Lactation causes significant hypoestrogenemia, which has negative effects on calcium and phosphate metabolism. Studies suggest that the decrease in bone mineral density averages 4 to 6 percent during the first six months of lactation. Polatti and colleagues evaluated variations in bone mineral density, with and without calcium supplementation, during lactation and during the 12 months after cessation of lactation.

The lactation cohort included women who intended to breast feed for at least six months following vaginal term delivery. There were three groups in the study: 139 women who breast fed and received 1 g of calcium supplementation daily; 135 women who breast fed but did not receive calcium supplementation; and 153 women who did not breast feed and did not receive calcium supplementation. The three groups were comparable in all relevant variables.

Bone mineral density of the radius and spine was measured within 10 days of delivery (baseline) and again at three, six, 12 and 18 months postpartum. Bone mineral density was also measured one month after cessation of lactation.

Bone mineral density progressively declined during lactation in the first six months postpartum but recovered thereafter to reach higher-than-baseline levels by 18 months. In the lactating women who received calcium supplementation, bone mineral density in the spine showed a decrease of 3.4 ± 0.4 percent at three months and 4.0 ± 0.3 percent at six months. Comparable figures in the lactating women who did not receive calcium supplementation were decreases of 3.7 ± 0.3 percent at three months and 4.4 ± 0.4 percent at six months. By 18 months, bone mineral density of the spine increased 1.8 ± 0.4 percent in the lactating women who did not receive calcium supplementation. In those who received calcium supplementation, bone mineral density at 18 months increased 2.0 ± 0.4 percent.

In nonlactating women, bone mineral density of the spine showed an increase of 0.8 ± 0.4 percent at three months and 1.4 ± 0.3 percent at six months. At 18 months, the increase in bone mineral density of the spine was 1.9 ± 0.4 percent in this group. The increase in the nonlactating women might be explained by the resumption of ovarian function.

Measurements of bone mineral density 12 months after cessation of lactation revealed no significant differences among the three groups. The effect of calcium supplementation on bone loss was transient.

The authors conclude that lactation for six months is associated with a progressive decrease in bone mineral density, but the decrease is followed by recovery of bone mass by 18 months postpartum, regardless of calcium supplementation.