Effects of high vs low glycemic index of dietary carbohydrate on cardiovascular disease risk factors and insulin sensitivity: the OmniCarb randomized clinical trial

Abstract

Importance: Foods that have similar carbohydrate content can differ in the amount they raise blood glucose. The effects of this property, called the glycemic index, on risk factors for cardiovascular disease and diabetes are not well understood.

Objective: To determine the effect of glycemic index and amount of total dietary carbohydrate on risk factors for cardiovascular disease and diabetes.

Design, setting, and participants: Randomized crossover-controlled feeding trial conducted in research units in academic medical centers, in which 163 overweight adults (systolic blood pressure, 120-159 mm Hg) were given 4 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 first participant was enrolled April 1, 2008; the last participant finished December 22, 2010. For any pair of the 4 diets, there were 135 to 150 participants contributing at least 1 primary outcome measure.

Interventions: (1) A high-glycemic index (65% on the glucose scale), high-carbohydrate diet (58% energy); (2) a low-glycemic index (40%), high-carbohydrate diet; (3) a high-glycemic index, low-carbohydrate diet (40% energy); and (4) a low-glycemic index, low-carbohydrate diet. Each diet was based on a healthful DASH-type diet.

Main outcomes and measures: The 5 primary outcomes 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: 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 = .002); increased LDL cholesterol from 139 to 147 mg/dL (6%, P ≤ .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 = .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 did lower triglycerides from 111 to 86 mg/dL (-23%, P ≤ .001).

Conclusions and relevance: 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.

Trial registration: clinicaltrials.gov Identifier: NCT00608049.

Figure 1

Participant Screening, Enrollment, and Follow-up in the OmniCarb Study Carb indicates carbohydrate; GI, glycemic index.

Figure 2

Effect of the Study Diets on Blood Glucose and Insulin Levels Over 12 Hours In the morning after a 10- to 12-hour fast and during the fourth or fifth week of each dietary period, the participants were given breakfast, lunch, and dinner that had the food and nutrient composition of the assigned diet period. Blood was sampled before breakfast, usually at 8:00 am, 8:30 am, 9:00 am, and hourly, ending at approximately 7:30 pm. See eTable 3 in Supplement 2 for data on glucose and insulin area under the curve and statistical testing. A self-selected subgroup of participants were included. Carb indicates carbohydrate; GI, glycemic index. To convert glucose to mmol/L, multiply by 0.0555; insulin to pmol/L, multiply by 6.945.

Figure 3

Effect of Study Diets on Main Outcomes The primary outcomes were systolic blood pressure, insulin sensitivity, and levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides. Diastolic blood pressure was a secondary outcome. Additional data related to these outcomes are presented in Table 3 and eTable 3 in Supplement 2. Apolipoproteins and other lipid outcomes are in eTable 4. Carb indicates carbohydrate; GI, glycemic index. To convert cholesterol to mmol/L, multiply by 0.0259; triglycerides to mmol/L, multiply by 0.0113. ^aFor the 5 primary outcomes on the primary diet contrast (insulin sensitivity, triglycerides, HDL cholesterol, LDL cholesterol, and systolic blood pressure), we plot and tabulate 99% CI to achieve nominal 95% coverage.