What is the effectiveness of low glycemic index diets for improving the control of diabetes?
The low glycemic index diet (sometimes called a low glycemic load diet) lowered the A1C level by a weighted mean difference of 0.5 percent when compared with a high glycemic index (or glycemic load) diet. Fewer episodes of hypoglycemia occurred with a low glycemic index diet compared with a high glycemic index diet.
The goal of diabetes care is to regulate blood glucose to normal or near normal levels as safely as possible. For most patients, the American Diabetes Association (ADA) recommends A1C control below or near 7 percent in nonpregnant adults for the prevention of microvascular and neuropathic complications.1
A recent Cochrane review identified studies that compared a low glycemic index diet with a high glycemic index diet or a diet using measured carbohydrate exchange in persons with less than optimally controlled type 1 and type 2 diabetes. The review included 11 relevant randomized controlled trials (n = 402), with each trial lasting four to 12 months. The end point evaluated was A1C level. Low glycemic index diets improved A1C by a weighted mean difference of 0.5 percent when compared with high glycemic index diets (95% confidence interval [CI], −1.0 to −0.1; P = .03) and a measured carbohydrate exchange diet (95% CI, −0.9 to −0.1; P = .02). In one of the trials included in the review, hypoglycemic events were significantly decreased in low glycemic index diets compared with high glycemic index diets (difference of −0.8 episodes per patient per month; P < .01). Also, fewer participants on a low glycemic index diet had more than 15 hypoglycemic episodes per month compared with participants on a measured carbohydrate exchange diet (35 versus 66 percent; P = .006).
The A1C reduction in the Cochrane review is consistent with the results of a meta-analysis from 2003 that compared low glycemic index diets with high glycemic index diets in children and adults with type 1 and type 2 diabetes.2 The mean difference in participants' A1C end-point values showed that low glycemic index diets reduced A1C by 0.43 absolute percentage points more than the A1C reduction produced by high glycemic index diets (95% CI, 0.72 to 0.13), assuming independence.2
Medical nutritional therapy is an essential component in the management of diabetes. Much research has focused on the effects of the type and quantity of carbohydrates in the diet. The glycemic index compares the postprandial effects of food items. Foods with a low glycemic index exert a more linear effect on blood glucose and appear to improve glycemic control. The ADA noted that for persons with diabetes, the use of the glycemic index and glycemic load may provide a modest additional benefit for glycemic control over that observed when total carbohydrate count is considered alone.1 It is unclear whether this has any effect on clinical outcomes such as cardiovascular mortality, neuropathy, or renal function. Nevertheless, the reduction in A1C in the Cochrane review is significant and comparable with that found with the use of several oral diabetes medications. Moreover, the low glycemic index diet reduced episodes of hypoglycemia. Successful interventions involved having patients consume more servings of high-fiber foods (e.g., lentils, beans, oats), which they chose from a provided list. This focus gives the patients flexibility in food choices that can work within any culture.
The Cochrane review did not evaluate fasting blood glucose or postprandial glucose. The metabolic effect of a low glycemic index diet cannot be evaluated in this review. However, the low glycemic index diet provided a safe and clinically proven method to assist patients in meeting their A1C goals. It is a useful tool for glycemic control in conjunction with other modalities.