Partitioning of nutrients during pregnancy and lactation: a review of mechanisms involving homeostasis and homeorhesis.

Control of metabolism during pregnancy and lactation involves two types of regulation-homeostasis and homeorhesis. Homeostasis control involves maintenance of physiological equilibrium or constancy of environmental conditions within the animal. Homeorhesis is the orchestrated or coordinated control in metabolism of body tissues necessary to support a physiological state. Regulation of nutrient partitioning during pregnancy involves homeorhetic controls arising from the conceptus. This assures growth of the conceptus (fetus and fetal membranes) and gravid uterus as well as development of the mammary gland. With the onset of lactation many--perhaps even most--maternal tissues undergo further adaptations to support rates of lipogenesis and lipolysis in adipose tissue are examples of important homeorhetic controls of nutrient partitioning that are necessary to supply mammary needs for milk synthesis. The interactions between homeorhesis and homeostasis during pregnancy and lactation and possible endocrine control are discussed. While not definitively established, roles for placental lactogen and prolactin are attractive possibilities in homeorhetic regulation of maternal tissues to support pregnancy and the initiation of lactaion, respectively.