118 Chapter 6 Methods Study cohort The study cohort consisted of a convenience sample of 65 preterm infants born between 2015 and 2018, with a gestational age of 24 to 32 weeks, admitted to the neonatal intensive care unit of the Amsterdam University Medical Centers, location Vrije Universiteit University Medical Center. The preterm infants were part of the NUTRIE study, a longitudinal observational study on nutrition in relation to the endocrine regulation of preterm growth and body composition. The NUTRIE study was powered to detect a medium size effect (r = 0.35) of Insulin-like growth factor 1 on fat mass percentage.. No power calculations were done for the primary outcomes presented in this paper. To demonstrate that the maximum allowed difference in fat mass measured by two different methods is < 200 g, 10 pairs would be needed based on a mean difference in the population of 100 g (+ 25) (8). Informed consent was obtained in the first week of life and participants were followedup frombirth to two years corrected age. Infantswith substantial congenital anomalies based on a chromosomal disorder or syndrome were excluded. The study was approved by the medical research ethics committee of the Vrije Universiteit University Medical Center and was conducted according to the good clinical practice guidelines and in line with the Declaration of Helsinki. The study was registered at the Dutch Trial Register where an audit trail of changes to the design was kept (www.trialregister.nl; NTR5311). Assessment of growth and body composition Growth and body composition were assessed on the same day in the same order in all participants. Follow-up at term equivalent age was planned between 38 and 46 weeks postmenstrual age (mean 43.8 + 1.9 weeks). SFT were measured first, followed by ADP and finally DXA. Infants were fed before the DXA in case the child was too agitated. Growth was assessed from birth until 36 weeks postmenstrual age and at term equivalent age. Measurements of weight, length, and head circumference were taken by two investigators. Infants were weighed nude on an electronic scale to the nearest 5 gram and length was measured with a length board to the nearest 0.5 cm. Occipital-frontal head circumference was measured to the nearest 0.1 cm with a non-stretchable measuring tape. Standard deviation scores (SDS) of weight, length and head circumferences were calculated according to Fenton (17). Small for gestational age (SGA) was defined as a birth weight below the tenth percentile (-1.3 SD) and postnatal growth restriction was assumed if, at 36 weeks postmenstrual age, there was a decrease in weight z-score of more than 1 SD compared to the birth weight z-score (18).
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