Effect of estrogen treatment and vitamin D status on differing bioavailabilities of calcium carbonate and calcium citrate

The authors hypothesized that estrogen treatment or vitamin D status may affect the bioavailability of two common calcium supplements differently. Using data derived from a recent trial in 25 postmenopausal women, the authors found that ΔAUC of serum calcium after subtraction of placebo was significantly higher after calcium citrate (median, 0.85; 25th to 75th percentile, 0.70 to 3.15) than after calcium carbonate (0.25; -0.58 to 1.00) in non-estrogen-treated patients. There was no difference in the bioavailability of calcium between the two calcium formulations in estrogen-treated patients. Bioavailability was also significantly higher with the citrate salt for the subgroups with lower serum 25-hydroxyvitamin D and higher serum 1,25-dihydroxyvitamin D concentrations. In summary, bioavailability of calcium from the calcium carbonate product was more dependent on estrogen treatment and vitamin D status than that of calcium citrate. This may explain the variable results of reported calcium supplementation studies.