Low Glycaemic Index Diet Fails to Reduce Cardiovascular Risk Factors, Study Shows

An article by Susan Mayor in the British Medical Journal (BMJ):

Overweight people eating a low glycaemic index diet show no difference in cardiovascular risk factors or insulin sensitivity compared with those consuming a high glycaemic index diet, a randomised trial has shown.

The glycaemic index is a property of some carbohydrate containing foods determined by how much they raise blood glucose concentration. Some nutrition policies recommend low glycaemic index diets but the effects on cardiovascular disease risk factors are unclear.

Researchers, led by Frank Sacks, professor of cardiovascular disease prevention at Harvard School of Public Health in Boston, Massachusetts, randomised 163 overweight adults to one of four diets, each consumed for five weeks and based on a healthy balance of whole grains, proteins, vegetables, fruits, and low fat foods (the Dietary Approaches to Stop Hypertension diet) (Sacks et al., 2014).

The four diets were high glycaemic index plus high carbohydrate, low glycaemic index plus high carbohydrate, high glycaemic index plus low carbohydrate, and low glycaemic index plus low carbohydrate. Each participant completed at least two of the study diets, resulting in 135-150 people for any pair of the four diets to be compared.

The findings were somewhat unexpected. With high carbohydrate diets, the low glycaemic index option decreased insulin sensitivity by 20% (from 8.9 to 7.1 units, P=0.002), increased low density lipoprotein cholesterol by 6% (from 139 to 147 mg/dL, P≤0.001), and had no effect on high density lipoprotein cholesterol, triglycerides, or blood pressure compared with the high glycaemic diet.

Comparing diets with low carbohydrate content, the outcomes were no different between the low and high glycaemic index options apart from a 5% decrease in triglycerides with the low glycaemic index diet (from 91 to 86 mg/dL, P=0.02). Comparing the low carbohydrate, low glycaemic index diet with the high carbohydrate, high glycaemic index diet gave similar results, with the only difference being a 23% decrease in triglycerides (from 111 to 86 mg/dL, P≤0.001).

“In the context of an overall DASH [Dietary Approaches to Stop Hypertension] type diet, using glycaemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance,” the study said.

“The unexpected findings suggest that the concept of glycaemic index is less important than previously thought, especially in the context of an overall healthy diet,” said Robert Eckel, University of Colorado, in an editorial accompanying the research report (Eckel, 2014).

References (In Article)

Eckel, R.H. (2014) Role of Glycaemic Index in the Context of an Overall Heart-healthy Diet. Journal of the American Medical Association. 312, pp.2508-2509.

Sacks, F.M., Carey, V.J., Anderson, C.A., Miller, E.R., Copeland,T., Charleston, J., et al. (2014) Effect of High vs Low Glycemic Index of Dietary Carbohydrate on Cardiovascular Disease Risk Factors and Insulin Sensitivity. Journal of the American Medical Association312, pp.2531-2541.

Reference (Whole Article)

BMJ 2014;349:g7719

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