The research found that composition of diet, the sources of calories, can affect physiological adaptations that defend body weight. On a low-glycemic load diet, resting energy expenditure (REE) decreased less than with the low-fat diet, which could amount to several pounds of weight change per year, given this effect would persist over a long term. For comparative purposes, a similar effect on caloric expenditure could be obtained by walking a mile per day (80 kcal/d).

Reduction in glycemic load may aid in the prevention or treatment of obesity, cardiovascular disease, and type 2 diabetes. For example, insulin resistance decreased by more than twice as much with weight loss in the low-glycemic load vs. the low-fat group.

"Composition of diet may impact how dieters respond to weight loss," said lead researcher, Mark Pereira, Ph.D., assistant professor in epidemiology at the University of Minnesota. "On a typical low-fat diet, the participants tended to experience more perceived hunger and a slower metabolic rate, which may make it more difficult to stay on the diet, while those on the low-glycemic load diet did not feel as hungry and had a faster metabolic rate."

Pereira and colleagues at Harvard University designed a randomized parallel-design study of 39 over-weight or obese young adults aged 21 to 40 years who received an energy-restricted diet, either low-glycemic load or low-fat. Participants' body composition, REE, blood cholesterol, blood pressure and blood glucose and insulin levels were measured and studied before and after 10 percent weight loss.

The research found that physiological adaptations that serve to defend baseline body weight can be modified by dietary composition. REE declined by 80 kcal/d less and hunger was less on the low-glycemic load diet vs. the low-fat diet during weight loss, similar to results from a prior short-term study in the American Journal of Clinical Nutrition. In addition, the low-glycemic load diet had beneficial effects on several obesity-related risk factors compared with a low-fat diet that was consistent with current nutritional guidelines.

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