Diets High in Carbohydrates but Low GI May Help Promote Weight Loss

25/07/06 "Good carbs" -- those which have lower glycemic index values -- are good for you after all, according to recent research published in the July 24 issue of the Archives of Internal Medicine, one of the JAMA/Archives journals.

The study found that 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. High or low protein intake, when combined with a low-glycemic index diet, did not appear to affect weight loss dramatically.

The theory behind low-glycemic index diets, such as the popular South Beach Diet, 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."

The authors encourage doctors to help patients understand what the glycemic index is, and teach the patients how to identify foods low on the index.

"Typically, foods with a low degree of starch gelatinization, such as pasta, and those containing a high level of viscous soluble fiber, such as whole grain barley, oats and rye, have slower rates of digestion and lower glycemic index values," editorialst Dr. Liu writes. "Without any drastic change in regular dietary habits, for example, one can simply replace high-glycemic index grains with low-glycemic index grains and starchy vegetables with less starchy ones and cut down on soft drinks that are often poor in nutrients yet high in glycemic load."