Determinants of leptin in human breast milk: results of the Ulm SPATZ Health Study

Background Leptin in human breast milk has been implicated as a potential regulator of early-life metabolic programming. We comprehensively investigated the influence of maternal body mass index (BMI) and non-adiposity associated determinants on breast milk leptin concentration at 6 weeks, 6 months, and 1 year postpartum. Methods The Ulm SPATZ Health Study consists of 1006 newborns and their mothers recruited from the general population in the University Medical Center Ulm, Southern Germany, in 2012/2013. Leptin concentration was measured in skimmed breast milk using commercially available ELISA (R&D System). Generalized estimating equations (GEE) accounting for repeated measures were used to calculate beta coefficients and 95% confidence intervals for association of potential demographic and lifestyle related determinants of hormone concentration with BMI-standardized leptin z-scores. Results Leptin concentration was measured in breast milk samples obtained from 694 mothers at approximately 6 weeks (n = 668), 6 months (n = 445), and 1 year (n = 69) postpartum. Differences in crude leptin concentrations between collection times were mostly explained by changes in BMI, breastfeeding frequency, and breast milk fat concentration. Positive associations between BMI and leptin were nonlinear and stronger among lower BMI subjects. Upon standardization, residual leptin concentrations were associated with maternal birth country, parity, age, and smoking history. Conclusions Breast milk leptin concentration is primarily determined by adiposity-related factors. Studies using BMI as a proxy measure for adiposity should account for an observed nonlinear association with leptin, which may be especially important in determining causal associations with health outcomes from this and other adiposity-related hormones in breast milk.