Role of milk-borne vs endogenous insulin-like growth factor I in neonatal growth

Neonatal pigs are characterized by a high efficiency of nutrient utilization and rapid growth rate. The utilization of dietary protein for lean tissue growth is particularly efficient in neonatal pigs and is associated with a high rate of skeletal muscle protein synthesis and deposition. In support of these high growth rates, neonatal pigs consume a milk diet that has a high biological value and is abundant in growth factors, including insulin and IGF-I. During the neonatal period, there are developmental changes in the circulating concentrations of, and tissue responsiveness to, hormones, particularly insulin, IGF-I, and growth hormone that play a central role in growth regulation. Our goal has been to characterize the dietary factors and specific aspects of endocrine function that are responsible for the anabolic stimulus that helps to sustain the high rates of protein deposition in neonatal pigs. Our results suggest that, despite the abundance of growth factors in milk and colostrum, the intake of nutrients is the primary anabolic stimulus for protein synthesis and this response declines with age. There is, however, a nonnutritive and as-yet-unidentified component in colostrum that provides a specific anabolic stimulus for skeletal muscle in newborns, but this is probably neither insulin nor IGF-I. Our studies also indicate that circulating concentration of IGF-I are not a primary stimulus of skeletal muscle protein synthesis and that the primary endocrine signal that mediates the response to nutrient intake may be insulin. Future research should address how the local expression of IGF and the function of insulin and IGF receptors affect the responsiveness of anabolic processes to nutrient intake and hence the efficiency of neonatal growth.