Time-course of leptin levels in term and preterm human milk

Objective: To compare the time-course of breast milk leptin levels between term and preterm pregnancy. Design: Open longitudinal prospective randomised study. Methods: RIA of leptin levels in milk from 33 mothers (term pregnancy: n = 24; preterm: n 9) at three postpartum intervals: 2-3 days, 4-5 days and 6 weeks (intervals A, B and C), combined with serum in 2 3 mothers (term: n = 17: preterm: n = 6) in interval A. Milk samples were sonicated before incubation. Results: Interval A leptin levels were approximately tenfold higher in serum than in milk (term: 13.24 +/- 2.48 vs 1.34 +/- 0.14 ng/ml, P < 0.0001; preterm: 4.46 +/- 1.05 vs 0.63 +/- 0.18 ng/ml. P < 0.0005), and higher in term than in preterm serum (P = 0.03). Milk levels were higher in the term vs preterm group in intervals A (P < 0.01) and B (P < 0.05). In the term group they declined significantly from interval A to interval B (P < 0.05) but did not vary significantly in the preterm group. Serum levels correlated with maternal body mass index; milk levels showed only moderate correlation with maternal and infant weight or body mass index. Conclusion: The reasons for the presence and differential longitudinal expression of leptin in human milk after term and preterm pregnancy remain unknown. A hypothesis, requiring further study, is that persistently lower leptin levels in preterm milk act as a compensatory release of a brake on neonatal growth.