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• W2189000770 abstract "Women are often advised to consume additional calcium during pregnancy and lactation. The assumption in the 10th edition of the Recommended Dietary Allowancesis that calcium intake should be increased from 20 to 30 mmol (800 to 1200 mg)/d during both pregnancy and lactation (1). The rationale is that in pregnancy this amount of calcium is needed for the mineralization of fetal bone, into which <750 mmol Ca is deposited at a rate of 5-6.25 mmol (200-250 mg)/d in the last trimester. However, early work with 48 Ca in adolescent girls showed that at least some of the fetal demand for calcium was likely to be met from increased maternal absorption that started early in pregnancy (2). During lactation, the recommended intake of calcium is increased by the same amount as in pregnancy to replace calcium secreted into breast milk, the upper limit of which is taken to be 7.5 mmol (300 mg)/d. In 1991 the equivalent recommendation in the United Kingdom was that additional dietary calcium was not required during pregnancy because mobilization of maternal bone supplied adequate calcium during the first 3 mo (3). During lactation it was thought that although the spontaneous increase in maternal food intake would supply adequate calcium to replace that secreted into breast milk, there was insufficient evidence that maternal calcium metabolism is adapted during this period. Furthermore, data were available showing that bone density is diminished during the first 3 mo of lactation, so that intake should increase by 14.3 mmol (550 mg)/d. In recent years more has been learned about the remarkable changes in calcium homeostasis that occur during pregnancy and lactation. Calcium absorption can now be measured more easily with stable calcium isotopes. Another development has been the availability of accurate methods of measuring bone mineral density and bone mineral content, including dual-energy X-ray absorptiometry (DXA) and quantitative computerized tomography. These methods permit the detection of small changes in the calcium content of bone. Although there have been numerous studies of calcium status in pregnant and lactating women, for logistical reasons pregnant subjects are usually recruited well after conception and therefore baseline values are not available. Lactation measures are usually limited to earlier periods to avoid the confounding effect of early mixed feeding and weaning. In both situations it is impossible to evaluate whether or when maternal calcium status returns to baseline values. The two articles on maternal calcium metabolism in this issue of the Journal are very helpful in terms of elucidating the source of calcium for the fetus and breast milk (4, 5). The study by Ritchie et al (4) is unique in that 14 women were studied from before conception until 5 mo after menstruation resumed—until 13 mo postpartum on average. Throughout this period, calcium absorption was followed by using stable isotopes; total-body and lumbar spine mineral content were measured by DXA and quantitative computerized tomography, respectively; urinary and breast milk calcium were assessed; and calciotropic hormones and biochemical markers of turnover were analyzed to explore possible mechanisms involved in the calcium changes. The investigators showed that the calcium required for fetal bone mineralization can be obtained by an increased efficiency of maternal calcium absorption in pregnancy, with no detectable mobilization of maternal bone for this purpose. Dietary calcium intake also increased. Even though urinary calcium was <50% higher in the third trimester, the <15 mmol (600 mg)/d absorbed at this time should still be adequate to supply the calcium needs of the fetus. At its earliest investigation in lactation (2 mo postpartum), calcium absorption had returned to prepregnancy values and urinary calcium losses were less than half those at baseline. The source of the breast-milk calcium was predominantly maternal spinal trabecular bone, with the reduction in urinary calcium contributing to calcium retention. Between 1 and 2 wk postpartum and 5 mo after the resumption of menstruation, bone mineral density was significantly lower in the total body and arms. The density of trabecular bone of the spine fell between 2 wk and 2 mo of lactation but returned to prepregnancy values by 5 mo after menses resumed, although total-body bone mineral density had not. Although many dramatic changes in calcitropic hormones were observed during both pregnancy and lactation, none could definitively explain the shifts in calcium flux. In the study by Ritchie et al (4), a combination of several factors made the measures taken during late lactation somewhat difficult to interpret. Of the 14 subjects, 5 took oral contraceptives postpartum and 4 were only partially breast-feeding by 2 mo. These problems are to be expected in subjects who are recruited before conception. In the study by Laskey et al (5) in this issue, bone changes during the first 3 mo of lactation were studied by DXA in 47 breast-feeding British women (5). These authors also found a substantial decrease in whole-body bone mineral content and located this loss in the spine and femoral neck. This loss did" @default.
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• W2189000770 title "Women’s dietary calcium requirements are not increased by pregnancy or lactation" @default.
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• W2189000770 doi "https://doi.org/10.1093/ajcn/67.4.591" @default.
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