To clarify the role of the intestine, kidney, and bone in
maintaining calcium homeostasis during pregnancy and lactation and
after the resumption of menses, a longitudinal comparison was
undertaken of 14 well-nourished women consuming 1200 mg Ca/d.
Measurements were made before conception (prepregnancy), once during
each trimester of pregnancy (T1, T2, and T3), early in lactation at 2
mo postpartum (EL), and 5 mo after resumption of menses. Intestinal
calcium absorption was determined from the enrichment of the first
24-h urine sample collected after administration of stable calcium
isotopes. Bone mineral of the total body and lumbar spine was measured
by dual-energy X-ray absorptiometry and quantitative computerized
tomography, respectively. Twenty-four--hour urine and fasting serum
samples were analyzed for calcium, calcitropic hormones, and
biochemical markers of bone turnover. Despite an increase in calcium
intake during pregnancy, true percentage absorption of calcium
increased from 32.9 ± 9.1% at prepregnancy to 49.9 ± 10.2%
at T2 and 53.8 ± 11.3% at T3 (P < 0.001). Urinary
calcium increased from 4.32 ± 2.20 mmol/d at prepregnancy to
6.21 ± 3.72 mmol/d at T3 (P < 0.001), but only minor
changes in maternal bone mineral were detected. At EL, dietary
calcium and calcium absorption were not significantly different from
that at prepregnancy, but urinary calcium decreased to 1.87
± 1.22 mmol/d (P < 0.001) and trabecular bone mineral
density of the spine decreased to 147.7 ± 21.2 mg/cm3
from 162.9 ± 25.0 mg/cm3 at prepregnancy
(P < 0.001). Calcium absorption postmenses increased
nonsignificantly to 36.0 ± 8.1% whereas urinary calcium decreased to
2.72 ± 1.52 mmol/d (P < 0.001). We concluded that fetal
calcium demand was met by increased maternal intestinal absorption;
early breast-milk calcium was provided by maternal renal c
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Contact: Lori Barber
lbarber@ascn.faseb.org
301 530-7038
American Journal of Clinical Nutrition
2-Apr-1998