Low-Versus High-Glycemic Index Diets in Women: Effects on Caloric Requirement, Substrate Utilization and Insulin Sensitivity

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Abstract
Background: Lowering dietary glycemic index appears to have positive health effects in obese and/or insulin resistant individuals. However, detailed studies in lean young men show no effect. Methods: This study was designed to test the null hypothesis that a diet rich in low-glycemic carbohydrate has no effect on lipid profile, caloric requirements, fat oxidation, or insulin sensitivity in adult women when compared to one rich in high-glycemic carbohydrate. The metabolic feeding protocol used was conducted in both a free-living and in-patient setting using a randomized crossover design. Seven women were studied on each of 2 diets in which 60% of the calories were from either high- or low-glycemic carbohydrate sources. Each diet lasted 20 days with measurements of caloric requirement, resting metabolic rate, glucose and insulin responses to diet and activity, insulin sensitivity, and lipid profile over the last 7 days. Caloric requirement was determined by bomb calorimetry. Other techniques included indirect calorimetry, hydrodensitometry, stable isotope tracers, and the euglycemic clamp. Results: On the low-glycemic index diet the women's caloric requirements were 11% ± 1% higher, fat oxidation at fasted rest supplied an average of 45% ± 4% versus 28% ± 5% of oxidative requirements, average glucose and insulin levels were approximately 40% lower, low density lipoproteins (LDL) and leptin concentrations were lower, and various indices of insulin sensitivity were > 20% higher. Conclusions: In this group of adult women, a diet that lowered glycemic index well below that typically found in western diets increased both daily caloric requirement and fat oxidation, decreased insulin and glucose concentrations and increased insulin sensitivity.