Role and regulation of metabolism in adipose tissue during lactation

The metabolism of energy-yielding compounds in adipose tissues has a pivotal role in supplying the energy demands of lactation. In most mammals, the relatively high ratio of milk energy production to maintenance demands a cycle of energy storage during pregnancy, lipid mobilization 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-energy supply, the stored lipid can be critical to establishment of lactation and maintenance of maternal health. There is a commonality among mammals in the adaptative responses during lactation and in their general regulation. However, in rodents and most domestic species the magnitude of metabolic response is greater than in women. The importance of adipose tissue to lactation is demonstrated by the number of highly coordinated and redundant control elements that regulate the adaptations of metabolism. This coordination is carried out by the central nervous system through the endocrine organs and the sympathetic nervous system. Recently, insights have been gained into the quantitation of the adaptations due to rate of milk production, stage of lactation, and intake of energy-yielding compounds and the physiological mechanisms of action of several regulatory factors. The intake of nutrients and demand for milk precursors have differential effects on the enzymes of lipid synthesis and release from adipose tissue, and the equations describing these chemical interconversions vary with stage of lactation, nutrient intake, and genetic propensity for milk production. Regulatory mechanisms that are now better understood include those of the interactions of growth hormone and insulin to control lipogenesis and the activity of the sympathetic nervous system to regulate lipolysis. Improvements in understanding and managing lactational energy metabolism have been limited by the complexity of the chemical interconversions of nutrients and their regulation. For women specifically, the severe lack of information on adaptations at the tissue level hinder further advancement. Improvement in this area will require a coordinated effort to study both physiological control mechanisms and quantitative parameters of lipid metabolism.