Researchers from the Johns Hopkins University School of Medicine and Harvard Medical School said food with similar carbohydrate content could differ in the amount they raised blood glucose, and the relationship between this GI property – a measure of how quickly carb-containing foods raised glucose levels in the bloodstream – and cardiovascular disease and diabetes risk factors was not fully understood.
According to their five-week controlled feeding study published in the Journal of the American Medical Association (JAMA), diets with a low GI of dietary carbohydrate did not result in improved insulin sensitivity, lipid levels or systolic blood pressure when compared to a high GI carb diet.
Study co-director Professor Lawrence Appel said they had been “really surprised” by the results. "We did not detect any clear benefit of the low glycemic index diets on the major risk factors for heart disease, and we found no evidence of benefit for diabetes prevention."
The trial, which involved 163 overweight adults with normal systolic blood pressure of between 120 and 159 mmHg, prescribed participants one of four study diets. The four different diets, each covering meals, snacks and calorie-containing beverages, were: a high GI (65% on glucose scale), high-carb diet (58% energy); a low GI (40%), high-carb diet; a high GI, low-carb diet (40% energy); and a low GI, low-carb diet.
Participants consumed one of the diet meals – which were based on a healthy Dietary Approaches to Stop Hypertension (DASH) approach - at the research center and took the remaining two home with them. Once they had completed one five-week diet they would be switched at some point to another, with each participant completing at least two of the four diets in total. The first participant was enrolled in April 2008 and the last participant finished in December 2010.
Unwarranted focus of obesity research
“In the context of an overall DASH-type diet, using glycemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance,” the researchers wrote in their conclusion.
Appel said evidence around low GI foods and sustained weight loss had been “inconsistent”, and these latest results suggested that the narrow focus on GI within obesity strategy research was “unwarranted".
He said instead the key to good health was simple. "Get back to the basics that most people already know. Don't drink sugar-sweetened drinks. Try to eat fruits, vegetables and whole grains. Try to avoid sweets, salt, and foods high in saturated and trans fats. People who follow these principles will reap the benefits."
The researchers said the findings could be applied broadly since 51% of participants were female, and 52% were African-American.
Source: The Journal of the American Medical Association
Vol.312, Iss. 23, pp. 2531-2541, doi:10.1001/jama.2014.16658
“Effects of High vs Low Glycemic Index of Dietary Carbohydrate on Cardiovascular Disease Risk Factors and Insulin Sensitivity”
Authors: F. M. Sacks, V. J. Carey, C. A. M. Anderson, E. R. Miller, T. Copeland, J. Charleston, B. J. Harshfield, N. Laranjo, P. McCarron, J. Swain, K. White, K. Yee, L. J. Appel