32 Chapter 2 rate enabling the infant to reach a body size comparable to that of healthy infants at a corresponding age. Also further maturation of the neuroendocrine axes and target organs may improve feedback mechanisms and thus optimize growth control. Hypothesizing more into detail eventually a point may be reached where growth velocity is fixed regardless of the previous growth pattern. This would then correspond with the absence of a correlation between IGF I levels and subsequent growth. At this point the neuroendocrine axes and target organsmay be completely programmed setting the growth rate at a fixed point. Concluding these diverse associations may possibly illustrate a regulatory effect to direct growth towards the mean. For IGF I to function optimally, i.e. to enable growth to the full potential of each individual, certain conditions are paramount. The neuroendocrine axes and target organs need to be matured to such an extent that feedback mechanisms can reliably control growth, i.e. correct too slow as well as too fast growth. Furthermore the environment needs to reinforce a steady growth rate; meaning that sufficient nourishment needs to be available and stressful factors, such as comorbidities, should be limited. Hypothesizing after term age there may be an optimal combination of these conditions, creating a window of opportunity to catch up in growth. In comparison, in children who are adopted from impoverished and stressful situations growth restriction occurs. When they are placed in a more nurturing environment through adoption catch-up growth is observed. In a study by Miller et al. adopted children with the lowest IGF I had 4.9 times higher odds (95%CI: 1.1 – 22.9) of showing catch up growth in height than children with the highest IGF I (mean age at adoption 20.1 months + 9.8) (27). Simplified, preterm infants show three growth patterns: small size at birth and persistent small size at term age (small for gestational age infants), appropriate size at birth but small size at term age (appropriate for gestational age infants with postnatal growth restriction) and appropriate size at birth and appropriate size at term age (appropriate for gestational age infants without postnatal growth restriction). In analogy with children following adoption it is hypothesized that infants small at term age come into a less stressful period after term age which enables them to catch up in growth. Indeed some infants, but not all, show ‘catchup’ growth. The majority of this ‘catch-up’ growth occurs in the first 6 to 12 months of life (28). However there is a concern that rapid growth leads to an unfavourable metabolic and cardiovascular outcome in later life. Indeed De Jong et al. found increased length gain between 6 and 12 months corrected age and weight gain between term and 2 years corrected age to be associated with increased systolic blood pressure at 2 years corrected age (29). In addition Singhal and colleagues found a poorer endothelial function in preterm born adolescents with the highest rate of weight gain in the first two weeks after birth (30). In comparison in term born young adults rapid weight gain in the first three months of life has also been associated with decreased insulin sensitivity, a higher percentage of body fat and more central adiposity (31). Moreover Hovi et al. demonstrated that a decrease in
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