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

Abstract

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 less than 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 mumol/liter GF (mean +/- SEM), Con 1.19 +/- 0.04, P less than 0.001] and affecting bone formation (plasma alkaline phosphatase, Lac 81.9 +/- 2.5 IU/liter, Con 53.5 +/- 2.7, P less than 0.001, and serum osteocalcin, Lac 14.0 +/- 0.7 microgram/liter, Con 7.3 +/- 0.4, P less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium economy in human pregnancy and lactation

“…Longitudinal studies in both BF and NBF women have demonstrated elevations in markers of both bone formation and resorption in the first weeks postpartum compared with measurements made in the same individual before pregnancy (39,67,104,109) , in late pregnancy (112) , 1 year after delivery (113) , postweaning (71,87) or 1 year after resumption of menses (114) . In cross-sectional studies higher concentrations of bone turnover markers than in NPNL women have been reported during the first weeks postpartum (93,104,115,116) . The patterns of change in the various bone turnover markers postpartum are influenced by lactation; at the same time points BF women have higher concentrations of all markers than NBF mothers (72,91,95,116,117) .…”

“…However, BF mothers nursing twins have elevated plasma concentrations of both PTH and 1,25(OH) 2 D compared with those nursing single infants (193) . Elevated PTH and 1,25(OH) 2 D have been reported in BF women relative to early lactation and to NPNL women during the later stages of lactation and after breast-feeding stops (83,95,109,113,115,144,190,191) , although the pattern is not consistent. The increases in PTH and 1,25(OH) 2 D may play a role in the replenishment of bone mineral postlactation through their effects on intestinal absorption and renal retention of Ca.…”

“…No differences have been found between women who have recently ceased breast-feeding and either non-lactating control women or those who have not recently been pregnant (95,115,147) . It is considered probable that a key regulator of Ca and bone metabolism during the first weeks of lactation is PTHrP.…”

“…The decrease in urinary Ca output partly reflects the reduction in glomerular filtration rate after parturition (150) . Some studies, but not all (151) , have shown that urinary Ca output of BF mothers during the first 3 -6 months of lactation is lower than that of NBF mothers at the same stage postpartum or of NPNL women (91,115,152) .…”

“…Post-lactation has been associated with decreases in urinary Ca excretion (115,144) or no difference (95) and with increases in intestinal Ca absorption efficiency (148) . Differences in the timing of the return of menses may complicate these findings (3) ; these effects are not apparent several months after breast-feeding has ceased (154) .…”