Calcium supplementation suppresses bone resorption in early postmenopausal women

In order to establish whether calcium supplementation suppresses bone resorption in early postmenopausal women and whether any response is related to calcium absorption status, we studied 22 healthy women (median age 52 years) all within 5 years of the menopause. Urine was collected between 9.00 p.m. and 9.00 a.m., and 9.00 a.m. and 9.00 p.m., (2 days) and a fasting blood and spot urine sample was obtained at 9 a.m. On the first day, 5 microCi of 45Ca in 250 ml water with 20 mg calcium carrier as the chloride was given at 9.00 a.m. and a further blood sample was obtained at 10.00 a.m. to measure calcium absorption. A 1 g calcium load was given at 9.00 p.m., immediately before the second 24-hour urine collection. There was a rise in plasma ionized calcium (1.18 +/- 0.010 mmol/liter versus 1. 21 +/- 0.011 mmol/liter, P < 0.01) and a fall in plasma PTH (4.2 +/- 0.34 pmol/liter versus 3.5 +/- 0.31 pmol/liter, P < 0.01) from baseline after the calcium load, and a trend for the magnitude of the change in PTH to be inversely related to calcium absorption (r = -0.33, P = 0.13). In the fasting spot urine samples, there were falls in hydroxyproline (OHPr/Cr; 14.6 +/- 0.71 versus 12.6 +/- 0.83, P < 0.001), pyridinoline (Pyr/Cr; 75 +/- 2.8 versus 70 +/- 3.5, P < 0.05), and deoxypyridinoline (Dpd/Cr; 22.7 +/- 1.2 versus 19.5 +/- 1. 1, P < 0.005) after the calcium load. The calcium load suppressed urinary Dpd/Cr between 9.00 p.m. and 9.00 a.m. (P < 0.005), but not between 9.00 a.m. and 9.00 p.m. We conclude that acute administration of a 1 g calcium load suppresses bone resorption in early postmenopausal women, probably by decreasing PTH secretion.