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Effects of High vs Low Glycemic Index of Dietary Carbohydrate on Cardiovascular Disease Risk Factors and Insulin Sensitivity: The OmniCarb Randomized Clinical Trial | Journal Scan
Study Questions:
Does the effect of glycemic index and amount of total dietary carbohydrate affect risk factors for cardiovascular disease and diabetes?
Methods:
This was a randomized crossover-controlled feeding trial, which included overweight adults. Participants were provided four complete diets that contained all their meals, snacks, and calorie-containing beverages, each for 5 weeks. Participants had to complete at least two study diets. Enrollment started in April 2008, and was completed in December 2010. For any pair of the four diets, there were 135-150 participants contributing at least one primary outcome measure. The diets included a high–glycemic index (65% on the glucose scale), high-carbohydrate (58% energy) diet; a low–glycemic index (40%), high-carbohydrate diet; a high–glycemic index, low-carbohydrate (40% energy) diet; and a low–glycemic index, low-carbohydrate diet. Each diet was based on a healthful Dietary Approaches to Stop Hypertension (DASH)-type diet. The primary outcomes of interest were insulin sensitivity (determined from the areas under the curves of glucose and insulin levels during an oral glucose tolerance test); levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides; and systolic blood pressure.
Results:
A total of 163 participants completed two or more diets and were included in the analysis. For any pair of diets, there were 135-150 participants. The trial ended when at least 160 participants completed at least two diets, as planned. Women comprised 52% of the participants; 51% were black. Hypertension was present in 26% of participants; obesity (body mass index ≥30 kg/m2) in 56%; LDL cholesterol of ≥130 mg/dl in 68%; triglycerides of ≥150 mg/dl in 17%; and fasting blood glucose of ≥100 mg/dl in 30%. At high dietary carbohydrate content, the low– compared with high–glycemic index level decreased insulin sensitivity from 8.9 to 7.1 units (−20%, p = 0.002); increased LDL cholesterol from 139 to 147 mg/dl (6%, p ≤ 0.001); and did not affect levels of 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 from 91 to 86 mg/dl (−5%, p = 0.02). 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 lowered triglycerides from 111 to 86 mg/dl (−23%, p ≤ 0.001).
Conclusions:
The authors concluded that in this 5-week controlled feeding study, diets with low–glycemic index of dietary carbohydrate, compared with high–glycemic index of dietary carbohydrate, did not result in improvements in insulin sensitivity, lipid levels, or systolic blood pressure. 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.
Perspective:
These data suggest that type of carbohydrate, and in particular a diet based on glycemic index, may not be clinically useful for improving cardiovascular risk parameters. Thus, a generally healthy diet high in fruits and vegetables and healthy grains with appropriate portion size continues to be the best advice for patients.
Keywords: Beverages, Blood Glucose, Blood Pressure, Body Mass Index, Cardiovascular Diseases, Edible Grain, Cholesterol, Diabetes Mellitus, Diet, Carbohydrate-Restricted, Dietary Carbohydrates, Fruit, Glucose Tolerance Test, Glycemic Index, Hypertension, Insulin Resistance, Insulins, Obesity, Portion Size, Risk Factors, Snacks, Triglycerides, Vegetables
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