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 relative absorbability of various sources. Absorbability of calcium from the carbonate 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 increment in urinary calcium, and at the low load by the tracer method only. Subjects were 37 healthy adult men and women, studied as outpatients, and each tested on both salts at the same load. Mean tracer absorption (+/- SD) for both salts combined was 36.0% at the 300 mg load and 28.4% at the 1000 mg load. In both experiments the observed mean difference in absorption between salts was very small. By the tracer method the within-subject difference (carbonate less citrate) was +3.3% +/- 1.2% of the ingested dose (mean +/- SEM; P < 0.05) at the high load, and at the low load, 3.6% +/- 2.7% (NS). Combining the two experiments yielded zero difference between sources. By the urinary calcium increment method, the mean difference between salts at the 1000 mg load was 1.8 +/- 4.1 mg (NS). Side-by-side comparisons of the two methods revealed that the tracer method was 3 times more sensitive than the urinary increment method. We conclude that, when taken with food, calcium from the carbonate salt is fully as absorbable as from the citrate, and that the urinary increment method is not sufficiently sensitive to be useful in comparing sources in free-living subjects.