Leptin measurement in urine in children and its relationship to other growth peptides in serum and urine

OBJECTIVE: Leptin has been implicated in the interaction between nutrition, energy balance and sexual maturation in humans. A non-invasive method of measuring leptin would greatly facilitate longitudinal studies of changes in leptin in normal children. The aim of this study was to evaluate the use of urinary leptin as a surrogate for serum leptin measurements. DESIGN: We have modified and validated a serum immunoradiometric assay (IRMA) kit for the measurement of leptin in urine, and subsequently investigated the relationship between urinary leptin and other growth-related proteins. METHODS: Cross-sectional study: urinary leptin, measured in the first morning urine voided and expressed as ng excreted overnight, and serum concentrations of leptin, IGF-I, IGF-II, IGFBP-3 and IGFBP-1 were determined in a cohort of 188 healthy schoolchildren aged 5-19 years (88 males, 100 females). Height, weight and pubertal status were assessed in all children. Longitudinal study: urinary levels of leptin, IGF-I and GH were measured daily in two adults (one male, one female) over a period of 6 weeks. RESULTS: The detection limit of this modified assay was 0.59 ng/L. The intra- and interassay coefficients of variation range was 4-8% and 4-9%, respectively. The recovery of recombinant leptin added to urine was 98-108%, and the assay had a recovery rate for serial dilution in the range of 106-112%. Urinary leptin correlated significantly with serum leptin (r = +0.65, P <0.01). Urinary leptin showed similar changes through puberty to those of serum leptin, with levels rising in females throughout puberty, whereas in males levels peaked at G2/G3 then decreased. BMI SDS was the main determinant of urinary leptin, as it was for serum leptin. In the cross-sectional study urinary leptin correlated significantly with serum IGF-I (r = +0.41, P = 0.001), IGF-II (r = +0.19, P = 0.05), IGFBP-3 (r = +0.29, P = 0.001) and IGFBP-1 (r = -0.25, P = 0.001). In the adult study, leptin was also detected in urine with similar night-to-night variability to that found for IGF-I and GH. CONCLUSION: Urinary leptin is a valid marker of serum leptin concentrations, and therefore this non-invasive assay would be a useful tool for longitudinal assessment of changes in leptin in children.