Insulin and body mass index decrease serum soluble leptin receptor levels in humans.

PURPOSE
Test how serum soluble leptin receptor (sOb-R) is influenced by glucose, insulin, body fat, body mass index (BMI), food intake and physical activity.


METHODS
We performed an epidemiological triangulation combining cross-sectional, interventional and Mendelian randomization study designs. In five independent clinical studies (n = 24-823), sOb-R was quantified in serum or plasma by commercial ELISA kits using monoclonal antibodies. We performed mixed models regression and two-sample Mendelian randomization.


RESULTS
In pooled, cross-sectional data, levelling on study, sOb-R associated inversely with body mass index (BMI) (beta [95% CI] -0.19 [-0.21 to -0.17]), body fat (-0.12 [-0.14 to -0.10) and fasting C-peptide (-2.04 [-2.46 to -1.62]). sOb-R decreased in response to acute hyperinsulinaemia during euglycaemic glucose clamp in two independent clinical studies (-0.5 [-0.7 to -0.4] and -0.5 [-0.6 to -0.3]), and immediately increased in response to intensive exercise (0.18 [0.04 to 0.31]) and food intake (0.20 [0.06 to 0.34]). In two-sample Mendelian randomization, higher fasting insulin and higher BMI were causally linked to lower sOb-R levels (inverse variance weighted, -1.72 [-2.86 to -0.58], and -0.20 [-0.36 to -0.04], respectively). The relationship between hyperglycaemia and sOb-R were inconsistent in cross-sectional studies, non-significant in intervention studies, and two-sample Mendelian randomization suggested no causal effect of fasting glucose on sOb-R.


MAIN CONCLUSION
Both BMI and insulin causally decreased serum sOb-R levels. Conversely, intensive exercise and food intake acutely increased sOb-R. Our results suggest that sOb-R is involved in short-term regulation of leptin signalling, either directly or indirectly, and that hyperinsulinaemia may reduce leptin signalling.