REGULATION OF NUTRIENT PARTITIONING AND METABOLISM DURING PRE- AND POSTNATAL GROWTH

Abstract

Regulation of whole-body flux rates and in vivo tissue metabolism of specific nutrients is discussed in relation to three developmental stages: prenatal, birth to weaning and postweaning. The marked decline in relative growth rate and weight-specific uptake of oxygen, glucose and amino acids of the fetus between mid and late gestation may be due in part to recently demonstrated limitations in placental transport capacity during late pregnancy. In fetal, as in neonatal and weaned, animals, insulin is an important regulator of metabolic homeostasis, but evidence for its role in chronic regulation of nutrient partitioning and growth at any stage of development is equivocal. Somatotropin-dependent regulation of skeletal growth, mediated by insulin-like growth factor I, does not develop until soon after birth, but in fetal and postnatal animals somatotropin depresses lipid deposition in adipose tissue by antagonizing stimulation of lipogenesis by insulin, thereby sparing utilization of lipogenic nutrients and promoting net mobilization of nonesterified fatty acids. These and other long-term actions, including effects on mineral metabolism, are consistent with the postulated role of somatotropin as an important homeorhetic regulator of postnatal metabolism and growth of key tissues. The complex integration of tissue metabolism and its regulation in vivo must be studied systematically in meat animals at different stages of development, grown at optimal and suboptimal rates. Existing concepts and methodology provide a foundation for beginning this task; further progress will be impossible without significant conceptual and technical innovation.