Randomized trial on the effects of a 7-d low-glycemic diet and exercise intervention on insulin resistance in older obese humans

Background: The optimal combination of diet and exercise that produces the greatest reversal of obesity-related insulin resistance is unknown. Objectives: We examined the effects of a combined 7-d low-glycemic index (low-GI) diet and exercise training intervention on insulin sensitivity in older obese humans. Design: Participants [n = 32; mean (±SEM) age: 66 ± 1 y; body mass index (in kg/m2): 33.8 ± 0.7] were randomly assigned to a parallel, double-blind, controlled-feeding trial and underwent supervised aerobic exercise (EX; 60 min/d at 80-85% maximum heart rate) in combination with either a low-GI (LoGI + EX: 41.1 ± 0.4) or a high-GI (HiGI + EX: 80.9 ± 0.6) diet. All meals were provided and were isocaloric to individual energy requirements. Insulin sensitivity and hepatic glucose production were assessed with a 40-mU · m-2 · min-1 hyperinsulinemic euglycemic clamp combined with a [6,6-2H 2]-glucose infusion. Results: After the intervention, small decreases were observed in body weight (-1.6 ± 0.2 kg; P < 0.0001) and fat mass (21.7 ± 0.9%; P = 0.004) in both groups. Maximal aerobic capacity (V̇O2max) also improved slightly (0.06 ± 0.02 L/min; P = 0.004). Resting systolic blood pressure, fasting glucose, insulin, triglycerides, and cholesterol all decreased after the study (all P < 0.05). Larger changes in systolic blood pressure and V̇O2max were seen in the LoGI + EX group. Insulin-stimulated glucose disposal (P < 0.001), insulin suppression of hepatic glucose production (P = 0.004), and postabsorptive fat oxidation (P = 0.03) improved equally in both groups after the intervention. Conclusions: These findings suggest that the metabolic improvements after short-term exercise training in older obese individuals are dependent on increased physical activity and are not influenced by a low-GI diet. However, a low-GI diet has added benefit in alleviating hypertension, thus reducing the risk of diabetic and vascular complications. © 2009 American Society for Nutrition.