In a new study published in the Journal of the American Medical Association, diets with low glycemic index of dietary carbohydrate do not appear to lead to a reduction in cardiovascular disease risk or improvement in insulin sensitivity. The study was funded by the National Heart, Lung, and Blood Institute of the National Institutes of Health (NIH).

In the OmniCarb Study (a randomized crossover-controlled trial), 163 overweight adults aged ≥30 with systolic blood pressure (BP) 120–159mmHg, diastolic BP 70–99mmHg, and body mass index (BMI) ≥25 were randomized to a sequence of the four study diets: high-glycemic index (65% on the glucose scale), high-carbohydrate (58% energy); low-glycemic index (40%), high-carbohydrate diet; high-glycemic index, low-carbohydrate diet (40% energy); and low-glycemic index, low-carbohydrate diet. Each diet was modified from the healthy dietary patterns established in the Dietary Approaches to Stop Hypertension (DASH) and Optimal Macronutrient Intake to Prevent Heart Disease (OmniHeart) studies. Each diet was administered for five weeks, separated by a break of at least two weeks during which the patients ate their self-selected diet. The primary outcomes were insulin sensitivity, systolic blood pressure, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglyceride levels. Secondary outcomes included diastolic blood pressure, fasting, two-hour blood glucose and insulin, and other lipoprotein parameters.

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In the primary diet comparison, the low-glycemic index, low-carbohydrate diet did not affect insulin sensitivity, systolic blood pressure, LDL cholesterol, or HDL cholesterol compared with the high-glycemic index, high-carbohydrate diet. It did, however, reduce triglycerides from 111 to 86mg/dL (−23%, P≤0.001).

While lowering glycemic index may improve weight loss or maintenance, the authors conclude that using glycemic index to select specific foods in an overall DASH-type diet may not improve cardiovascular risk factors or insulin resistance.

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