Intestinal absorption of Ca(H2PO4)2 and Ca citrate compared by two methods.

The purpose of this study was to compare the absorption of calcium from two soluble Ca salts and determine if the oral Ca load test can be used as a simple measure of relative Ca bioavailability. Eight normal subjects ingested 750 mg Ca as solution of Ca(H2PO4)2 or Ca citrate (two occasions) tagged with 47Ca tracer. Absorption of Ca from the two salts was estimated from both the increase in forearm radioactivity and rise in urinary Ca (oral Ca load test). Both measures indicated that Ca was absorbed at least twice as well from Ca citrate as from Ca(H2PO4)2. However, the increment in urinary Ca 2-4 h after the oral load correlated with fractional absorption when only Ca citrate (r = 0.803, p less than 0.02) was ingested. For group comparisons the oral Ca load test is a rough index of Ca availability but cannot substitute for more direct measures of absorption in an individual.