Abstract
Low calcium intake and parity may be factors affecting calcium homeostasis during pregnancy and lactation. In this study, changes of urinary indices related to bone and renal adjustments of calcium homeostasis were measured in each trimester of pregnancy and in the lactation period, in primiparous (n=35) and multiparous (n=25) women with habitual calcium intakes about half the RDA for this nutrient, in comparison to neverpregnant (control) women (n=23) with similar calcium intakes. Calcium and phosphorous urinary excretion did not change during pregnancy but decreased in the lactation period, similarly in primiparas and mutiparas (p<0.01). Calcium urinary excretion was not different between pregnant and control women, indicating that renal conservation may contribute to calcium homeostasis during pregnancy with sub-adequate as opposed to adequate calcium intakes. In lactation, urinary calcium excretion was 32% lower than in the controls, indicating renal calcium conservation also in this period. Hydroxyproline urinary excretion increased during pregnancy and decreased in the lactation period, both in primiparas (p<0.001) and multiparas (p<0.05). Excretion was, on average, 152% and 80% higher than excretion in the control women, in the third trimester of pregnancy and in lactation, respectively, consistent with increased bone mobilization in these periods. Considering all pregnant women, hydroxyproline excretion in the third trimester correlated weakly but significantly with pregnancy weight gain (r=0.275, p=0.04). Multiparity did not affect the renal handling of calcium and phosphorous during pregnancy and lactation, neither hydroxyproline excretion in lactation. However, hydroxyproline excretion in the third trimester of pregnancy was 58% higher in primiparas compared to multiparas, in spite of similar weight gains during pregnancy. This could be due to differences between primiparas and multiparas in the composition of tissue gained during pregnancy or to an adaptive mechanism in multiparas to protect maternal bone from excessive calcium loss during pregnancy.
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