Human lactation: Forearm trabecular bone loss, increased bone turnover, and renal conservation of calcium and inorganic phosphate with recovery of bone mass following weaning

The calcium (Ca) metabolism of established human lactation was studied in 40 adult women (mean age 32.4 years) who had been breast-feeding for 6 months (Lac) and in 40 age-matched controls (Con) using fasting urine and blood biochemistry and forearm single-photon bone mineral densitometry (BMD). Serial studies were performed up to 6 months after weaning in Lac women and repeated once in Con women. During lactation the significant findings were (1) a selective reduction (7.1%, P < 0.03) in BMD at the ultradistal site containing 60% trabecular bone, but not at two more proximal, chiefly cortical bone sites; (2) increased bone turnover affecting bone resorption [fasting hydroxyproline excretion, Lac 2.22 + 0.12 μmol/liter GF (mean + SEM), Con 1.19 + 0.04, P < 0.001] and affecting bone formation (plasma alkaline phosphatase, Lac 81.9 + 2.5 IU/liter, Con 53.5 + 2.7, P < 0.001, and serum osteocalcin, Lac 14.0 + 0.7 μg/liter, Con 7.3 + 0.4, P < 0.001); and (3) renal conservation in the fasting state of both Ca and inorganic phosphate (Pi) with a resultant moderate increase in plasma Pi but not in plasma Ca (total or ionized). There were no differences between the groups in serum parathyroid hormone (PTH, intact and midmolecule assays), 25-hydroxy- and 1,25-dihydroxyvitamin D, nephrogenous cyclic AMP production, or plasma creatinine. In 25 of these Lac women restudied at one or more of the times 2, 4, or 6 months after weaning, the findings were (1) an early (2 months) normalization of bone resorption and renal Pi handling with continuing increased bone formation and renal Ca conservation, associated with the onset of increased intact PTH levels; and (2) a recovery, within 4-6 months in ultradistal BMD associated with normalization of bone formation but with persisting renal Ca conservation and elevated intact PTH levels. We conclude that in established adult human lactation there is increased bone turnover with an accompanying loss of trabecular bone, despite renal conservation of Ca and Pi. After weaning, the deficit in trabecular bone is made up during a period of imbalance between a normal bone resorption rate and an elevated bone formation rate. The moderately elevated PTH after weaning may play a role in the recovery of bone mass by maintaining renal Ca conservation and by an anabolic action on bone.