ROLE AND REGULATION OF METABOLISM IN ADIPOSE-TISSUE DURING LACTATION

The metabolism of energy-yielding compounds in adipose tissues has a p ivotal role in supplying the energy demands of lactation. In most mamm als, the relatively high ratio of milk energy production to maintenanc e demands a cycle of energy storage during pregnancy, lipid mobilizati on and increased feed intake during lactation, and restoration of body fat in late lactation or after weaning. In well-fed women, the energy balance usually remains positive. However, in situations of low-energ y supply, the stored lipid can be critical to establishment of lactati on and maintenance of maternal health. There is a commonality among ma mmals in the adaptative responses during lactation and in their genera l regulation. However, in rodents and most domestic species the magnit ude of metabolic response is greater than in women. The importance of adipose tissue to lactation is demonstrated by the number of highly co ordinated and redundant control elements that regulate the adaptations of metabolism. This coordination is carried our by the central nervou s system through the endocrine organs and the sympathetic nervous syst em. Recently, insights have been gained into the quantitation of the a daptations due to rate of milk production, stage of lactation, and int ake of energy-yielding compounds and the physiological mechanisms of a ction of several regulatory factors. The intake of nutrients and deman d for milk precursors have differential effects on the enzymes of lipi d synthesis and release from adipose tissue, and the equations describ ing these chemical interconversions vary with stage of lactation, nutr ient intake, and genetic propensity for milk production. Regulatory me chanisms that are now better understood include those of the interacti ons of growth hormone and insulin to control lipogenesis and the activ ity of the sympathetic nervous system to regulate lipolysis. Improveme nts in understanding and managing lactational energy metabolism have b een limited by the complexity of the chemical interconversions of nutr ients and their regulation. For women specifically, the severe lack of information on adaptations at the tissue level hinder further advance ment. Improvement in this area will require a coordinated effort to st udy both physiological control mechanisms and quantitative parameters of lipid metabolism.