Study: Low glycemic diet does not improve insulin sensitivity or cardiovascular risk factors

New research indicates that a low glycemic diet does not improve insulin sensitivity, lipid levels or blood pressure

Nutrition experts are continually debating the nutritional value of carbohydrate-containing foods and whether some are healthier than others. High carbohydrate foods are classified by how much they increase blood sugar; known as glycemic index. In new findings led by researchers at Brigham and Women's Hospital (BWH) in Boston and Johns Hopkins University (JHU) in Baltimore, researchers looked at glycemic index' effect on cardiovascular disease (CVD) and diabetes and found that low glycemic diets did not improve insulin sensitivity or cardiovascular risk factors. These findings are published in JAMA on December 17, 2014.

"The study results were very surprising. We hypothesized that a low glycemic index would cause modest, though potentially important improvements in insulin sensitivity and CVD risk factors," explained Frank M. Sacks, MD, a physician and researcher in BWH's Channing Division of Network Medicine BWH and lead author of this study. "Our findings demonstrated that using glycemic index to select specific foods did not improve LDL cholesterol, HDL cholesterol, triglycerides, blood pressure or insulin resistance."

The researchers conducted a randomized controlled trial in which 163 overweight adults with elevated blood pressure were given one of four complete diets that contained all of their meals, snacks and beverages, each for five weeks. Each diet was a variation of a healthful dietary pattern as recommended by our national dietary guidelines, such as the DASH and Mediterranean diets. Researchers measured five outcomes including insulin sensitivity, levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides and systolic blood pressure. Additionally, the effects of glycemic index when total carbohydrate is high, as in the DASH diet, or low, such as the OmniHeart diet or Mediterranean diet, were observed.

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

The researchers note that future studies are needed to see if low glycemic index diets are helpful with type 2 diabetes, or for long-term weight loss. Previous research has shown inconsistent results on whether low glycemic index helped people lose weight.