Diets that classify high-carb foods by how much they increase blood sugar don't improve risk factors for diabetes or cardiovascular disease as previously thought, according to a new study.

"The study results were very surprising," says lead author Frank M. Sacks, MD, a physician and researcher in Brigham and Women's Hospital's Channing Division of Network Medicine. "Our findings demonstrated that using glycemic index to select specific foods did not improve LDL cholesterol, HDL cholesterol, triglycerides, blood pressure or insulin resistance."

In the study, Dr. Sacks and his team worked with 163 overweight adults, all of whom had elevated blood pressure, in a randomized, controlled trial.

Over the course of five weeks, participants were given one of four complete diets, each of which was a variation of a diet such as the DASH or the Mediterranean diet that are recommended by US national dietary guidelines.

Insulin sensitivity, levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides and systolic blood pressure were the outcomes that interested the researchers the most.

They also looked at the effects of glycemic index when total carbohydrate intake is high, as it is on the DASH diet, or low, as is the case with the OmniHeart diet or Mediterranean diet.

"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, as well as maintained baseline body weight," says Dr. Sacks.

"We found that composing a healthful diet with low-glycemic index carbohydrate containing foods rather than high-glycemic index foods did not improve insulin sensitivity, HDL or LDL cholesterol levels or systolic blood pressure."