Bone mineral density changes during lactation: maternal, dietary, and biochemical correlates.

The objectives of this study were to characterize the effects of lactation and weaning on maternal bone mineral density (BMD) and on biochemical markers of bone turnover, and to determine the effects of dietary intake, milk output, and other maternal factors on changes in BMD. Twenty-six fully lactating and eight nonlactating women were followed longitudinally through 7 mo postpartum; the lactating women were followed through postweaning. Maternal dietary and supplement intake data, infant milk intake measurements, blood and urine samples, and midradius and L2-L4 vertebral BMD measurements were obtained 0.5, 3, 5, and 7 mo postpartum. Biochemical analyses included measurements of calciotropic hormones, 24-h urinary excretion of calcium, markers of bone formation and resorption, estradiol, and prolactin. Estimated maternal demands for calcium excretion in milk were met by a combination of high calcium intake (from diet and supplements, 1500 +/- 460 mg/d at 0.5 mo for lactating women) and a decline of approximately 4% in vertebral BMD between 0.5 and 3 mo postpartum. Postweaning BMD (n = 15) at this site approximated initial values. Two factors were positively associated with vertebral BMD, estradiol (P < 0.001) and calcium intake (P = 0.03), whereas two factors were negatively associated, parity (P = 0.03) and protein intake (P = 0.01). In these well-nourished women, the results suggest that the extent of bone loss associated with lactation and its recovery postweaning are negatively influenced by parity. The results also suggest that the bone loss may be attenuated by a generous dietary ratio of calcium to protein.