Kinetics of calcium metabolism in lactation

The impact of mammalian reproduction on cancellous bone architecture

Pregnancy and lactation make demands on maternal calcium homeostasis which may affect bone strength. Recently, changes in cancellous architecture have been described in iliac crest bone biopsies from normal pregnant women but the rarity of such human material means an animal model is essential. The microanatomy of cancellous bone was compared in uniparous and multiparous rats using undecalcified histological sections of lumbar and caudal vertebrae and also proximal femora. An automated trabecular analysis system (TAS) measured a comprehensive range of structural variables including the trabecular number, connectivity and width. In the first pregnancy cycle an early stimulation of bone formation (which quadrupled at some sites) was indicated by an increase in the skeletal uptake and spacing of double calcein labels and the immediate generation of thicker more numerous and interconnected trabeculae. A 40% increase in cancellous bone volume was observed in the lumbar spine in comparison with age-matched virgin controls. In contrast, a rapid succession of 3 pregnancy cycles (including lactation) culminated in cancellous atrophy of 15% at the same site, with a loss in trabecular number ranging from 20% (caudal vertebra) to 30% (lumbar vertebrae). In comparison, the proximal femur lost 40% of its struts but, nevertheless, uniquely sustained its cancellous bone volume. When lactation was excluded the number of struts lost was halved although trabecular thinning then took place which was sufficient to maintain the previous 15% deficit in bone volume. It was concluded that a single pregnancy strengthens the cancellous component of the maternal skeleton while a quick succession of pregnancies weakens it. Lactation influences the pattern of bone loss but not its amount.

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

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.

Role of vitamin D in maternal skeletal changes during pregnancy and lactation: a histomorphometric study

The effect of vitamin D on bone changes during the reproductive cycle in female rats has been investigated. One group of female rats was maintained on a vitamin D-deficient diet and another group of a vitamin D-replete diet from weaning. Both groups were mated with normal males and changes in their bones were determined histomorphometrically during pregnancy, lactation, and after weaning. All vitamin D-deficient rats had bone changes typical of rickets. Pregnancy caused significant reductions in mineralized tissue of trabecular and cortical bone in the vitamin D-deficient rats. Lactation caused further significant reductions in mineralized tissues of cortical and trabecular bone in both the vitamin D-deficient and vitamin D-replete animals, with the greatest changes seen at weaning. Some restoration of mineralized tissues occurred following weaning. There was an increase in tetracycline-labeled bone surface in the vitamin D-replete animals during lactation, likely due to an increase in bone formation rates. In the vitamin D-deficient animals during lactation, there was a decrease in tetracycline-labeled bone surface, likely due to severely depressed bone mineralization. These results indicate that the mobilization of calcium from bone to maintain pregnancy and lactation occurs by a mechanism independent of vitamin D.