INTESTINAL CALCIUM-ABSORPTION OF WOMEN DURING LACTATION AND AFTER WEANING

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.