ABSTRACT. Objective. Leptin, a hormone present in breast milk, is involved in energy regulation and metabolism. The purpose of this investigation was to determine whether leptin is present in either preterm breast milk (PBM) or preterm formula (PF). The effects of delivery methods and pasteurization on leptin levels also were evaluated.Methods. PBM samples were obtained from 29 mothers who delivered infants at between 23 and 34 weeks' gestation. Leptin levels were measured in PBM and PF with the use of a radioimmunoassay specific for human and bovine leptin, respectively. Milk samples were pasteurized by fast-and slow-heating methods. PBM and PF spiked with human leptin were delivered through catheters by bolus and continuous administration to determine the effects of delivery method on recoverable leptin levels.Results. Median PBM leptin concentration was 5.28 ng/mL (intraquartile range: 24.79). Birth gestational age, birth weight, and gender of the infant did not significantly influence PBM leptin levels. Neither bolus nor continuous feeding practices affected leptin levels in PBM or spiked PF. However, pasteurization significantly reduced the amount of detectable leptin in PBM.Conclusions. PBM leptin levels were highly variable and similar to levels reported for term breast milk. There was no effect of postnatal age on PBM leptin concentrations. Sterilization decreased detectable leptin levels, whereas feeding practices had no adverse effect on the quantity of leptin delivered. Although no infant formula contained leptin, leptin could be added to formula and delivered through various feeding methods without loss. Pediatrics 2001;108(1). URL: http://www.pediatrics.org/ cgi/content/full/108/1/e15; delivery method, infant formula, leptin, pasteurization, preterm breast milk.ABBREVIATIONS. PBM, preterm breast milk; PF, preterm formula; RIA, radioimmunoassay. L eptin can no longer be viewed as solely an antiobesity hormone. Although leptin plays an important role in modulating adaptation to energy regulation and utilization in the fasting state, 1 it also affects angiogenesis, 2 wound healing, 3 hematopoiesis, 4,5 bone metabolism systems, 6 and the neuroendocrine 7 and immune systems. 8 In utero, the fetus is exposed to leptin derived primarily from the placenta 9,10 and from its own tissues. 11 Premature delivery separates the infant from its principal source of leptin before the late-gestation rise in leptin levels. 9 Premature infants have significantly lower serum leptin levels than full-term infants. 12 This has significant implications for the premature infant, who is in a catabolic state.Breast milk and formula are the only sources of nutrition and growth factors for the infant in the postnatal environment. Mammary epithelial cells produce leptin, 13 and leptin is secreted into term breast milk. [13][14][15] A previous study 15 showed that leptin can pass from mother's milk into the circulation of rat pups, suggesting that term breast milk is an exogenous source of leptin. Whether preterm breast milk (PBM) ...