Visceral adipose tissue impairs insulin secretion and insulin sensitivity but not energy expenditure in obesity

In obesity, a central pattern of fat distribution is mostly associated with hyperinsulinemia, insulin resistance, and hyperlipemia, thus promoting the development of non-insulin-dependent diabetes mellitus and cardiovascular disease. In addition, in obesity, changes in energy expenditure are hypothesized to be involved in the development or maintenance of excessive body fat storage. In this study, abdominal fat distribution by computed tomographic (CT) scan was used to study the relation between the visceral fat depot, insulin secretion, and insulin sensitivity in a group of obese subjects with normal glucose tolerance (n = 26; body mass index [BMI], 39 ± 1 kg/m2) and a group of normal-weight control subjects (n = 9; BMI, 23 ± 1 kg/m2). The minimal model method was used to assess insulin sensitivity, SI, and first-phase (Φ1) and second-phase (Φ2) β-cell sensitivity from plasma glucose, insulin, and C-peptide concentrations measured during an intravenous glucose tolerance test ([IVGTT] 0.33 g/kg body weight). Moreover, we evaluated the relationships between these parameters and the resting metabolic rate (RMR) and glucose-induced thermogenesis (GIT) measured by indirect calorimetry. The data show the following: (1) in obese subjects, (Φ1) is greater but not statistically different from the value in control subjects (252 ± 41 v 157 ± 25 dimensionless 109); (2) Φ2 is significantly higher in obese subjects (27 ± 4 v 14 ± 2 min−1 × 109, P < .05), with a positive correlation between the amount of visceral adipose tissue (VAT) and Φ2 (r = .49, P < .05); (3) Sl is decreased in the obese group (2.8 ± 0.3 v 9.7 ± 1.6 10−4 · min−1/(μU · mL−1), P < .0001), with a negative correlation of Sl, with the adiposity index BMI (r = −.67, P < .0001) and VAT (r = −.56, P < .05); (4) RMR, expressed in absolute terms, was significantly increased in obese versus lean subjects (5.9 ± 0.2 v 4.6 ± 0.3 kJ/min, P < .01), whereas when RMR was adjusted for fat-free mass (FFM), the difference between the two groups disappeared (0.09 ± 0.003 v 0.09 ± 0.002 kJ/min · kg FFM). We did not observe any difference in GIT between lean and obese subjects. Moreover, GIT was significantly correlated with FFM (r = .69, P < .005), but not with BMI. The amount of VAT did not correlate with RMR or GIT. In conclusion, these results suggest that in obese subjects with normal glucose tolerance, insulin sensitivity is impaired and the β-cell hyperresponse to glucose is mainly due to an enhanced second-phase β-cell secretion. The degree of visceral fat deposition seems to affect insulin secretion and worsens insulin sensitivity, but does not influence energy expenditure.