Relationship between insulin sensitivity, obesity, body fat distribution and β-endorphinaemia in obese women

OBJECTIVE: To study the associations of obesity (as body mass index (BMI)), of body fat distribution (as waist to hip ratio (WHR)) and of beta-endorphinaemia (beta-EP-aemia) with fasting insulin and glucose concentrations, with insulin secretion (as first phase insulin response (FPIR)) and with insulin sensitivity (S-I) in obese women. DESIGN: a cross-sectional study of insulin sensitivity in obese women. SUBJECTS: 45 obese women (age: 20-70 y, BMI: 27-50). MEASUREMENTS: Frequently sampled intravenous glucose tolerance test (FSIGTT), FPIR, fasting glucose, fasting insulin, BMI, body fat topography (WHR), beta-EP-aemia, plasma ACTH. RESULTS: In univariate analysis the following positive associations were observed: fasting glucose with age and WHR, fasting insulin with BMI and WHR, beta-EP plasma concentration with WHR; S-I was negatively associated with BMI, WHR and beta-EP plasma concentrations. This pattern of associations remained unaltered in multivariate analysis including age, BMI and WHR as independent variables. The contribution of beta-EP plasma concentrations to S-I variability was corroborated by a stepwise multiple regression analysis: 53.8% of S-I variation could be explained by BMI (30.7%), by beta-EP plasma concentrations (17.2%) and by WHR (5.9%). Finally, women were divided into two groups according to whether they had a peripheral (P-BFD, WHR less than or equal to 0.80, n = 24) or an abdominal (A-BFD, WHR greater than or equal to 0.85, n = 16) body fat distribution. After adjustment for age and BMI, S-I values were lower while beta-EP and ACTH plasma concentrations were higher in the A-BFD compared to the P-BFD group, In this latter group, 54.8% of S-I variation was explained by the same variables as in the whole group. In the A-BFD group, higher WHR was associated with lower FPIR. CONCLUSIONS: 1) The major finding of this study is that, in non-diabetic obese women (especially those with a P-BFD), higher beta-EP plasma concentrations are associated with lower insulin sensitivity, This association is independent of both the magnitude of obesity and the pattern of fat distribution, although these two parameters are strong predictors of S-I. 2) The major reduction in S-I observed in women with A-BFD probably results from the additive effects of obesity, of elevated beta-EP plasma concentrations and of metabolic and endocrine alterations in relation with the central pattern of fat distribution.