VITAMINS AND MINERALS
As a component of prenatal care, micronutrient supplementation might reduce maternal morbidity and mortality directly by treating a pregnancy-related illness or indirectly by lowering the risk of complications at delivery. Nevertheless, the
effectiveness of supplementation programs—notably of iron and folate—has tended to focus on infant outcomes, perinatal mortality, preterm delivery, and low birth weight (11). When infant outcomes are discussed, however, a distinction needs to
be made between the teratogenic effects of deficiency at periconception and deficiency in later pregnancy. The former includes folate deficiency resulting in fetal neural tube defects (12, 13) and iodine deficiency that causes cretinism (14),
whereas the latter includes intrauterine growth retardation (15,16) and other pregnancy-related and delivery complications (17–21). This distinction is important because many pregnancies in developing countries are unplanned; the use of prenatal care services, in which iron or iron and folate supplements are
generally provided or prescribed, is less than optimal (22); and the cost-effectiveness of the programs promoted needs to be considered. Even in developed countries, prenatal care providers have reservations about the widespread and indiscriminate use of prophylactic minerals and vitamins because of unproven benefits in well-nourished populations, risks of teratogenicity, and side effects (23–25). These concerns need to be considered in the context of developing countries.
Data on vitamin and mineral metabolism and requirements during pregnancy are scanty, and determining the consequences of apparently deficient or excessive intakes is not easy (1). In pregnancy, maternal metabolism is altered by hormones that mediate the redirecting of nutrients to the placenta and mammary gland as well as the transfer of nutrients to the developing infant. Kidney function changes to handle the clearance of both fetal and maternal metabolic waste, which is associated with increased urinary excretion of water-soluble vitamins (eg, folate) (2). Blood volume and composition also change; by the third trimester, blood volume increases by 35–40% over the nonpregnant state, largely because of a 45–50% expansion of plasma volume and a 15–20% expansion of red blood cell mass.Nutrient requirements during pregnancy are usually calculated by adding an increment to the value for nonpregnant and nonlactating women that covers the cost of fetal growth and development and the associated changes in maternal tissue metabolism.
This factorial approach, however, may not necessarily be correct because it does not take into account metabolic changes in absorption or excretion that may compensate for the additional nutrient requirements without the need for an increase in intak