Acute effects of an oral calcium load in pregnancy and lactation: findings on renal calcium conservation and biochemical indices of bone turnover.

An oral calcium (Ca) tolerance test was used to compare the acute calcaemic, calciuric, parathyroid and bone turnover responses in 21 women at 36 weeks of pregnancy, 27 breast-feeding women studied 22 weeks postpartum and 27 control women. In all groups the oral Ca load increased plasma Ca and urinary calcium excretion (CaE), reduced intact PTH concentration (and consequently reduced renal phosphate and cyclic AMP excretion) and reduced hydroxyproline excretion (HypE, a biochemical index of bone resorption). There were no changes in the biochemical indices of bone formation, serum osteocalcin (elevated in the lactating group) and alkaline phosphatase, in any group. The pregnant women had the same fall in HypE and a greater calcaemic response than the controls. These results suggest that there is increased intestinal Ca absorption efficiency and a normal rate of bone resorption in late pregnancy. In contrast, the lactating women had a greater fall in HypE (from a baseline twice that of controls) and a significantly lower (p less than 0.001) rise in CaE, despite a calcaemic response similar to that of controls. Therefore, in lactation there is increased bone turnover and an increased capacity to reabsorb Ca in the renal tubule, compared to controls. An oral calcium supplement may benefit breast-feeding women, by reducing lactation-related elevated rate of bone resorption and consequent loss of trabecular bone.