Glycemic Index Might Not Impact Weight Loss

Data generated during a study at Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts University, Boston, suggests that calorie-restricted diets with divergent glycemic loads (GL) can result in comparable long-term weight loss. The research was published in the April 2007 issue of the American Journal of Clinical Nutrition (Long-term effects of 2 energy-restricted diets differing in glycemic load on dietary adherence, body composition, and metabolism in CALERIE: a 1-y randomized controlled trial. Das SK, Gilhooly CH, Golden JK, Pittas AG, Fuss PJ, Cheatham RA, Tyler S, Tsay M, McCrory MA, Lichtenstein AH, Dallal GE, Dutta C, Bhapkar MV, DeLany JP, Saltzman E, Roberts SB).

The first phase of the multi-center Comprehensive Assessment of Long-term Effects of Restricting Intake of Energy (CALERIE) trial was carefully designed to account for other issues that might influence the outcome, including dietary factors that affect hunger and satiety, such as palatability, dietary variety and fiber, and laboratory techniques that measure adherence. It also provided a complete set of meals and snacks to its participants. Because there was careful attention to factors that influence hunger and satiety, participants were generally satisfied on a calorie-restricted diet, says corresponding author Susan Roberts, Ph.D., director of the USDA HNRCA's Energy Metabolism Laboratory.

The 34 overweight, but otherwise healthy, men and women in the study achieved and maintained comparable weight loss after one year, whether they were on a low-glycemic-load or a high-glycemic-load diet. The goal was for both groups to restrict calories by 30% and, after one year, both groups had lost an average of 8% of their original body weight, says Roberts. We found that the two groups did not differ significantly in their average body fat loss, energy intake, metabolic rate, or reports of hunger and satiety.

Both diets restricted calories by 30%, compared to the individuals baseline energy requirements and provided the recommended levels of vitamins, minerals and essential fatty acids. Even so, both groups ate more calories than the study provided; at six months the high-glycemic-load (HG) group averaged a 16% calorie-restricted diet and the low-glycemic-load (LG) group averaged a 17% calorie-restricted diet. However, the degree of caloric nonadherence was not significantly different between the two groups. After six months, the subjects on a LG diet (40% carbohydrate, 30% fat and 30% protein) had lost an average of 10.4% body weight, while the HG diet (60% carbohydrate, 20% fat and 20% protein) group had lost an average of 9.0% body weight. By 12 months, though, those in both the LG and HG groups had lost the same 8% average.

Unlike several other long-term studies, which have reported greater weight loss with low GL diets at six months but no differences by 12 months, our data show no significant short-term or long-term differences, says Sai Das, Ph.D., scientist at the USDA HNRCA and study author. However, we did detect a greater tendency for weight and body-fat regain among LG participants. This finding suggests that reduced calorie intake may be harder to sustain on LG diets over time.

The research was supported by a grant from the National Institute on Aging, part of the National Institutes of Health; the USDA; and the Boston Obesity Nutrition Research Center. For more information about recruitment for the second phase of the CALERIE study, visit http://hnrc.tufts.edu/studies/2520.shtml.