic control, as assessed by HbA1c [2].

This claim deserves closer inspection. Fundamental differences in the designs between the 7 randomized controlled trials of conventional MNT reviewed in the editorial [2,36,37] and the 14 studies of low-GI interventions included in the meta-analysis [1] obscure comparisons. First, the MNT interventions are heterogeneous. Some described using energy restriction [38], while others described using different combinations of energy restriction [39], carbohydrate counting [40], increased meal frequency [39,40], or physical activity [39,40] with the addition of non-nutritional interventions such as increased treatment intensity [39,41], multidisciplinary care [42], behavioral modification [38,40,42], group education [40], or blood glucose self-monitoring [38–40,42] to achieve reductions in HbA1c. To complicate matters further, two of the seven MNT interventions included aspects of low-GI interventions. One study included a low-GI curriculum as part of a larger 12-week intensive nutrition and exercise program [43]. Another study, in addition to energy restriction, included low-GI principles emphasizing decreased intake of refined rapidly absorbed starches and sugars and increased intake of soluble fibre [44]. In contrast, the low-GI interventions included in the meta-analysis applied consistent guidelines for low-GI diets [5,7], where the low-GI maneuver was the sole intervention [1]. Second, the selected MNT interventions reviewed are poorly controlled. One MNT study was a "before versus after" treatment analysis without a reference group [44], while the other studies compared an MNT intervention with either no intervention [38,43] or the same MNT intervention with a different format [40] or usual care [39,41,42] unmatched for energy and macronutrient profile of the diets. Even oral agent and/or insulin therapy were allowed to increase in the MNT intervention compared with the control in two of the seven studies reviewed [37,40]. Conversely, the low-GI intervention studies consistently compared low-GI with high-GI diets, matched, at a minimum, for energy and macronutrient profiles with oral agent and insulin therapy held constant. The only exception was the study by Gilbertson, in which the difference in GI between the two diets was very small (77 versus 79, respectively). A number of these low-GI intervention studies also represented partial metabolic feeding trials [1]. Taken together, the study designs assessing MNT interventions make it difficult to disentangle the effects of the nutritional interventions from the other aspects of conventional care, whereas the studies of the low-GI interventions isolate the effectiveness of the low-GI diets within the context of conventional care.

These important differences in design alter the conclusions that can be drawn. If low-GI interventions included in the meta-analysis permitted similar multifactorial approaches with medication changes and were compared with the same unmatched controls as in the reviewed MNT intervention studies, then they might also have enjoyed such large effect sizes. Arguably, their effects could be considered additive. The high-GI interventions used as controls are consistent with conventional MNT interventions in terms of intensity of care and macronutrient recommendations. Comparisons in these studies can be considered as low-GI conventional MNT versus conventional MNT alone. By this rationale, if conventional MNT interventions have been shown to yield ~1–2% unit [2,36,37] reductions while low-GI interventions yield ~0.4% unit [1] reductions in HbA1c, then it can be argued that low-GI interventions actually yield ~1.4–2.4% unit reductions. In this regard, the MNT intervention that included a low-GI curriculum as part of a larger 12-week intensive nutrition and exercise program reduced HbA1c by 1.8% units compared with a control group that received no intervention [43].

In conclusion, better-controlled and comparative studies that test the hypothesis that specific aspects of conventional dietary interventions are more effective than low-GI interventions are required before the claim can be made that conventional MNT interventions that do not include low-GI are more effective nutrition therapies. In the meantime, the Brand Miller and coworker analysis [1] should satisfy the skeptics that low-GI interventions are clinically efficacious in diabetes therapy over the mid to longterm. This acknowledgment renders the other arguments regarding its variability, calculation, application to mixed meals, ease of use, etc. academic. That is, it works despite its limitations. The debate continues.

Received September 30, 2003. REFERENCES

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