, Volume 69, Issue 5, pp 1121-1126
Date: 16 Nov 2012

Calcium bioavailability of calcium L -threonate in healthy Chinese subjects measured with stable isotopes (44 Ca and 42 Ca)

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Calcium L-threonate is a novel drug that was developed for the treatment of osteoporosis and as a calcium supplement. However, calcium bioavailability of this drug is unknown due to lack of effective evaluation methods. In this study, we sought to measure the bioavailability of calcium L-threonate with a double-label stable isotope method.


Fourteen healthy Chinese subjects were enrolled in the clinical study and were given 300 mg calcium L-threonate tablets containing 40 mg 44Ca after an intravenous injection of 4 mg 42Ca solution (as calcium chloride). Fractional urine samples were collected at the following time intervals: 0–3, 3–6, 6–9, 9–13, 13–24, 24–36 and 36–48 h. The abundance ratios of 44Ca/40Ca and 42Ca/40Ca in the urine were determined with thermal-ionization mass spectrometry (TI-MS). The calcium bioavailability was estimated by calculating the true fractional calcium absorption (TFCA) using the abundance ratios of 44Ca/40Ca and 42Ca/40Ca.


The bioavailability of calcium L-threonate in 14 healthy Chinese subjects was 26.49 ± 9.39 %. There was good agreement between TFCA from the 24 to 36 h and the 36 to 48 h urine pool, indicating that calcium balance was achieved at 24 h after dosing. The TFCA of the subjects did not statistically correlate with total urinary calcium excretion (0–48 h). There were no serious adverse events in this study.


The bioavailability of calcium L-threonate in humans was successfully determined by estimating TFCA with the double-label stable isotope method, thus providing a useful approach for the evaluation of bioavailability of calcium formulations.