Glycemic index may not be key to improved cardiovascular health

Researchers exploring glycemic index effects on cardiovascular health in overweight and obese patients found that a low glycemic diet did not improve risk. Nor did it positively alter risks for diabetes.

The study enrolled 163 overweight or obese patients at Johns Hopkins Medical Institutions in Baltimore and Brigham and Women’s Hospital in Boston. Patients were given two of four diets for five weeks: either high-glycemic, high-carbohydrate; high-glycemic, low-carbohydrate; low-glycemic, high-carbohydrate; or low-glycemic, low-carbohydrate. Meals for the five-week span were provided by the research team. Enrolled patients were not diabetic.

Frank M. Sacks, MD, of the Harvard School of Public Health in Boston, and colleagues expected to find that lower glycemic diets positively influenced markers for cardiovascular disease and diabetes, including insulin sensitivity, low-density lipoprotein cholesterol, high-density cholesterol, triglycerides and blood pressure. Instead, they did not see improvements in patients' lipid levels, systolic blood pressure or insulin sensitivity.

Triglycerides were largely the only marker altered over the course of the study. Between low-carbohydrate diets, low-glycemic foods reduced triglycerides by 5 percent over high-glycemic foods. Low-carbohydrate, low-glycemic diet reduced triglycerides 20 percent compared with high-glycemic index, high-carbohydrate diet, but no other markers were affected.

The only apparent impression on glycemic index itself was seen between high-carbohydrate content diets, representing a 20 percent reduction between low- and high-glycemic diets; however, in this comparison, no other markers, including triglycerides, were affected.

Robert H. Eckel, MD, University of Colorado Anschutz Medical Campus in Aurora, wrote in an editorial that these finding suggested that glycemic index had less impact on cardiovascular health than previously believed. “These findings should therefore direct attention back to the importance of maintaining an overall heart-healthy lifestyle, including diet pattern,” Eckel wrote.

This diet, outlined by 2013 American College of Cardiology and American Heart association guidelines, emphasizes fruits, vegetables and whole grains with some low-fat dairy, fish, legumes and poultry, nontropical oils and nuts, and emphasizes limited portions of red meat, sodium, sweets and sugar sweetened beverages.

In a press release, Sacks emphasized that the results of diets that varied already “healthful” diets, such as DASH and Mediterranean diets, were surprising. However, “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,” Sacks stated.

Sacks et al noted that this study was not conducted among diabetic patients and should not be applied to their care. Future research should explore the benefits of both glycemic controlled diets in diabetic patients and among patients needing long-term weight loss and control.

The study was published Dec. 17 in JAMA.