The absorption of tricalcium phosphate and its acute metabolic effects

The use of calcium (Ca) supplements by postmenopausal women is growing rapidly. A commercial preparation of tricalcium phosphate (TCP) is available in the USA. Depending on the relative absorption of Ca versus phosphate, a rise in serum phosphorus (P) could stimulate parathyroid hormone (iPTH) secretion. We therefore compared Ca absorption and the metabolic responses following TCP to that of Ca carbonate (CC) on separate occasions in each of 10 women, aged 22-40 years. The subjects were fasted overnight for 12 hours while good hydration was maintained. Following a 2-hour baseline-urine collection, 1200 mg calcium (as CC or TCP) was ingested and two 2-hour postload urine collections were made. Blood was drawn at 1, 2, and 4 hours after the Ca load. Serum (S) and urine (U) Ca, P, and creatinine, and U cyclic AMP (cAMP) were determined. iPTH levels following TCP were also measured. Ca absorption was determined by the postload rise in Uca above baseline. Uca excretion increased significantly and was accompanied by significant rises in Sca after both preparations. Following TCP, S and U phosphorus increased. Urinary cAMP did not change after either preparation, and iPTH levels fell after oral TCP. We conclude that Ca taken as TCP is absorbed adequately and, thus, despite a rise in the S phosphorus level does not stimulate parathyroid activity.