Bone changes due to pregnancy and lactation: influence of vitamin D status

Abstract

The effects of pregnancy and lactation on endosteal bone formation and resorption were evaluated in vitamin D-depleted (-D) and vitamin D-repleted (+D) rats. Pregnancy induced a marked stimulation of osteoclastic bone resorption and of static and dynamic parameters of bone formation and mineralization. Bone resorption increased independently of vitamin D status and did not correlate with plasma 1,25-dihydroxyvitamin D3 [1,25(OH)2D] levels, but it was associated with increased plasma immunoreactive parathyroid hormone (iPTH) concentrations. Stimulation of the endosteal bone formation rate was mainly impaired in D-depleted rats, resulting in trabecular bone loss, which, in -D mother rats, was associated with decreased bone ash and total bone calcium. Lactation further stimulated bone resorption and reduced the trabecular bone volume; ash weight and bone calcium content were also decreased independently of the vitamin D status and changes in plasma iPTH levels. In presence of vitamin D, the bone formation rate increased fourfold during lactation but was unchanged in -D lactating rats. During lactation, vitamin D-depleted rats lost twofold more calcified bone than +D rats because of impaired mineralization. Thus, the present study shows that both the endosteal bone resorption and formation are stimulated by pregnancy and lactation and that vitamin D is required for normal bone mineralization during the reproductive period.

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

“…Consistent with this, an increase in femoral ash weight and calcium content has been found in rats and sheep at mid-pregnancy and term (69,629), which correlates with an increasing calcium content of the diet (818), while a progressive 10 -15% increase in total body mineral content during pregnancy has been observed by DXA in normal Black Swiss mice (296,478,827,994). However, not all studies are consistent with this since no change in femoral or tibial ash weight or calcium content was observed in several studies of rats (362,598,628,972), decreases in lumbar spine BMD by DXA and histomorphometry have been observed at mid-pregnancy and term in rats (408), and BMD of the whole body and lumbar spine decreases by DXA in C57BL/6J mice (477,580). Some of these differences are likely due to differing calcium contents of the diet, with dietary calcium restriction consistently causing reduced skeletal mineral content by the end of pregnancy in rats (64,271,740) and goats (55).…”

Calcium Metabolism of Pregnant Rats Fed a Vitamin D-Depleted Diet

“…However, in pregnant rats, intestinal Ca absorption doubles despite the absence of VD [1,6]. Pregnancy induces VD-independent bone resorption and VD-dependent bone mineralization [11,13]. These previous observations seem to indicate that VD is not always necessary to sustain maternal and fetal Ca homeostasis during pregnancy.…”

“…Sci. 69(4): 441-443, 2007 Pregnancy induces dramatic changes in the homeostatic mechanisms for calcium (Ca) and other minerals; intestinal transport Pregnancy induces VD-independent bone resorption and VD-dependent bone mineralization [11,13]. These previous observations seem to indicate that VD is not always necessary to sustain maternal and fetal Ca homeostasis during pregnancy.…”

“…Pregnancy induces dramatic changes in the homeostatic mechanisms for calcium (Ca) and other minerals; intestinal transport [1] and bone resorption [11] increase to meet fetal requirements. The alterations in Ca metabolism during pregnancy are accompanied by elevated circulating levels of 1,25-dihydroxyvitamin D [1,25(OH) 2 D], a more active physiological form of vitamin D (VD) metabolites [5].…”

“…Therefore, it seems likely that the elevated levels of plasma PTH reduced urinary Ca excretion due to stimulation of renal Ca resorption. Bone metabolism was considered to reflect the increase in PTH-induced bone resorption and the decrease in 1,25(OH) 2 D-dependent bone formation [11,13].…”