Low glycemic index carbohydrate diets did not improve cardiovascular risk factors for insulin resistance in overweight and obese patients with prehypertension or stage 1 hypertension, according to a new study in JAMA.
The glycemic index measures how a carbohydrate-containing food raises blood glucose. A food with a high glycemic index raises blood glucose more than a food with a medium or low glycemic index. Even though some nutrition policies advocate consumption of low-glycemic index foods and even promote food labeling with glycemic index values, the independent benefits of glycemic index and its effect on risk factors for cardiovascular disease and diabetes are not well understood.
Participants in the study were adults older than 30 who were overweight with elevated blood pressure (but not treated with medication). Participants had no diabetes, cardiovascular disease or chronic kidney disease. They were given two of four different diets to complete during the study period, all of which had a large contrast in glycemic index, while controlling total carbohydrates and other key nutrients, such as fatty acids, potassium and sodium. Calories were adjusted for each participant to prevent weight change.
The diets were based on the Dietary Approaches to Stop Hypertension (DASH) diet, which is rich in fruits, vegetables and low-fat dairy foods, and low in saturated and total fat. The investigators compared four modified versions of the DASH diet: a high-glycemic index, high-carbohydrate diet; a low-glycemic index, high-carbohydrate diet; a high-glycemic index, low-carbohydrate diet; and a low-glycemic index, low-carbohydrate diet.
The study found that in participants consuming the diets with high-carbohydrate content, the low-glycemic index diet compared to the high-glycemic index diet decreased insulin sensitivity, increased low-density lipoprotein (LDL) cholesterol, and did not affect levels of high-density lipoprotein (HDL) cholesterol, triglycerides or blood pressure.
For participants consuming low carbohydrate content, the low-glycemic index diet compared to the high-glycemic index diet did not affect outcomes at the end of the five week dietary intervention, except for decreasing triglycerides. A diet low in glycemic index and carbohydrates, compared to a diet high in glycemic index and carbohydrates, did not affect insulin sensitivity, systolic blood pressure, LDL or HDL cholesterol.
“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 study said. However, “it is also meaningful to note that every DASH-type diet studied in previous trials and this trial lowered blood pressure and LDL cholesterol levels of the participants from baseline when they were eating their usual diets.”
While it remains unclear how physicians should council their non-diabetic patients regarding glycemic index food choices, the study reaffirms the benefits of a DASH-style diet in lowering blood pressure and LDL cholesterol. In this study, an average decrease of 7-9 mm Hg systolic blood pressure and 4-6 mm Hg diastolic blood pressure occurred for participants. This can reinforce to physicians the benefits of making nutritional recommendations to their patients. Partnering with patients on healthy living guidelines can help improve chronic disease. For example, one Texas doctor helped a patient shed more than 200 pounds, which in turn helped the patient better manage her type 2 diabetes.
Partnering with patients is a key facet of the AMA’s Improving Health Outcomes initiative, which is taking new approaches in the treatment of type 2 diabetes and cardiovascular disease: Focusing on health as well as medical care, and strengthening links between the clinic and community through novel strategies and collaborations.
