Low cerebrospinal fluid insulin levels in obese humans

Abstract

To the Editor: In healthy men, insulin concentrations in cerebrospinal fluid (CSF) increase after acute elevations of plasma insulin levels, indicating that the brain content of insulin is normally finely tuned to circulating insulin [1]. Central nervous insulin promotes weight loss [2] and attenuates peripheral insulin resistance via inhibition of hepatic gluconeogenesis [3]. With increasing body weight, plasma insulin levels rise as a result of elevated insulin resistance. It is not yet known whether in hyperinsulinaemic obese subjects this increase is accompanied by parallel increases in CSF insulin levels that would act to promote weight loss in these patients. We compared insulin concentrations in plasma and CSF in a group of 45 subjects (23 women, age 19–80 years) with a wide body weight range (BMI 16.24–38.10 kg/m). Five subjects were newly diagnosed as diabetic by elevated fasting plasma glucose levels (>7.0 mmol/l). Since their exclusion did not alter the results, their data were included in the analyses. After an overnight fast, subjects reported to the laboratory for simultaneous sampling of blood and CSF (1 ml) via lumbar puncture after local anaesthesia (2 ml mepivacain-HCl 1%). BMI and waist-to-hip ratio were assessed, and body composition was measured by standard bioelectrical impedance analysis (BIA 2000-M; Data Input, Frankfurt, Germany). Insulin was determined using a commercial competitive double-antibody RIA (Pharmacia Insulin RIA 100; Pharmacia Diagnostics, Upsalla, Sweden). To increase the sensitivity of the insulin assay for the expected lower concentrations in CSF, the standard procedure was slightly modified. In brief, insulin was determined using 100 μl of CSF, 50 μl of [I]insulin diluted with buffer at a ratio of 1:3, and 50 μl of insulin antiserum diluted at a ratio of 1:2. The incubation time was increased from 2 h in the standard method to 3 h at room temperature. With this procedure, threshold sensitivity was reliably lowered to 1.8 pmol/l. Intra-assay variation was <4.5% for determination in CSF and plasma. Insulin plasma and CSF concentrations were measured in duplicate and quadruplicate, respectively. The lowest standard used was 1.8 pmol/l, which could be reliably measured with <4.5% variation. Therefore, all measures below this standard were set to 1.8 pmol/l for analysis. Thus, the actual CSF:plasma insulin ratio may even be slightly lower than reported, although analyses using the measurements below the lowest standard did not alter the results. Additionally, plasma and CSF glucose concentrations were measured (Beckman glucose analyser II; Beckman Instruments, Fullerton, CA; coefficient of variation <1.1%). Insulin resistance was estimated according to the homeostasis model assessment (HOMA) formula, with high scores denoting a high degree of insulin resistance. Bivariate correlation coefficients were determined and a stepwise multiple regression analysis was performed to detect associations Diabetologia (2006) 49:2790–2792 DOI 10.1007/s00125-006-0409-y