Absorption of calcium as the carbonate and citrate salts, with some observations on method

Calcium supplement use has increased and there is confusion about the relat ive absorbability of various sources. Absorbability of calcium from the car bonate and citrate salts was compared at 300 mg and 1000 mg calcium loads, ingested as part of a light breakfast meal. Absorption was measured at the high load both by tracer appearance in serum and by the absorptive incremen t in urinary calcium, and at the low load by the tracer method only. Subjec ts were 37 healthy adult men and women, studied as outpatients, and each te sted on both salts at the same load. Mean tracer absorption (+/- SD) for bo th salts combined was 36.0% at the 300 mg load and 28.4% at the 1000 mg loa d. In both experiments the observed mean difference in absorption between s alts was very small. By the tracer method the within-subject difference (ca rbonate less citrate) was +3.3% +/- 1.2% of the ingested dose (mean +/- SEM I P<0.05) at the high load, and at the low load? 3.6% +/- 2.7% (NS). Combin ing the two experiments yielded zero difference between sources. By the uri nary calcium increment method, the mean difference between salts at the 100 0 mg load was 1.8 +/- 4.1 mg (NS). Side-by-side comparisons of the two meth ods revealed that the tracer method was 3 times more sensitive than the uri nary increment method. We conclude that, when taken with food, calcium from the carbonate salt is fully as absorbable as from the citrate, and that th e urinary increment method is not sufficiently sensitive to be useful in co mparing sources in free-living subjects.