Low-glycemic index diet promotes weight loss

A diet high in carbohydrates but low on the glycemic index, which measures the impact of carbohydrates on blood sugar levels, may help promote weight loss, decrease body fat and reduce cardiovascular disease risk, according to a report in the July 24 issue of Archives of Internal Medicine.

Carbohydrates have been at the center of recent debates about the best diet for weight loss, according to background information in the article. Public attention has focused on low-glycemic index and high-protein regimens. Clinicians continue to recommend low-fat, high-carbohydrate plans; concerned that many sources of protein are high in saturated fats, physicians and nutrition experts have called for more research into the benefits and risks of each diet. The theory behind low-glycemic index diets holds that rapidly digested, high-glycemic index carbohydrates cause fluctuations in blood glucose (sugar) and insulin levels, contributing to hunger and preventing the breakdown of fat.

Joanna McMillan-Price, M.Nutr.Diet., University of Sydney, Australia, and colleagues recruited 129 obese or overweight young adults (ages 18 to 40 years) and randomly assigned them to one of four reduced-calorie, reduced-fat diets for a 12-week period. Two were high-carbohydrate diets and two were high-protein diets; one of each had a high glycemic load (the total contribution of all foods in the diet to blood glucose levels) and the others had low glycemic loads. At the beginning and end of the study, participants underwent body composition testing. They were weighed weekly on electronic scales and blood samples were taken at weeks six and 12.

At the end of the study, participants on all four diets had lost weight, and there were no significant differences between diets in the reduction in body fat, decrease in waist size or amount of weight loss (an average of 4.2 to 6.2 percent of body weight). There were significant differences, however, in the likelihood of reaching the clinical goal of 5 percent weight loss. Between the two high-carbohydrate diets, lowering the glycemic index nearly doubled fat loss; this effect was stronger in women and did not occur among those on high-protein diets. Participants on the high-protein, high-glycemic index diet had increased levels of total and LDL (bad) cholesterol, while those on the high-protein, low-glycemic index diet and high carbohydrate, low-glycemic index diet experienced reductions in total and LDL cholesterol. All other cardiovascular risk factors, including levels of HDL (good) cholesterol, free fatty acids and C-reactive protein, were similar among the four groups.

"In conclusion, at least in the short term, our findings suggest that dietary glycemic load, and not just overall energy intake, influences weight loss and postprandial glycemia [blood sugar levels after eating]," the authors write. "Moderate reductions in glycemic load appear to increase the rate of body fat loss, particularly in women. Diets based on low-glycemic index whole grain products (in lieu of whole grains with a high glycemic index) maximize cardiovascular risk reduction, particularly if protein intake is high. Reassuringly, this advice can optimize clinical outcomes within current nutrition guidelines, without the concerns that apply to low-carbohydrate diets."