Objective: Restoration of insulin secretion is critical for the treatment of type 2 diabetes. Exercise and diet can alter glucose-induced insulin responses, but whether this is due to changes in beta-cell function per se is not clear. The mechanisms by which lifestyle intervention may modify insulin secretion in type 2 diabetes have also not been examined but may involve the incretin axis.
Research design and methods: Twenty-nine older, obese (aged 65 +/- 1 years; BMI 33.6 +/- 1.0 kg/m(2)) subjects, including individuals with newly diagnosed type 2 diabetes (obese-type 2 diabetic) and individuals with normal glucose tolerance (obese-NGT), underwent 3 months of nutritional counseling and exercise training. beta-Cell function (oral glucose-induced insulin secretion corrected for insulin resistance assessed by hyperinsulinemic-euglycemic clamps) and the role of glucose-dependent insulinotropic polypeptide (GIP) were examined.
Results: After exercise and diet-induced weight loss (-5.0 +/- 0.7 kg), oral glucose-induced insulin secretion was increased in the obese-type 2 diabetic group and decreased in the obese-NGT group (both P < 0.05). When corrected for alterations in insulin resistance, the change in insulin secretion remained significant only in the obese-type 2 diabetic group (1.23 +/- 0.26 vs. 2.04 +/- 0.46 arbitrary units; P < 0.01). Changes in insulin secretion were directly related to the GIP responses to oral glucose (r = 0.64, P = 0.005), which were augmented in the obese-type 2 diabetic group and only moderately suppressed in the obese-NGT group.
Conclusions: After lifestyle-induced weight loss, improvements in oral glucose-induced insulin secretion in older, obese, nondiabetic subjects seem to be largely dependent on improved insulin sensitivity. However, in older obese diabetic patients, improved insulin secretion is a consequence of elevated beta-cell function. We demonstrate for the first time that changes in insulin secretion after lifestyle intervention may be mediated via alterations in GIP secretion from intestinal K-cells.