Making the Glycemic Index More Palatable
Making the Glycemic Index More Palatable
Abstract & Commentary
By Dónal P. O'Mathúna, PhD . Dr. O'Mathúna is Senior Lecturer in Ethics, Decision- Making & Evidence, School of Nursing, Dublin City University, Ireland; he reports no financial relationship to this field of study.
Synopsis: In patients with stable type 2 diabetes, a low-glycemic index diet for six months gave a modestly better reduction in HbA1c concentration compared to a high-cereal fiber diet. A high withdrawal rate raises questions about the practicality of adhering to the diet.
Source: Jenkins DJ, et al. Effect of a low-glycemic index or a high-cereal fiber diet on type 2 diabetes: A randomized trial. JAMA 2008;300:2742-2753.
Clinical trials using antihyperglycemic medications to improve glycemic control have not demonstrated the anticipated cardiovascular benefits. Low-glycemic index diets may improve both glycemic control and cardiovascular risk factors for patients with type 2 diabetes, but debate over their effectiveness continues due to trial limitations. The objective of this study was to test the effects of low-glycemic index diets on glycemic control and cardiovascular risk factors in patients with type 2 diabetes.
The investigators conducted a randomized, parallel-group study at a Canadian university hospital research center with 210 type 2 diabetics treated with antihyperglycemic medications. Subjects were recruited by newspaper advertisement and randomly assigned to one of two treatment arms (a high-cereal fiber diet or low-glycemic index dietary advice) for six months. The main outcome measure was absolute change in glycated hemoglobin A1c (HbA1c), with fasting blood glucose and cardiovascular disease risk factors as secondary measures.
In the intention-to-treat analysis, HbA1c decreased by -0.18% absolute HbA1c units (95% confidence interval [CI], -0.29% to -0.07%) in the high-cereal fiber diet compared with -0.50% absolute HbA1c units (95% CI, -0.61% to -0.39%) in the low-glycemic index diet (P < 0.001). There was also an increase of high-density lipoprotein cholesterol in the low-glycemic index diet of 1.7 mg/dL (95% CI, 0.8-2.6 mg/dL) compared with a decrease of high-density lipoprotein cholesterol by -0.2 mg/dL (95% CI, -0.9 to 0.5 mg/dL) in the high-cereal fiber diet (P = 0.005). The reduction in dietary glycemic index related positively to the reduction in HbA1c concentration (r = 0.35; P < 0.001) and negatively to the increase in high-density lipoprotein cholesterol (r = -0.19; P = 0.009).
In patients with type 2 diabetes, six-month treatment with a low-glycemic index diet resulted in moderately lower HbA1c levels compared to a high-cereal fiber diet.
Diet and lifestyle changes have been shown to effectively prevent or help manage type 2 diabetes in high-risk patients. Such patients are also at risk for cardiovascular diseases. Controversy exists as to whether medications and diets that improve glycemic control in such patients also improve cardiovascular outcomes. The World Health Organization and the Diabetes Associations of the United States, Canada, and the United Kingdom have given qualified support for the glycemic index concept, but many health professionals consider the index too complex and variable for use in clinical practice.1
Glycemic index is a method of ranking carbohydrates from 0 to 100 based on the extent to which they raise blood sugar levels immediately after eating compared to reference foods. High-glycemic index foods lead to a peak in blood sugar within 30-60 minutes of eating, followed by a rapid decline. Low-glycemic index foods usually have a later and lower peak, spread out over up to two hours.
A new and more complete set of glycemic index tables was published at the end of 2008.1 Low-glycemic index foods have a score of 55 or less when compared to glucose as the reference food (bread is another common reference standard). Common foods with low scores include most varieties of legumes, pasta, fruits, and dairy products. Part of the practical difficulty with this index is that breads, breakfast cereals, rice, and snack products, including whole-grain versions, are available in high-glycemic index forms (70 or greater) and low-scoring forms. However, many manufacturers do not put these values on their labels, and the values can vary from country to country. Highly processed, convenience, and sugary foods tend to have high-glycemic index values.
The glycemic index should not be used in isolation. For example, chocolate has a low-glycemic index, but its high saturated fat content must also be taken into consideration.
Low-glycemic index diets are claimed to improve both glycemic control and cardiovascular risk factors. A 2009 Cochrane systematic review of trials using a low glycemic index diet found reduced levels of glycated proteins in diabetes patients.2 The clinical trial reviewed here sought to address questions regarding the impact of a low-glycemic index diet on cardiovascular risk factors. Previous trials have reported conflicting results, although these were generally of shorter duration and enrolled small numbers of patients. The trial reviewed here sought to correct this by enrolling more than 200 participants and being conducted over six months. The participants had stable type 2 diabetes with HbA1c concentrations between 6.5% and 8.0%. A previous one-year trial of a low-glycemic diet that found no beneficial effect on HbA1c levels was criticized for enrolling participants with a mean concentration of 6.1%, thus making it difficult to detect beneficial reductions.3 Participants in the trial being reviewed here were taking oral antihyperglycemic medications. Those taking acarbose were ineligible. Acarbose is an alpha-glucosidase inhibitor that is required for carbohydrate digestion; it basically creates a low-glycemic index diet by slowing the rate of carbohydrate absorption from the intestinal tract.
The participants were randomly assigned to a low-glycemic index diet or a diet high in cereal fiber. The carbohydrate servings were calculated to deliver 42%-43% of total dietary calories. Different calorie totals were permitted based on participants' weight-loss goals. Although participants were informed this was not a weight-loss trial, most began the trial overweight and wished to lose weight. Participants were given lists of either low-glycemic index foods or high-cereal fiber foods. Daily records of food intake were maintained. Monthly visits were scheduled to check on participants' compliance with their assigned diet.
The study adhered to high standards in reporting. Power calculations were presented, along with an unplanned interim analysis. While conducting the trial, a new report demonstrated a smaller than anticipated HbA1c reduction from a similar diet. The interim analysis found a similarly small reduction, allowing the researchers to revise their power analysis and increase the number of participants required. Several statistical tests were conducted to check for various confounding variables. One limitation with this trial was the high drop-out rate (21%), although this is comparable to similar studies.
Results were analyzed on an intention-to-treat basis. As listed above, significantly greater improvements were found for the low-glycemic index diet in HbA1c and HDL levels, but no significant differences in LDL or total cholesterol levels. The two diets did not differ significantly in their impact on blood pressure or C-reactive protein.
This well-conducted trial provides further evidence for the effectiveness of low-glycemic index diets in improving glycemic control and reducing certain cardiovascular disease risk factors. Although the reduction in HbA1c concentration was modest, the authors concluded that it was clinically significant. The reduction falls within the range given by the FDA as clinically meaningful in the development of new medications. Guiding diabetic patients on selecting low-glycemic index foods thus can be helpful as part of overall management of type 2 diabetes.
1. Atkinson FS, et al. International tables of glycemic index and glycemic load values: 2008. Diabetes Care 2008;31: 2281-2283.
2. Thomas D, Elliott EJ. Low glycaemic index, or low glycaemic load, diets for diabetes mellitus. Cochrane Database Syst Rev 2009;(1):CD006296.
3. Wolever TM, et al. The Canadian Trial of Carbohydrates in Diabetes (CCD), a 1-y controlled trial of low-glycemic-index dietary carbohydrate in type 2 diabetes: No effect on glycated hemoglobin but reduction in C-reactive protein. Am J Clin Nutr 2008;87:114-125.In patients with stable type 2 diabetes, a low-glycemic index diet for six months gave a modestly better reduction in HbA1c concentration compared to a high-cereal fiber diet. A high withdrawal rate raises questions about the practicality of adhering to the diet.
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