Immaturity or starvation? Longitudinal study of leptin levels in prematureinfants

Objective: Leptin, the protein product of the ob gene, is a potential place ntal growth factor and is integral to the body's system of energy regulatio n as shown in animal models. Premature infants are especially vulnerable to changes in energy regulation, and several studies have demonstrated a rapi d fall in leptin values at birth. The purpose of the present investigation was to measure leptin levels in premature infants throughout hospitalizatio n. Methods: Eligible infants were less than 32 weeks' gestation, appropriat e for gestational age, and hospitalized at Christiana Hospital Special Care Nursery. Serum samples for leptin analysis were drawn within 24 h of birth and twice a week thereafter until discharge. Concurrent growth measurement s were obtained with each leptin sample. Body mass index, ponderal index, a nd midarm circumference/head circumference ratios were calculated to assess growth. Results: Leptin levels were low and remained low for the duration of the premature infants' hospitalization (mean SD = 1.35 +/- 0.63 ng/ ml/m l, range 0-3.06). After controlling for weight, there was a small (r(2) = 0 .1, p < 0.00001) but significant correlation between leptin and postnatal a ge after 4 days of age. Despite an increase in caloric intake during the st udy period, there was no relationship between leptin and caloric intake. Th ere were significant negative correlations between measurements of growth a nd both leptin and the leptin/weight ratio. Maternal diabetes and the use o f steroids had small but significant effects on the leptin/weight ratio. Co nclusion: In this population of predominantly female premature infants, lep tin levels were very low as compared to term infants, children and adults, and did not change appreciably over the study period. The low leptin levels seen in these premature infants are similar to those levels seen in malnou rished adults, anorexics, and in animal models of starvation. We speculate that a critical adipose store needs to be reached before increased amounts of leptin can be adequately produced. Persistently low leptin levels may al so reflect an immaturity in the hypothalamic-pituitary-adrenal axis. Copyri ght (C) 2001 S. Karger AG, Basel.