Fuel selection: the maternal adaptation to fetal nutrient demand.

The necessity to accommodate the developmental and fuel requirements of the fetus confers upon the pregnant mother a considerable nutritional burden. Pregnancy is thus a state in which the maternal response to feeding and fasting is substantially modified to ensure a constant supply of glucose for the developing fetus, while maintaining the glucose and energy requirements of the mother. One of the major adaptations to pregnancy involves a decreased threshold and an increased response of the pancreatic f i cell to glucose [l]. This adaptation, through facilitating insulin hypersecretion during meals [2], effectively compensates for maternal insulin resistance at the level of glucose uptake by skeletal muscle [3-51. It also permits storage of excess dietary glucose as glycogen in maternal muscle, thereby dampening excursions in glycaemia which might be considered to be detrimental to the fetus. The recent observations that the placenta can secrete leptin [6,7] and leptin levels are raised in pregnant women [6,8] have raised the intriguing possibility that leptin may play an important role in nutritional signalling from the mother to the fetus in pregnancy. During the progression from the fed to the fasted state, metabolic adaptations to pregnancy include postabsorptive hypoglycaemia [9123 and resistance of lipolysis to the action of insulin in vivo at low concentrations [S]. This results in an altered balance between glucose and lipid-derived fuels which impinges on maternal fuel selection. Further impetus to gaining an understanding of maternal fuel homeostasis during pregnancy has been provided by the hypothesis that environmental factors operating in fetal life are important in the development of several adult onset diseases, including hypertension, insulin resistance, Type 2 diabetes and Syndrome X (insulin resistance syndrome, the cluster of pathological states including insulin resistance, hypertension and hypertriglyceridaemia)