Is milk-borne insulin-like growth factor-I essential for neonatal development?

Insulin-like growth factor-I (IGF-I), a polypeptide growth factor found in milk, is hypothesized to play a functional role in the growth and development of neonates, particularly the gastrointestinal tract. Considerable evidence, based on direct tracer studies with 125I-labeled IGF-I and measurements of circulating IGF-I concentrations in neonatal animals fed a range of IGF-I doses, indicates that the intestinal absorption of IGF-I and the possible effect on metabolism and somatic growth are negligible. However, studies in neonatal animals indicate that oral administration of pharmacological doses of IGF-I increases small intestinal mucosal growth, whereas oral IGF-I provided within the physiological range may enhance the development of intestinal lactase. Therefore, clinical trials exploring the therapeutic use of oral IGF-I as an intervention for preterm neonates and those with compromised intestinal function seem warranted. However, milk-borne IGF-I may not be essential for normal healthy infants, perhaps because endogenous IGF-I provides a sufficient stimulus for maintenance of gastrointestinal structure and function. Future studies should explore the significance of endogenous IGF-I and whether milk-borne IGF-I may be important under pathological conditions in which the endogenous IGF-I production may be compromised.