A number of widely-followed diets have been based on the idea that focusing on foods with a low-glycemic index might improve cardiovascular risk factors and lower the risk of developing diabetes. But an NIH-funded study suggests that using the glycemic index to select foods may not improve insulin sensitivity, lower HDL or LDL lipid levels or reduce blood pressure levels. Results of the study appeared online in the Journal of the American Medical Association.

The study, funded by the NIH’s National Heart, Lung and Blood Institute, included 163 overweight adults in a controlled feeding study. Participants had systolic blood pressure levels of between 120 and 159 mmHg, which means participants at the higher end of this range were considered to have high blood pressure.

Participants spent five weeks eating each of the four diets being tested. All foods and calorie-containing drinks were provided to participants for the duration of each five-week period on the test diets. Each diet used the DASH eating plan with variations in carbohydrate and glycemic index levels. The DASH eating plan emphasizes vegetables, fruits, and whole grains; includes fat-free or low-fat dairy products, fish, poultry, beans, nuts, and vegetable oils; and limits sweets, sugary beverages, and red meats.

The diets compared were:
High-glycemic index/high carbohydrate;
Low-glycemic index/high carbohydrate;
High-glycemic index/low carbohydrate;
Low-glycemic index/low carbohydrate.

The low-glycemic index/low-carbohydrate diet resulted in similar insulin sensitivity, systolic blood pressure and HDL and LDL cholesterol levels when compared to the high-glycemic index/high-carbohydrate diet.