Rapid suppression of bone resorption and parathyroid hormone secretion by acute oral administration of calcium in healthy adult men.

Acute effects of oral calcium supplementation have been studied mainly in pre- and post-menopausal women, whereas very few data concerning men are available. We investigated the effects of 1.2 g of oral calcium administered for 4 days in 18 healthy young men. The day before the first calcium dosing (day -1) and the day of the last calcium dosing (day 4) total and ionized serum calcium and intact PTH were measured at multiple time-points up to 24 h; calcium, C-terminal telopeptide (CTX), pyridinoline (PYD) and deoxypyridinoline (DPD) were measured in urine collected every six h. On day 4, total and ionized serum calcium increased during the 6 h following oral calcium: the maximum increase over baseline was 5.04 and 7.4% respectively, occurring after 2.9 +/- 1.9 h (mean +/- SD) and 2.6 +/- 0.9 h. The AUC was significantly higher on day 4 than on day -1 for both total serum calcium (+4.6%, p<0.02) and ionized serum calcium (+9.2%, p<0.0001). Twenty-four h urinary excretion of total calcium increased significantly on day 4 (+15.1%, p<0.02), mainly as a consequence of increased excretion during the first 6 h. Serum PTH was suppressed by oral calcium, with a significant reduction of AUC on day 4 (-15.1%, p<0.05). Serum concentrations of intact PTH dropped from 26.4 +/- 9.8 pg/ml at time zero to a minimum mean value of 14.9 +/- 7.6 pg/ml at time +2 h. Bone resorption markers significantly decreased on day 4 (CTX -33.2%, p<0.001; PYD -28.5%, p<0.05; DPD -35.8%, p<0.02). Most of the effect was seen in the first 6 h after oral calcium load. These data support the concept that acute suppression of PTH and bone resorption induced by calcium administration is not gender specific and provide the rationale to further assess also in men the long-term effects of oral calcium in the prevention and treatment of osteoporosis.