Basing diet for overweight people on the so-called glycaemic index offers no benefits, according to a new study.
The index seeks to identify foods which boost blood glucose levels most rapidly.
The index gives a high score to white bread and white rice but low scores to whole grains and sweet fruits.
But a study of 163 adults found that using the index to decide which carbohydrates to eat made no difference to a range of indicators of ill health.
Dr Frank Sacks of Brigham and Women’s Hospital and Harvard Medical School, Boston, USA, and colleagues looked at the effectiveness of low glycaemic index diets on insulin sensitivity, cholesterol levels and blood pressure.
They used figures from 163 overweight adults who had slightly raised blood pressure. Participants were split into four groups and followed a specific diet for five weeks. The diets varied on their carbohydrate content and the glycaemic index of their foods.
The effectiveness of the glycaemic index approach for cardiovascular disease and diabetes is unclear, particularly as all the carbohydrate content of a food is broken down to glucose eventually, the researchers say.
The study found that a low- compared with high GI diet had no effect on insulin sensitivity, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, or blood pressure.
The only benefit of a low GI diet was a reduction in triglycerides (blood fats), when it was combined with a low carbohydrate diet.
Full details appeared in the Journal of the American Medical Association.
The authors conclude: "Using glycaemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance."
Researcher Dr Lawrence Appel commented: "We did not detect any clear benefit of the low glycaemic index diets on the major risk factors for heart disease, and we found no evidence of benefit for diabetes prevention."
Sacks, F. M. et al. Effects of High vs Low Glycaemic Index of Dietary Carbohydrate on Cardiovascular Disease Risk Factors and Insulin Sensitivity: The OmniCarb Randomized Clinical Trial. JAMA 17 December 2014 doi:10.1001/jama.2014.16658
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