Growth factors, body composition and energy expenditure in late preterm and term infants during the first 4 months of life: a prospective cohort study.

Abstract

Background: Late preterm infants (34-36 weeks gestation) represent the majority of preterm births and are often assumed to follow similar postnatal growth trajectories as term infants. However, the postnatal hormonal environment and body composition development in this group remain underexplored. This prospective observational study aimed to analyze and compare growth, body composition, energy expenditure, hormonal, and metabolic responses in healthy late preterm and term infants in the first four months of life.

Results: Anthropometry, body composition, energy expenditure, metabolic biomarkers and growth factors were measured in 94 term infants (gestational age: 39.6 ± 1.3 weeks, birth weight 3330 ± 570 g) and 18 late preterm infants (35.0 ± 1.0 weeks, 2520 ± 660 g) at three time points (0-5, 55-65 and 115-125 days of life). The onset of fat mass accretion occurred directly after birth resulting in higher percent fat mass in late preterm infants in early life. Late preterm infants reached a similar percent fat mass approximately five weeks earlier in postmenstrual age than term infants. In contrast, fat-free mass developed along similar trajectories in both groups, indicating preserved lean tissue growth in late preterm infants. Energy expenditure doubled during the first two months and was closely linked to fat-free mass accretion. Insulin-like growth factor (IGF)-1 and IGF-2 levels increased postnatally, with slightly higher concentrations in late preterm infants. Increase of percent fat mass paralleled leptin and IGF levels in both groups. IGF-1 and IGF-2 levels were higher in formula-fed infants, supporting the influence of nutritional composition on growth-related hormonal regulation.

Conclusions: Birth may initiate changes in hormonal levels and acceleration of fat mass accrual, resulting in higher fat mass in late preterm-born infants at term age when compared to term-born infants. Next to hormonal shifts, these changes appear to be driven by nutritional factors in the early postnatal period. The results suggest that growth targets for late preterm infants may need to be reconsidered, particularly in the early postnatal period. Future studies should provide evidence on individual growth targets and nutritional guidelines for preterm infants to account for the physiological differences to term infants.