BONE CHANGES AFTER 3 MO OF LACTATION - INFLUENCE OF CALCIUM INTAKE, BREAST-MILK OUTPUT, AND VITAMIN-D-RECEPTOR GENOTYPE

Factors influencing the change in bone mineral after 3 mo of lactation were investigated in 47 breast-feeding mothers, 11 formula-feeding mo thers, and 22 nonpregnant, nonlactating control subjects. At 6-8 wk po stpartum, the breastfeeding group had a mean (+/-SD) calcium intake of 34.8 +/- 13.2 mmol/d and breast-milk volume, calcium concentration, a nd calcium output of 0.865 +/- 0.230 L/d, 7.41 +/- 1.25 mmol/L, and 6. 41 +/- 2.00 mmol/d, respectively. There was no relation between calciu m intake and any breast-milk variable. Dual-energy X-ray absorptiometr y of the whole body, spine, hip, and forearm was performed at 0.5 and 3 mo. There were significant decreases in bone mineral content at the spine (3.96%; 95% CI: 4.86%, 3.06%), femoral neck (2.39%; 95% CI: 3.61 %, 1.17%), total hip (1.51%; 95% CI: 2.45%, 0.60%), and whole body (0. 86%; 95% CI: 1.29%, 0.43%) in breast-feeding mothers but not in formul a-feeding mothers or nonpregnant, nonlactating women. These changes we re not related to calcium intake, breast-milk calcium concentration, v itamin D-receptor genotype, postpartum weight change, or use of the pr ogesterone-only contraceptive pill. After adjustment for bone area, br east-milk volume and height were identified as significant predictors at the spine, such that greater decreases were associated with taller mothers (P = 0.007) and those with greater breast-milk volume (P = 0.0 01). This finding suggests that the marked bone mineral changes observ ed in breast-feeding mothers represented a physiologic response to lac tation that was independent of dietary calcium supply.