Calcium supplementation is recommended as a prophylaxis against bone loss. This study was performed to determine the dose dependency of calcium absorption in an attempt to derive an optimum dose schedule. Using the well-described oral calcium load technique, we measured the calcium absorption from three different calcium doses (0.5, 1.0, and 2.0 g) of both calcium carbonate and calcium citrate administered to 21 normal subjects (4 men and 17 women, 22-60 years). Nine subjects underwent two additional loads with 0.2 g of elemental calcium as calcium carbonate and as calcium citrate. The intestinal calcium absorption from calcium carbonate and calcium citrate was estimated from the rise in urinary calcium following oral ingestion of the respective calcium salt. The increment in urinary calcium post-load, reflective of intestinal calcium absorption, rose rapidly from 0 to 0.5 g calcium loads with only slight subsequent increases from the 0.5 g to 2.0 g calcium doses. Thus, results indicate that 0.5 g of calcium is the optimum dose of either calcium salt. Moreover, the increment in urinary calcium post-load was higher from calcium citrate than from calcium carbonate at all four dosage levels. The increment in urinary calcium (during the second 2 hr) following calcium citrate load (0.5 g calcium) was 0.104 +/- 0.096 mg/dl glomerular filtrate (GF), which was higher than that of 0.091 +/- 0.068 mg/dl GF obtained from 2.0 g calcium as calcium carbonate. These results confirm the superior calcium bioavailability from calcium citrate as compared with calcium carbonate.(ABSTRACT TRUNCATED AT 250 WORDS)