1. The influence of citrate on intestinal calcium absorption (CaA) was studied in eight healthy males.

2. On separate occasions, either a load containing 5 mmol of calcium chloride and 21 mmol of citrate in the form of sodium potassium citrate or a citrate-free vehicle load corrected for pH and cations was ingested. CaA was measured over 3 h with a 47Ca–85Sr double tracer method.

3. After citrate administration, 10 min fractional CaA decreased significantly from 30 to 110 min post-load, and 3 h cumulative CaA dropped to 54.6 ± (sem)6.1% of the total dose as opposed to 76.3 ± 4.5% after vehicle administration (P < 0.002). Citrate administration raised serum and urinary citrate, but had little effect on blood acid–base status. After both loads, urinary specific activity of 47Ca significantly correlated with 3 h cumulative CaA, while citrate administration decreased urinary calcium excretion only slightly as compared with vehicle.

4. The results suggest that, in man, higher doses of oral citrate inhibit CaA, probably by way of intraluminal complexation of calcium by citrate.

5. The finding might help explain the fall in urinary calcium excretion observed in patients treated with alkali citrate for recurrent calcium urolithiasis.

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