A low-glycemic index diet combined with exercise reduces insulin resistance, postprandial hyperinsulinemia, and glucose-dependent insulinotropic polypeptide responses in obese prediabetic humans.

Am J Clin Nutr. 2010 Oct 27. [Epub ahead of print]

A low-glycemic index diet combined with exercise reduces insulin resistance, postprandial hyperinsulinemia, and glucose-dependent insulinotropic polypeptide responses in obese prediabetic humans.
Solomon TP, Haus JM, Kelly KR, Cook MD, Filion J, Rocco M, Kashyap SR, Watanabe RM, Barkoukis H, Kirwan JP.

Department of Pathobiology, Lerner Research Institute and Nutrition, Case Western Reserve University, Cleveland, OH.

Abstract
BACKGROUND: The optimal lifestyle intervention that reverses diabetes risk factors is not known.

OBJECTIVE: We examined the effect of a low-glycemic index (GI) diet and exercise intervention on glucose metabolism and insulin secretion in obese, prediabetic individuals.

DESIGN: Twenty-two participants [mean ± SEM age: 66 ± 1 y; body mass index (in kg/m(2)): 34.4 ± 0.8] underwent a 12-wk exercise-training intervention (1 h/d for 5 d/wk at ≈85% of maximum heart rate) while randomly assigned to receive either a low-GI diet (LoGIX; 40 ± 0.3 units) or a high-GI diet (HiGIX; 80 ± 0.6 units). Body composition (measured by using dual-energy X-ray absorptiometry and computed tomography), insulin sensitivity (measured with a hyperinsulinemic euglycemic clamp with [6,6-(2)H(2)]-glucose), and oral glucose-induced insulin and incretin hormone secretion were examined. Results: Both groups lost equal amounts of body weight (-8.8 ± 0.9%) and adiposity and showed similar improvements in peripheral tissue (+76.2 ± 14.9%) and hepatic insulin sensitivity (+27.1 ± 7.1%) (all P < 0.05). However, oral glucose-induced insulin secretion was reduced only in the LoGIX group (6.59 ± 0.86 nmol in the prestudy compared with 4.70 ± 0.67 nmol in the poststudy, P < 0.05), which was a change related to the suppressed postprandial response of glucose-dependent insulinotropic polypeptide. When corrected for changes in β cell glucose exposure, changes in insulin secretion were attenuated in the LoGIX group but became significantly elevated in the HiGIX group.

CONCLUSIONS: Although lifestyle-induced weight loss improves insulin resistance in prediabetic individuals, postprandial hyperinsulinemia is reduced only when a low-GI diet is consumed. In contrast, a high-GI diet impairs pancreatic β cell and intestinal K cell function despite significant weight loss. These findings highlight the important role of the gut in mediating the effects of a low-GI diet on type 2 diabetes risk reduction.