Free serum leptin but not bound leptin concentrations are elevated in patients with end-stage renal disease

Background. Leptin is a 16-kDa protein that is thought to be a regulator of food intake and body weight. Although total serum leptin levels have been reported to be elevated in obese and normal weight patients with end-stage renal disease (ESRD), it is not known whether serum-free leptin concentrati ons are also increased in patients with ESRD with no apparent nutritional p roblems. Furthermore, there are no data on how different dialysis modes (hi gh-flux haemodiafiltration and low-flux dialysis) influence serum leptin su bfractions. Methods. We measured fasting serum free and bound leptin levels in three gr oups of male subjects: patients on haemodiafiltration with high flux dialys ers (n = 11), patients on haemodialysis with low-flux dialysers (n = 17) an d healthy age (61+/-8 years) and BMI (23.8+/-3.1 kg/m(2)) matched control s ubjects (n = 28). Both leptin components were determined before and after a single dialysis session. Results. Body mass indices were correlated with serum free leptin levels in both patients (r = 0.69, P<0.001) and controls (r = 0.77, P<0.001). Mean ( SD) serum free leptin levels were significantly higher in ESRD patients tha n in control subjects (91+/-33 vs 41+/-21 pmol/l; P<0.01). Bound leptin lev els did not differ in both groups (0.67+/-0.12 vs 0.56+/-0.11 nmol/l, NS). Elevated serum-free leptin levels in ESRD patients could be reduced by haem odiafiltration with high-flux membranes, but not with low-flux haemodialysi s membranes. The former led to a reduction of initial serum free leptin val ues to 76+/-17% (P<0.01), whereas bound leptin remained unaffected. Conclusion. Serum-free leptin levels are elevated in ESRD without any appar ent effect on body weight. In contrast, serum bound leptin levels remain st able, thus central feedback regulation via the bound form of the hormone ma y serve as an alternative explanation in the regulation of food intake and energy expenditure in chronic patients on haemodialysis with no apparent nu tritional problems.