BACKGROUND: Caloric restriction in obese diabetic patients quickly improves glucose control, independently from weight loss. However, the early effects of a very-low-calorie diet (VLCD) on insulin sensitivity and insulin secretion in morbidly obese patients with type 2 diabetes are still unclear. OBJECTIVE: The objective was to study the relative contributions of insulin sensitivity, insulin secretion, or both to improvement in glucose metabolism, after 1 wk of caloric restriction, in severely obese diabetic patients. DESIGN: Hyperglycemic clamps were performed in 14 severely obese (BMI, in kg/m(2): >40) patients with type 2 diabetes in good glucose control (glycated hemoglobin < 7.5%) before and after 7 d of a VLCD (400 kcal/d). RESULTS: The VLCD caused a 3.22 ± 0.56% weight loss (P < 0.001), 42.0% of which was fat loss, accompanied by decreases in fasting plasma glucose (P < 0.05) and triglycerides (P < 0.01). In parallel, the Disposition Index, which measures the body's capability to dispose of a glucose load, increased from 59.0 ± 6.3 to 75.5 ± 6.3 mL· min(-1) · m(-2) body surface area (P < 0.01), because of improvements in indexes of both first- and second-phase insulin secretion (P < 0.02), but with no changes in insulin sensitivity (P = 0.33). CONCLUSION: The marked improvement in metabolic profile, observed in severely obese patients with type 2 diabetes after a 7-d VLCD, was primarily due to the amelioration of β cell function, whereas no contribution of insulin sensitivity was shown. This trial was registered at www.clinicaltrials.gov as NCT01447524.
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