Hj. Kalkwarf et al., INTESTINAL CALCIUM-ABSORPTION OF WOMEN DURING LACTATION AND AFTER WEANING, The American journal of clinical nutrition, 63(4), 1996, pp. 526-531
We investigated whether intestinal calcium absorption and serum 1,25-d
ihydroxycholecalciferol (calcitriol) concentrations are higher in wome
n during lactation and after weaning to compensate for calcium lost in
breast milk. Measurements were obtained at 4.6 mo postpartum in 24 la
ctating women and 24 nonlactating women, at 9.6 mo postpartum in 24 la
ctating women (2.6 mo after complete weaning) and 24 nonlactating wome
n. One-half of the women in each group were randomly assigned to recei
ve 1 g supplemental Ca/d as calcium carbonate. Fractional calcium abso
rption was measured by using stable isotopic tracers Ca-42 and Ca-44.
Fractional absorption was 0.32 +/- 0.02 (+/- SEM) in both lactating an
d nonlactating women, but was higher in lactating women after weaning
(0.37 +/- 0.02) compared with nonlactating postpartum control subjects
(0.31 +/- 0.02). These effects were independent of calcium intake. Ch
anges in serum calcitriol paralleled changes in fractional absorption.
There were no differences in calcitriol concentrations between lactat
ing and nonlactating women, but calcitriol was greater in women after
weaning compared with postpartum control subjects. Lactating women who
had resumed menses had higher fractional absorption and serum calcitr
iol than did lactating women who had not. Serum calcium and phosphorus
concentrations were greater in lactating compared with nonlactating w
omen; there were no differences between groups after weaning. We concl
ude that lactation stimulates increases in fractional calcium absorpti
on and serum calcitriol, but the responses are only apparent after wea
ning or the resumption of menses.