The mechanism by which calcium supplementation slows bone loss is probably through a reduction in serum parathyroid hormone (PTH). With age there is an increase in serum PTH and bone turnover due to the combined effects of reduced calcium intake and absorption and to vitamin D insufficiency. Low doses of calcium of 250 mg result in acute suppression of serum PTH and low habitual calcium intakes are associated with higher serum PTH levels than higher calcium intakes. The beneficial effects on bone of calcium supplements may therefore be mediated via an antiresorptive effect. In general the most consistent effects of calcium are observed in the appendicular skeleton, while the positive effect on trabecular bone appear to be transient. In a recent study, where citrate and calcium infusions were used to characterize the impact of age and gender on PTH secretion in normal subjects, ageing was associated with an increase in PTH secretory
response to changes in serum calcium, in both women and men