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Randomized trial on the effects of a 7-d low-glycemic diet and exercise intervention on insulin resistance in older obese humans^1,2,3

¹ From the Department of Pathobiology, Lerner Research Institute, Cleveland Clinic, Cleveland, OH (TPJS, JMH, KRK, MDC, and JPK); the Department of Physiology (JMH, M Riccardi, and JPK) and the Department of Nutrition (KRK, HB, and JPK), Case Western Reserve University School of Medicine, Cleveland, OH; the Department of Cardiovascular Medicine, Heart and Vascular Institute (M Rocco), the Department of Endocrinology, Diabetes, and Metabolism, Endocrinology and Metabolism Institute (SRK), and the Department of Gastroenterology-Hepatology, Digestive Disease Institute (JPK), Cleveland Clinic, Cleveland, OH.

² Supported by NIH grant RO1 AG12834 (JPK) and in part by the NIH, National Center for Research Resources, CTSA 1UL1RR024989, Cleveland, OH.

³ Address correspondence to JP Kirwan, Department of Pathobiology, Lerner Research Institute, Cleveland Clinic, 9500 Euclid Avenue NE-40, Cleveland, OH 44195. E-mail: kirwanj{at}ccf.org.

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 [6,6-²H₂]-glucose infusion.

Results: After the intervention, small decreases were observed in body weight (–1.6 ± 0.2 kg; P < 0.0001) and fat mass (–1.7 ± 0.9%; P = 0.004) in both groups. Maximal aerobic capacity (O₂max) 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 O_2max 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.