Micronutrients and the bone mineral content of the mother, fetus and newborn.

The fluxes of the primary bone-forming minerals, calcium, phosphorus, magnesium and zinc, across the placenta and through breast milk place considerable demands on maternal mineral economy. Increases in food consumption, elevated gastrointestinal absorption, decreased mineral excretion and mobilization of tissue stores are several possible biological strategies for meeting these extra mineral requirements. This paper presents a review of the evidence on the extent to which these strategies apply in the human situation, the mechanisms by which they occur, the limitations imposed by maternal diet and vitamin D status and the possible consequences for the growth of the infant and bone health of the mother. On the strength of current evidence it appears that pregnancy and lactation are associated with physiological adaptive changes in mineral metabolism that are independent of maternal mineral supply within the range of normal dietary intakes. These processes provide the minerals necessary for fetal growth and breast milk production without requiring an increase in maternal dietary intake or compromising maternal bone health in the long term. This may not apply to pregnant women whose mineral intakes or sunlight exposure are marginal. As a vehicle for promoting optimal growth and bone mineral content of infants, supplementation of lactating women with minerals or vitamin D is unlikely to prove effective. The situation in pregnancy is less certain. Until more studies have been conducted, a precautionary case can be made for targeted supplementation of pregnant women who have very low intakes of calcium or who are at risk of vitamin D deficiency.