Elevated parathyroid hormone-related peptide associated with lactation and bone density loss

Objective.-To investigate the hypothesis that parathyroid hormone-related peptide (PTHrP) may be involved with bone loss and recovery as a means of providing adequate calcium and phosphate to infants. Design.-An 18-month prospective cohort study. Setting.-General community setting with recruitment occurring at birthing education classes. Participants.-Volunteer sample of 115 postpartum healthy women aged 20 to 40 years, and 0 or 1 parity prior to parturition with no intent to breast-feed or intent to breast-feed at least 6 months. Main Outcome Measures.-Parathyroid hormone-related peptide, prolactin, estradiol, 1,25-dihydroxyvitamin D, 25-hydroxyvitamin D, femoral bone mineral density, and bone turnover markers were measured in 115 postpartum women at 2 weeks, 2 months, 4 months, 6 months, 12 months, and 18 months postpartum. Lumbar bone mineral density was measured at 2 weeks, 6 months, 12 months, and 18 months postpartum. Results.-Elevated PTHrP values were significantly associated (P<.001) with breast-feeding status, elevated prolactin levels, and lower serum estradiol levels, conditions occurring during lactation, Furthermore, elevated PTHrP levels were negatively and significantly associated (P<.01) over time with bone mineral density change at both the spine and the femoral neck, even after accounting for prolactin levels, breast-feeding status, return of menstruation, estradiol levels, PTH levels, 1,25-dihydroxyvitamin D levels, dietary calcium intake, physical activity, and body size. Conclusion.-These data clearly support the hypothesis that PTHrP is an alternative mechanism associated with bone loss and recovery during and subsequent to lactation.