30 Chapter 2 IGF I regulation IGF I is a small polypeptide which is mainly synthesized in the liver. It stimulates cell division, cell growth, cell motility, glucose uptake and protein synthesis. Furthermore IGF I inhibits apoptosis. Prenatally it is secreted into the blood under control of insulin. Postnatally growth hormone (GH) gradually takes over this role. In addition malnutrition and hypothyroidismnegatively influence IGF I plasma levels. IGF I is also synthesized in multiple peripheral tissues, e.g. kidney, bone and muscle, where it is released under control of GH and local factors. 99% of IGF I in plasma is bound to high affinity IGF binding proteins which control IGF I transportation and distribution. (12) The role of IGF I in growth and body composition Fetal IGF I levels gradually increase during pregnancy to reach approximately 46 to 90 ng/ml at term age (13). After preterm birth IGF I levels slowly increase (14). Meanwhile infants born at term show a quick surge in IGF-I levels (15). Figure 1 illustrates postnatal IGF I levels in preterm and term infants. In preterm infants IGF I levels at birth are positively correlated with birth weight (16). Until term age these infants IGF I levels’ are also positively correlated with their preceding as well as their subsequent weight gain, indicating higher previous as well as higher subsequent growth rate in those infants with higher IGF I levels (14). Figure 1. Postnatal Insulin-like growth factor I (IGF I) levels in preterm and term infants. Preterm infants: Hansen-Pupp (○) (14), Ohkawa (□) (17), Van de Lagemaat (△) (18), Giapros (◊) (19), Wang (▽) (20). Term infants: Iniguez (▲) (21), Kurtoglu (●) (15), Wang (◆) (20), Larnkjaer (■) (22), Hyun (▼) (23), Ong (+) (24)
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