Calcium fractional absorption and metabolism assessed using stable isotopes differ between postpartum and never pregnant women

Determining the fractional absorption (FA) of calcium using the incorporati on into urine of stable isotopes given intravenously (IV) and orally has be come a routine procedure. We investigated the FA of calcium in two groups o f (2-3 mo) postpartum women lactating (LACT) (n = 6) and nonlactating (PPNL ) (n = 6), and in never pregnant (NP) women (n = 7). The women consumed a c ontrolled diet containing 30-33 mmol/d calcium (Ca) for 21 d. On d 7 of the controlled diet, the women received 0.05 mmol Of Ca-42 IV and 0.25 mmol Ca -44 orally in milk. Urine samples (24-h) were collected for the next 14 d a nd morning blood samples were collected from fasting subjects before dosing and at 24 and 48 h after receiving the isotopes. Milk samples from the LAC T women were collected from each feeding beginning 24 h before to 72 h afte r dosing. There were no significant differences in the FA of calcium as mea sured by stable isotope incorporation into urine (23.8 +/- 2.9%), serum (24 .0 +/- 3.4%) or milk (23.6 +/- 3.6%) of LACT women. The fractional calcium absorption measured in urine of the postpartum women (LACT and PPNL, 23.8 /- 2.9% and 25.0 +/- 3.3%, respectively) did not differ but was greater (P < 0.028) than that of the NP women (17.3 +/- 1.3%). The postpartum LACT and PPNL women had a reduced urinary excretion of calcium (P < 0.01) compared with the NP women. There was a significantly greater incorporation (P < 0.0 01) by LACT women of the oral isotope dose into milk than into urine. Calci um FA can be determined from incorporation of stable isotopes into breast m ilk and serum as well as urine.