A trial in which 163 overweight adults with elevated blood pressure were given one of four complete diets that contained all their food for five weeks has found that a low glycemic diet did not improve insulin sensitivity, lipid levels or blood pressure. But it's important to note than all diets were based on a healthful DASH-type diet, varying only in their glycemic index and carbohydrate proportions.
So, the trial found that a healthy diet with low–glycemic index carbohydrate-containing foods didn't improve insulin sensitivity, HDL cholesterol levels, LDL cholesterol levels, or systolic blood pressure any more than the healthy diet with high–glycemic index foods It did reduce plasma triglyceride levels slightly more.
In fact, the low–glycemic index with high-carbohydrate diet was worse than the high–glycemic index, high-carbohydrate diet — decreasing insulin sensitivity and increasing LDL cholesterol and LDL apolipoprotein B levels. This finding was unexpected and inconsistent with other research. However, a meta-analysis of 28 trials found that lowering glycemic index did not affect HDL cholesterol or triglyceride levels and lowered LDL cholesterol level only if fiber content was also increased.
In line with the researchers' previous findings, the DASH diet had slightly better effects when its carbohydrate content was reduced. When the same calories were instead supplied by unsaturated fat and protein, triglycerides and VLDL levels were substantially lowered and diastolic blood pressure slightly lowered.
As in previous trials, all the DASH-type diets lowered blood pressure and LDL cholesterol levels.
Previous research has shown inconsistent results on whether low glycemic index helped people lose weight.
Sacks FM, Carey VJ, Anderson CM, et al. Effects of High vs Low Glycemic Index of Dietary Carbohydrate on Cardiovascular Disease Risk Factors and Insulin Sensitivity: The OmniCarb Randomized Clinical Trial. JAMA. 2014;312(23):2531-2541. doi:10.1001/jama.2014.16658.
Goff LM, Cowland DE, Hooper L, Frost GS. Low glycaemic index diets and blood lipids: a systematic review and meta-analysis of randomised controlled trials. Nutr Metab Cardiovasc Dis. 2013;23(1):1-10.