BIOCHEMICAL MARKERS OF BONE TURNOVER IN LACTATING AND NONLACTATING POSTPARTUM WOMEN

We measured two bone-formation markers, osteocalcin and bone-specific alkaline phosphatase, and one bone-resorption marker, N-telopeptide, i n a longitudinal study in order to describe levels of these markers in lactating and nonlactating women after parturition. This 18-month pos tpartum period included an initial 6 months in which a 5% short-term h one loss occurred at both spine and femoral neck among breast feeding women. The second part of the 18-month period was characterized by bon e recovery among women who had lost bone. These bone-change characteri stics provided an opportunity to evaluate the performance of biochemic al markers during both bone loss and recovery and to identify environm ental exposures during lactation associated with bone turnover. The el igible population comprised 115 women whose bone-turnover markers were measured at 2 weeks (baseline) and at 2, 4, 6, 12, and 18 months afte r parturition. Participants reported reproductive characteristics, die t, physical activity, use of medications, and infant-feeding practices at each contact. Women were grouped according to lactation duration: 0-1 months, 2-5 months, and 6 months or more. Women who breast-fed for at least 6 months had significantly different levels of all three bon e-turnover markers compared with the levels in bottle-feeding controls , which were indicative of substantially increased bone turnover. Fact ors that predicted the difference in biochemical markers from baseline to B-month values by regression analysis were lactation of 2-6 months duration and lactation for 6 months or more. Dietary calcium intake, physical activity level, and body size did not explain the differences in the change from the baseline level to the 6-month level, a period of time that corresponded with bone loss lp the lactating women. Facto rs that predicted the differences in bone-turnover markers between 6 a nd 18 months (the time of bone-mass recovery) were lactation status an d number of months to resumption of menses. By the 18-month observatio n, there was no difference in the mean values for the measured bone-tu rnover markers among the three lactation groups. This suggests that me nstrual activity, rather than diet or physical activity, is the primar y factor in bone-mass recovery after the bone loss of lactation.