A healthy diet that was modified to have a low glycemic index did not improve insulin sensitivity, lipid levels, or blood pressure in overweight and obese adults, compared with one with a high glycemic index, according to a report published online Dec. 16 in JAMA.
This unexpected finding indicates that, against a background diet that is already healthy, “using glycemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance,” said Dr. Frank M. Sacks of the department of nutrition, Harvard School of Public Health, Boston, and his associates.
The investigators performed a randomized, crossover feeding study in which 163 people aged at least 30 years were randomly assigned to one of four study diets for 5 weeks, then switched to a different diet for a further 5 weeks, after a 2-week washout period.
Dr. Frank M. Sacks
“We studied diets that had a large contrast in glycemic index, while at the same time we controlled intake of total carbohydrates and other key nutrients such as fatty acids, potassium, and sodium and maintained body weight. The background diets in which we manipulated glycemic index were healthful dietary patterns established in the Dietary Approaches to Stop Hypertension (DASH) and Optimal Macronutrient Intake to Prevent Heart Disease (OmniHeart) studies that are being recommended in dietary guidelines to prevent CVD,” they noted. About half of the study population were female and half were black, and there were high prevalences of obesity (56%), hypertension (26%), high cholesterol (68%), and impaired fasting glucose (30%). All meals, snacks, and beverages were provided on site. Adherence, which was closely monitored, was high: All study foods and no nonstudy foods were consumed on 96% of person-days on each diet.
Contrary to expectations, the two low-glycemic-index diets failed to improve insulin sensitivity, cholesterol levels, or blood pressure when compared with the two high-glycemic-index diets. “Composing a DASH-type diet with low-glycemic-index foods ... does not improve CVD risk factors and may in fact reduce insulin sensitivity and increase LDL cholesterol,” Dr. Sacks and his associates wrote (JAMA 2014 Dec. 16 [doi:10.1001/jama.2014.16658]).
They noted that these findings are especially important given that some nutrition policies advocate replacing high-glycemic-index with low-glycemic-index foods. Even though some experts go so far as to promote that all foods be labeled with their glycemic-index values, “the benefits of glycemic index are uncertain, especially with persons are already consuming a healthful diet rich in whole grains, vegetables, and fruits.”
The investigators emphasized that none of the study participants had type 2 diabetes, so these findings do not apply to patients with that disease.