185 Summary 9 be used to assess body composition in preterm infants. Nevertheless, chapter 6, demonstrated that commonly accepted methods show poor agreement. Fat mass percentage measured with ADP showed a mean difference of 4.5 ± 4.7% when compared to fat mass percentage measured with DXA. Moreover, there was a proportional bias: as the mean fat mass percentage increased, the absolute difference in fat mass percentage between the two methods increased. Furthermore, it was shown that the measurement of skinfolds could not accurately predict fat mass measured by ADP or DXA. A systematic review of the literature in chapter 7, showed that there are actually very few studies validating commonly accepted methods to measure body composition in preterm infants against one another. In line with chapter 6 body proportionality measures, skinfold thickness, bioelectrical impedance and ultrasound were found to be a poor representation of body composition in preterm infants. In current practice, DXA, ADP and isotope dilution, are accepted as accurate measures. Nevertheless, there is variation within and between these methods and our systematic review showed that the statistical agreement between ADP and isotope dilution may be interpreted as poor. Therefore, caution should be taken when comparing body composition measured with different methods. To facilitate future studies and support clinical practice it would be valuable for researchers and physicians to come to an agreement on which reference should preferentially be used to measure body composition in preterm infants. In our studies, we primarily used ADP and suggest the use of ADP or DXA or isotope dilution depending on local availability and expertise. Monitoring body composition remains important in the light of the potential increased cardiometabolic disease risk in adults born prematurely. Early life events and possible interventions in this period may have the potential of improving long term outcomes of preterm infants. Indeed, in chapter 5 it was shown that higher IGF-I levels in the first month of life are associated with increased fat free mass at term equivalent age. Likewise, increased growth in the first month of life was associated with an increased fat free mass percentage at term equivalent age. However, a greater increase in weight, length or head circumference SDS after this window of the first month of life was associated with a less favourable body composition, i.e. a lower fat free mass percentage at term equivalent age. These findings suggest that the window of opportunity to improve the body composition of preterm infants may be limited to the early postnatal phase. In this phase interventions to increase IGF-I levels may have the potential to improve health outcomes of infants born preterm. Meanwhile after this phase IGF-I levels would have reached a threshold value where IGF-I determines the growth rate and further enrichment of nutrition could lead to an unfavourable body composition due to increased fat mass deposition.
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