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Glycemic index did not affect CV risk factors, insulin resistance
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Diets containing foods with low glycemic index of dietary carbohydrate did not help patients with obesity or overweight achieve better insulin sensitivity, lipid levels or systolic blood pressure, according to research published in JAMA.
Choosing specific foods based on glycemic index may not improve cardiovascular risk factors or insulin resistance in the context of eating patterns established in the Dietary Approaches to Stop Hypertension (DASH), according to researchers.
“It is not necessary to learn the complex glycemic index system to select foods for health,” Frank M. Sacks, MD, of Harvard School of Public Health, told Endocrine Today. “Healthful dietary patterns, like DASH or Mediterranean, recommended in our national dietary guidelines do not use glycemic index to select foods and improve risk factors for CVD and reduce incidence of CV events.”
Frank M. Sacks
In research units at academic medical centers, Sacks and colleagues conducted a randomized crossover-controlled feeding trial involving 163 adults with overweight and systolic BP between 120 mm Hg and 159 mm Hg; enrollment started April 2008, and the last intervention ended December 2010.
Four DASH-based complete diets including meals, snacks and calorie-containing beverages were assigned for 5 weeks each: high-glycemic index (65% on glucose scale) and high-carbohydrate diet (58% energy); low-glycemic index (40%) and high-carbohydrate diet; high-glycemic index and low-carbohydrate diet (40% energy); and a low-glycemic index and low-carbohydrate diet.
Primary outcomes were insulin sensitivity (areas under the curves of glucose and insulin levels during an oral glucose tolerance test), LDL cholesterol levels, HDL cholesterol levels, triglycerides and systolic BP.
Participants completed at least two study diets; for any pair of diets, 135 to 150 participants contributed at least one primary outcome measure.
With high dietary carbohydrate content, low- vs. high-glycemic index level decreased insulin sensitivity from 8.9 to 7.1 units (−20%; P=.002) and increased LDL cholesterol from 139 mg/dL to 147 mg/dL (6%; P≤.001); levels of HDL cholesterol, triglycerides and BP were unaffected.
With low carbohydrate content, low- vs. high-glycemic index level did not affect outcomes with the exception of triglycerides, which decreased from 91 mg/dL to 86 mg/dL (−5%; P=.02).
The low-glycemic index, low-carbohydrate diet did not affect insulin sensitivity, systolic BP, LDL cholesterol or HDL cholesterol vs. the high-glycemic index, high-carbohydrate diet when comparing primary diets, but it did lower triglycerides from 111 mg/dL to 86 mg/dL (−23%; P≤.001).
“Foods that have the same carbohydrate content differ in how much they increase blood glucose,” Sacks said. “This could be important for patients with diabetes to help manage their blood sugar.”
In an accompanying editorial, Robert H. Eckel, MD, of the University of Colorado Anschutz Medical Campus, wrote that many of the findings from the research were contrary to expectations.
“When glycemic index was lower in the high-carbohydrate diet, insulin sensitivity not only did not increase but decreased,” Eckel wrote. “With the same diet pattern, levels of LDL cholesterol and apolipoprotein B (a secondary endpoint) increased, with no changes in HDL cholesterol or triglyceride level or blood pressure.”
Eckel said the concept of glycemic index appears to carry less importance than previously believed, suggesting a need to refocus. “These findings should therefore direct attention back to the importance of maintaining an overall heart-healthy lifestyle, including diet pattern.” – by Allegra Tiver
For more information:
Eckel RH. JAMA. 2014;doi:10.1001/jama.2014.15338.
Sacks FM. JAMA. 2014;doi:10.1001/jama.2014.16658.
Frank M. Sacks, MD, can be reached at Department of Nutrition, Harvard School of Public Health, 665 Huntington Ave., Boston, MA 02115; email: fsacks@hsph.harvard.edu.
Disclosure: One researcher reports serving as an expert witness in litigation involving
POM Wonderful, Hershey, Unilever and Keebler. The research was supported by a grant from the National Heart, Lung, and Blood Institute and received additional support from the NIDDK, the Harvard Clinical and Translational Science Center from the National Center for Advancing Translational Science and the general clinical research center at Brigham and Women’s Hospital.