Nutrient needs increase during pregnancy and lactation to support fetal growth and milk synthesis, respectively. Physiological adjustments that are made to meet those needs alter the fraction of ingested nutrient retained, or the bioavailability. Using stable isotopes as tracers, we measured calcium, zinc and selenium homeostasis in women during reproduction. The physiological response, and therefore the bioavailability, of these three minerals differed during reproduction. Calcium absorption

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... creased ϳ2-fold during pregnancy but dropped to values for nonpregnant women during lactation. The calcium needs for lactation were met by renal conservation and bone resorption. In women chronically consuming a low calcium diet, fractional calcium absorption increased to Ͼ80% during reproduction. Zinc absorption tended to increase during pregnancy and lactation; renal conservation was not evident at any time during the reproductive cycle. Selenium absorption was high, ϳ80% of intake, in both pregnant and nonpregnant women. Pregnant women conserved selenium by decreasing urinary selenium excretion. Studies defining the impact of maternal status and the dietary mineral source and amount on mineral bioavailability are needed to determine the potential benefits of mineral supplementation during reproduction. J. Nutr. 131: 1355S-1358S, 2001.