Main

Leptin regulates energy balance in experimental animals and is, apart from placenta, produced exclusively by adipose tissue (1–6). The circulating leptin concentrations reflect body fat content in mice (7), as well as in adult humans (8,9). In addition to its role in energy metabolism, leptin may contribute to other physiologic systems, such as reproduction (10,11), hematopoiesis (12), and fluid and electrolyte homeostasis (13). Recently, we (14) and others (5,15–17) have shown that immunoreactive leptin is detectable in cord blood in newborn infants and its concentration correlates closely with birth weight.

There is a gender difference in leptin concentrations in adult humans (18) and prepubertal children (19,20), with females having higher leptin concentrations per unit fat mass. However, the data regarding gender difference in leptin in full-term infants have been conflicting, with some studies indicating no gender difference (14,16) and others suggesting higher leptin concentrations in newborn females (5,17). It is possible that the development of a marked gender difference in leptin levels seen already in prepubertal children (19,20) is a consequence of the development of different gender-specific hormonal milieus, such as the early postnatal rise in serum testosterone concentration in male newborn infants (21).

The present study was undertaken to examine three aspects of leptin metabolism during the early postnatal period. First, we examined whether plasma leptin levels change when the infants adapt to extrauterine life and the nutrition of the newborn is transferred from the fetoplacental unit to external intake. Second, we studied whether plasma leptin concentration is associated with adipose tissue thickness as determined by ultrasound in newborn infants. Third, we examined a possible development of a gender difference in leptin levels during the first 3 postnatal days.

METHODS

Subjects. We studied 38 healthy newborn infants (20 male, 18 female; gestational age 39.7 ± 1.3 wk (mean ± SD), range 36.3 to 41.9 wk) born in the Helsinki City Maternity Hospital. Birth weight was 3470 ± 552 g (range 2470 to 4630 g). Relative birth weight, as determined by reference to a Finnish newborn population of 74 766 singletons born 1978-1982 (22), was -0.26 ± 1.1 SD, range -2.0 to +2.0 SD. The placental weight ranged from 370 to 850 g. Two infants were delivered by cesarean section. Two mothers had gestational diabetes treated with diet only. The BMI ranged from 11.2 to 16.1 kg/m2 in the newborns and from 15.8 to 43.8 kg/m2 in the mothers. The study was approved by the ethical committee of the Helsinki City Hospitals and was conducted according to the principles of the Declaration of Helsinki. Written informed consent of the parents was obtained before participation in the study.

Methods. A mixed blood sample was obtained from umbilical cord at birth, and a venous blood sample was taken when the infant was 3 d old (mean age 73 ± 9 h, range 60 to 100 h). Both samples were taken into tubes containing EDTA. The tubes were centrifuged at 3500 rpm for 10 min, and plasma was frozen and stored at -20°C until analysis. Plasma leptin concentrations were determined with RIA (Linco Research, St. Charles, MO) (23). The detection limit of this assay was 0.26 µg/L in our laboratory as determined by calculating 2 SDs (mean of 13 assays) from the zero reference point, and the intra-assay and interassay coefficients of variation at low concentration (2.8 µg/L) were 4.7% and 2.6% and at medium concentration (15.6 µg/L) 3.8% and 2.2%, respectively. Total plasma testosterone concentration was determined with RIA (Orion Diagnostica, Turku, Finland). Birth weight and length were recorded. At the age of 3 d, weight was recorded and the circumference of the proximal third of the left humerus was measured using a soft metric measuring tape. The thickness of s.c. adipose tissue at the same site was measured three times by 7 MHz linear ultrasound transducer (Acuson 128; Mountain View, CA). The coefficient of variation of three measurements was 6.0 ± 4.3%. This method has been validated previously using computer tomography as a reference standard (24).

Statistical analysis. Comparisons between unpaired and paired items were done using Mann-Whitney U and Wilcoxon rank sum tests, respectively. Correlation coefficients were calculated with the Spearman test. Analysis of covariance was used to adjust leptin levels for possible confounders. The results are given as mean ± SD. A value of p < 0.05 was considered statistically significant.

RESULTS

Demographic data of the newborns are given in Table 1. At birth, cord plasma leptin concentration was 9.7 ± 5.2 µg/L, with no gender difference between male (8.6 ± 4.6 µg/L) and female (10.9 ± 5.6 µg/L, p = 0.198, Mann-Whitney U test) infants (Fig. 1, Table 1). In the analysis of covariance, there was no statistically significant gender difference in leptin levels when adjusted for the thickness of the s.c. fat (p = 0.267). There was a slight gender difference in cord plasma leptin levels with higher levels in female infants when adjusted for infant BMI (p = 0.04) or birth weight (p = 0.013). There was no gender difference in plasma leptin/thickness of s.c. fat ratio (p = 0.35), whereas at birth, plasma leptin/birth weight ratio was higher in female than male newborns (Table 1).

Table 1 Demographic data and plasma testosterone and leptin concentrations of the newborns
Full size table
Figure 1
figure 1

Plasma leptin concentrations in cord blood and at 3 d of age in male (n = 20) and female (n = 18) newborn infants. Plasma leptin decreases in both male and female newborns by the third postnatal day (p < 0.001). At birth, leptin levels are similar in male and female newborns. The gender difference evolves by the third postnatal day, with females having higher plasma leptin concentrations (p = 0.01).

Full size image

Plasma leptin decreased in male (to 1.8 ± 0.4 µg/L, p < 0.001) and female (to 2.3 ± 0.8 µg/L, p < 0.001) infants by the third postnatal day (Fig. 1, Table 1). At this age, gender difference was statistically significant (p = 0.01, Mann-Whitney U test). In the third postnatal day, plasma leptin/thickness of s.c. fat ratio was similar in male and female newborns (p = 0.231), whereas in female newborns, plasma leptin/body weight ratio was higher than in males (Table 1).

In male newborns, cord plasma leptin concentration correlated with arm circumference, but the association of cord plasma leptin with BMI, the thickness of the s.c. fat, or birth weight did not reach statistical significance (Table 2). In 3-d-old male newborns, plasma leptin concentration did not correlate with any of these parameters (Table 2). In female newborns, cord plasma leptin concentration correlated with BMI, the thickness of the s.c. fat, and circumference of the arm, but not with birth weight (Table 2). In 3-d-old female newborns, leptin concentration correlated with weight and arm circumference (Table 2). There was no statistically significant association between cord plasma leptin concentration and placental weight either in female (r = 0.30, NS) or male (r = 0.37, NS) newborn infants.

Table 2 Spearman's correlation coefficients between leptin concentration at birth and at 3 d of age vs antrophometric data in male (n = 20) and female (n = 18) newborns
Full size table

The decrease in leptin concentration during the first 3 postnatal days correlated with BMI at birth (r = 0.63, p < 0.01), thickness of s.c. fat (r = 0.54, p < 0.05), and arm circumference (r = 0.72, p < 0.01) in female infants and with arm circumference (r = 0.50, p < 0.05) in male infants; but the association between the change in weight and the change in plasma leptin levels during the first 3 postnatal days was not statistically significant in female (r = 0.07, NS) or male (r = 0.43, NS) infants. Neither cord plasma leptin, plasma leptin at 3 d of age, nor the decrease in leptin levels correlated with the maternal BMI in male or female infants.

DISCUSSION

Our data confirm the results from previous studies that immunoreactive leptin is present in cord blood (5,14–17). Moreover, leptin concentrations in cord plasma correlate well with the adiposity of the female newborn, as determined directly by the measurement of the thickness of the s.c. fat by ultrasound. This association did not reach statistical significance in male newborns. During intrauterine development, the fetus is dependent on the maternal fuel and constituent supply, and the presence of a satiety signal would seem rather paradoxical. The IL-6-type cytokine receptor-like structure and signaling capabilities of the leptin receptor (25) and the expression of mRNA for leptin receptor isoforms in the lungs, liver, and kidneys of human fetuses (12) raise the possibility that leptin may function as a regulator of growth during fetal development. Recent data suggesting placenta as a source of leptin for the growing fetus (5,6) support the idea of leptin having a physiologic function in the maturation process. At present, it is impossible to distinguish whether the leptin in cord blood is of fetal or placental origin. However, there is a small (9%), albeit significant, arteriovenous difference with higher leptin concentrations in blood from umbilical artery (16), suggesting that at least part of the leptin in the fetoplacental circulation is of fetal origin.

Leptin concentration decreased markedly during the first 3 postnatal days. The decrease in infant weight during the same time, which is mostly explained by physiologic fluid loss (26), is unlikely to explain the fall in leptin, because we found no correlation between changes in weight and leptin. The basal metabolic rate increases within the first 2 postnatal days in the human neonate (27). The growth and development of the neonatal period becomes dependent on the external energy and constituent supply as the nutrition of the newborn is transferred from the fetoplacental unit to external intake. Thus, the postnatal decrease in leptin concentration may be a physiologic adaptation to these profound alterations in the metabolic environment and may serve to enhance food intake of the newborn. According to a recent study, the decrease in leptin concentrations occurs by 16 h after birth, and leptin levels remain low until at least 3 d after delivery (28). The mechanisms behind the postnatal fall in leptin levels remain at present unclear; we cannot exclude the possibility that part of the decline in plasma leptin concentration reflects the clearance of placental leptin.

There is a well-established gender difference in leptin concentrations in adult humans (18) and prepubertal children (19,20), with females having higher leptin concentrations per unit fat mass. However, data regarding gender difference in leptin in newborn infants have been conflicting (14,16,17); a small gender difference in leptin concentrations was found when 100 newborn infants were studied (5). In our study, the unadjusted leptin levels were not statistically different between male and female infants at birth. Recently, Helland et al. (29) suggested that the gender of the infant modifies the increase in maternal leptin levels from 18 to 35 wk of pregnancy: the increase in leptin levels was not significant in women carrying male fetuses. Moreover, leptin levels were lower in male infants at birth, and the gender difference became more apparent when leptin/birth weight ratio was analyzed (29). Similarly, female newborns had a higher plasma leptin/birth weight ratio than male newborns at birth in our study. It is thus possible that the contribution of placenta in leptin secretion (6) may have masked the gender difference in cord plasma leptin levels in our study.

By third postnatal day, however, a clear gender difference in leptin concentrations had developed, with higher concentrations in female newborns. The thickness of the s.c. fat was similar in male and female newborns in our study, and correlations between plasma leptin at 3 d of age and weight and arm circumference were observed in female but not in male newborns. These data suggest that factors other than adiposity, such as hormonal milieu, may interfere with the association between leptin and adiposity. There is no gender difference in serum estradiol concentrations during the early postnatal period, whereas a postnatal rise in serum testosterone concentrations occurs in male but not in female newborns (21,30). In adult males, there is an inverse relationship between serum leptin and testosterone concentrations that is independent of BMI (31). A negative correlation between serum leptin and testosterone concentrations was observed also in adult male insulin-dependent diabetes mellitus patients (32). In our study, male newborns had higher plasma testosterone and lower leptin concentrations than female newborns at 3 d of age. These data raise the possibility that the gender difference in leptin levels may be a result of testosterone downregulating leptin secretion. Indeed, a recent study has shown this to be the case: testosterone downregulates leptin production at both the protein and mRNA levels in human adipocytes (33). In leptin-deficient ob/ob mice, leptin treatment corrects the sterility attributed to insufficiency of hormones at the hypothalamic-pituitary level (10). Moreover, compared with controls, leptin treatment resulted in earlier maturation of the reproductive tract and earlier reproduction in mice (11). In humans, an increase in serum leptin levels occurs in both girls and boys before the appearance of other reproductive hormones, suggesting that leptin may be an important signal triggering the onset of puberty (34,35). Thus, higher leptin levels in females shortly after birth may be physiologically important and relate to leptin's role in reproduction.

In conclusion, our data suggest that circulating leptin concentrations correlate with adiposity at birth in female but not in male newborn infants. Leptin concentration decreases markedly in both genders by the third postnatal day, and a clear gender difference with higher leptin levels in females develops by that time. The postnatal decrease in plasma leptin concentration may be a physiologically feasible adaptation to extrauterine life.