110 Chapter 5 preterm infants at term equivalent age, implying IGF-I relates to overall growth rather than body composition. (11) Albeit, in relation to later measurements of body composition, others did find IGF-I to relate to body composition. For example, Cooper et al. reported a higher increase in IGF-I between hospital discharge and 1 year corrected age to be associated with a concurrent higher increase in fat free mass in infants born prematurely. (9) After correcting for gestational age at birth, more weight gain between birth and 36 weeks PMA, and in particular from 30 weeks PMA onwards, was associated with a lower fat free mass percentage at term equivalent age. Hypothesizing this would imply that, in contrast to weight gain in the first month of life, high rates of weight gain are not desirable up to term equivalent age. In line with that, we found that a higher SD score for length and head circumference at 36 weeks PMA and term equivalent age, was associated with a lower fat mass percentage. Furthermore, the increase in head circumference SDS from 30 weeks onwards was associated with a lower fat free mass percentage. Our data suggest that increased growth rates, in particular after the first month of life, are associated with a lower fat free mass percentage and thus a less favorable body composition. We speculate that it might be beneficial to prevent the decrease in SDS in the first weeks of life. Then there might be a less rapid increase in SDS afterwards and potentially a more favorable body composition at term equivalent age. Nevertheless, the fat deposition may be an adaption to extra-uterine life to enable adequate thermoregulation and provide energy stores.(16) In that light it might be valuable to develop normative data, with cut off points where fat accumulation becomes undesirable, for example based on later cardiometabolic outcomes. Study limitations Our study cohort had a relatively low incidence of postnatal growth restriction and a relatively highmean fat mass percentage. Others previously showed that preterm infants with postnatal growth restriction (weight SD at term age < -2 SD), had a lower fat mass percentage compared to preterm infants without postnatal growth restriction. (17, 18) This could partially explain the higher fat mass percentage in our cohort. Yet, it makes our results less generalizable. In addition, our effect size was limited. Despite the sample size of this cohort, the number of IGF-I measurements per week was low. Blood sampling took place on alternating weeks, and combined with a relatively small number of extremely preterm infants, this resulted in a sample size ranging between 1 infant at 26 weeks PMA to 33 infants at 33 weeks PMA. These conditions may have contributed to our findings.
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