In a study that included overweight and obese participants, those with diets with low glycemic index of dietary carbohydrate did not have improvements in insulin sensitivity, lipid levels, or systolic blood pressure, according to a study in the December 17 issue of JAMA.1
Foods that have similar carbohydrate content can differ in the amount they raise blood glucose, a property called the glycemic index. Even though some nutrition policies advocate consumption of low-glycemic index foods and even promote food labeling with glycemic index values, the independent benefits of glycemic index, and its effect on risk factors for cardiovascular disease and diabetes, are not well understood, according to background information in the article.
Frank M. Sacks, M.D., of Brigham and Women’s Hospital and Harvard Medical School, Boston, and colleagues conducted a trial in which 163 overweight adults with prehypertension or stage 1 hypertension were given 4 different complete diets that contained all of their meals, snacks, and calorie-containing beverages, each for 5 weeks; and completed at least 2 study diets. The four diets were (1) a high-glycemic index (65 percent on the glucose scale), high-carbohydrate diet (58 percent energy); (2) a low-glycemic index (40 percent), high-carbohydrate diet; (3) a high-glycemic index, low-carbohydrate diet (40 percent energy); and (4) a low-glycemic index, low-carbohydrate diet. Each diet was based on a healthful Dietary Approaches to Stop Hypertension (DASH)-type diet, which is rich in fruits, vegetables, and low-fat dairy foods, and low in saturated and total fat.
The researchers found that at high dietary carbohydrate content, the low- compared with high-glycemic index level decreased insulin sensitivity; increased low-density lipoprotein (LDL) cholesterol; and did not affect levels of high-density lipoprotein (HDL) cholesterol, triglycerides, or blood pressure. At low carbohydrate content, the low- compared with high-glycemic index level did not affect the outcomes except for decreasing triglycerides. In the primary diet contrast, the low-glycemic index, low-carbohydrate diet, compared with the high-glycemic index, high-carbohydrate diet, did not affect insulin sensitivity, systolic blood pressure, LDL cholesterol, or HDL cholesterol but did lower triglycerides.
“In the context of an overall DASH-type diet, using glycemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance,” the authors conclude.
Robert H. Eckel, M.D., of the University of Colorado Anschutz Medical Campus, Aurora, writes in an accompanying editorial that many of the results of this study were contrary to what had been expected.2
“When glycemic index was lower in the highcarbohydrate diet, insulin sensitivity not only did not increase but decreased. With the same diet pattern, levels of LDL cholesterol and apolipoprotein B (a secondary end point) increased, with no changes in HDL cholesterol or triglyceride level or blood pressure.”
“The unexpected findings of the study by Sacks et al suggest that the concept of glycemic index is less important than previously thought, especially in the context of an overall healthy diet, as tested in this study. These findings should therefore direct attention back to the importance of maintaining an overall heart-healthy lifestyle, including diet pattern.”