Lactation delays postpartum bone mineral accretion and temporarily alters its regional distribution in women.

The objective of this work was to compare long-term changes in bone mineral in lactating (L) and nonlactating (NL) women for 2 y postpartum. The 40 L women (mean duration of breastfeeding 345 +/- 177 d) and 36 NL women were enrolled during late pregnancy. Subjects were healthy and nonsmoking with a mean age of 28.8 +/- 4.1 y. Bone mineral content (BMC) was measured at 0.5, 3, 6, 12, 18 and 24 mo by dual-energy X-ray absorptiometry set for total body scan with regional analysis. BMC adjusted for bone area, weight and height (adj-BMC) decreased in L women at the lumbar spine (-3.1%, P < 0. 001) and pelvis (-0.9%, P = 0.03) by 3 mo, and at the total body (-0. 9%, P = 0.05) by 6 mo. Losses were recovered following onset of menses. Adj-BMC at the lumbar spine, pelvis, thoracic spine and total body increased over baseline by 24 mo in L women. In NL women, adj-BMC increased over baseline within 3 mo and continued to increase thereafter. Net total-body gains were greater in the 27 NL women who completed the final measurement than in their 26 L counterparts (+2.3% vs. +0.6%, P = 0.001). Net regional gains differed at the head, legs, and ribs, but not at the lumber spine, pelvis or thoracic spine. Duration of breastfeeding, parity, onset of menses and maternal age affected bone changes in L women. These results indicate that lactation delays bone mineral accretion and temporarily alters its regional distribution in postpartum women.