Bone mineral density and calcium metabolism of Hong Kong Chinese postpartum women - a 1-y longitudinal study

Objective: This paper reports longitudinal changes in bone mineral density (BMD), calcium homeostasis and dietary calcium intake in a group of Hong Kong breastfeeding women during the first year postpartum. Design and subjects: Nine mothers who breastfed exclusively or almost exclusively for at least 3 months and 14 formula feeding mothers aged 20 - 40 y were interviewed after delivery, 2 and 6 weeks, 3, 6 and 12 months postpartum. BMD at L2 - L4 lumbar spine (LS), trochanter (Tro) and femoral neck (FN), serum intact parathyroid hormone (iPTH), serum bone-specific alkaline phosphatase (b-ALP), urinary deoxypyridinoline ( Dpd), serum and urinary calcium (Ca) and phosphorus ( P) and dietary intake of macronutrients were assessed. Results: Compared to the formula feeding group, BMD assessed at LS, Tro and FN decreased significantly in the breastfeeding group over the first 6 months, with rebound to approximate baseline values at 12 months for the latter two sites. Serum iPTH increased in both groups, whereas serum b-ALP was consistently higher in the breastfeeders. Urinary Ca and P excretion decreased early postpartum in both groups, but the breastfeeders had higher excretion at 3 and 6 months. Breastfeeding mothers consumed significantly more Ca than the formula feeding mothers in the early postpartum. Conclusions: Increased calcium requirement during early lactation is affected through mobilisation of bone and renal calcium conservation. Bone mineral loss during lactation is temporary. Further studies are warranted to investigate the effects of diet and other hormonal factors on the calcium homeostasis during lactation.