One-month of calcium supplementation does not affect iron bioavailability: a randomized controlled trial

Calcium (Ca) and iron (Fe) are essential minerals for normal growth and development. Although previous studies have shown that Ca inhibits acute Fe absorption, there is no evidence of the possible long- or medium-term effects of Ca supplementation on Fe bioavailability. The aim of this study was to determine the effect of 34 d of Ca supplementation on heme Fe and non-heme Fe bioavailability in non-pregnant women of ages 33 to 47 y.

This was a prospective, randomized, double-blind, placebo-controlled trial. Twenty-six healthy women (40 ± 5 y) were randomly assigned to receive either 600 mg of elemental Ca/d as CaCO3 (Ca group, n = 13) or a placebo (P group, n = 13) for 34 d. Heme Fe and non-heme Fe bioavailability were determined before and after treatment using (55)Fe and (59)Fe radioisotopes. A two-factor, repeated-measures analysis of variance was used to assess differences by treatment and timing.

The geometric mean (range ± 1 SD) of heme Fe bioavailability before and after treatment was 16.5% (8.3-32.8) and 26% (15.5-43.6) for the Ca group and 21.8% (13.0-36.6) and 25.1% (16.5-38.3) for the P group. Non-heme Fe bioavailability before and after treatment was 39.5% (19.9-78.7) and 34.1% (19.1-60.6) for the Ca group, and 44.6% (24.9-79.7) and 39.3% (24.3-63.4) for the P group. There were no differences in either heme Fe or non-heme Fe bioavailability either at baseline or after treatment.

The administration of calcium supplements for 34 d does not affect iron bioavailability. This trial is registered with Controlled-trials.gov, number ISRCTN 89888123.