14 Chapter 1 Background Embryonic and fetal development are an exceptional phase of human life: a onecelled zygote develops into an infant with roughly 200 different cell types, weighing on average 3.5 kg at birth. This exponential growth and differentiation rate will never be equaled in later life. Preterm delivery, however, abruptly interrupts this process and results in a very precarious situation. The now premature infant finds itself in an alien extra-uterine environment while it is yet to go through a major part of its’ development. As premature infants can no longer rely on the regulatory function of the placenta for nutrient supply, immunity andendocrine control, their growthanddevelopment depend on immature organ systems. The developing gastro-intestinal tract still has an impaired digestive and absorptive capacity.(1) Furthermore, the intestines have an inadequate mucosal barrier function andmaternal antibodies, which are largely transferred across the placenta in the last trimester, are lacking.(1, 2) Combined with an overall immature immune response and a general pro-inflammatory state, premature infants are left prone to infections. Moreover, the regulation of growth is disrupted by insufficient levels of growth factors.(3) Altogether, preterm infants are faced with the preposterous task of thriving in the face of comorbidities and insufficient nutrient and hormone supplies. Obviously, this is a process prone to error, making postnatal growth failure a common problem in preterm infants. (4, 5) After an initial phase of impaired growth, preterm infants are likely to show accelerated growth up to 2-3 years of age.(6) Growth patterns in infancy and early childhood have been linked to health outcomes. Notably, postnatal growth restriction is associated with an increased incidence of co-morbidities during hospitalization (7) and may lead to impaired neurodevelopment in later life (8). Furthermore, there are concerns that impaired growth triggers a thrifty phenotype with increased adipose tissue and adverse cardiometabolic outcomes in later life. (9, 10) Therefore, it would be of interest to gain more insight in factors determining growth and body composition in early life, as a means to enhance health outcomes in infants born preterm. The regulation of fetal and postnatal growth and body composition The regulation of fetal growth and body composition is an intricate process. There is an interplay between oxygen, nutrients, and hormones of fetal, placental and maternal origin. In addition, genetic factors determine a part of the growth potential. After preterm birth, however, a new balance needs to be found between substrates, endocrine and genetic factors.
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