Insulin-like growth factor (IGF-I) and its low molecular weight binding protein in human milk

Insulin-like growth factors (IGF-I and IGF-II) are mitogenic polypeptides bound in serum to specific binding proteins. This study reports the occurrence and concentrations of IGF-I and its low molecular weight IGF binding protein (IGF BP) in human milk and serum after delivery in 41 healthy women. The levels of IGF-I in milk declined between 4 and 92 hours postpartum from 49 +/- 17 micrograms/l to 29 +/- 27 micrograms/l (p less than 0.01). The level of IGF BP also declined from 165 +/- 80 micrograms/l to 97 +/- 70 micrograms/l (p less than 0.01), whereas the serum concentration of this protein remained unchanged. A positive correlation was observed between the individual levels of IGF-I and its binding protein in milk (r = 0.49, p less than 0.01), but no correlation was found between the individual levels of 34 kDa IGF BP in milk and serum. The relative abundance of IGF BP in the first milk compared to IGF-I is intriguing and its biologic significance remains to be clarified.