A dual stable calcium isotope method will be used to determine fractional calcium absorption. Two stable calcium isotopes will be administered to participants in order to determine fractional calcium absorption. 7 mg of 44Ca will be administered orally in 60 mL of liquid at the start of the study. 2 mg of 42Ca will then be administered intravenously. 24 hour urine collection will begin immediately following oral tracer administration for the assessment of urinary calcium excretion. The enrichment of 44Ca and 42Ca in urine will be determined by magnetic sector thermal ionization mass spectrometry. The isotopic ratios of 44Ca and 42Ca to 48Ca (another naturally occurring calcium isotope present in the human body) will be calculated (42Ca/48Ca and 44Ca/48Ca). The relative fraction of the oral to intravenously administered isotope in the 24 hour urine sample [(42Ca/48Ca) / (44Ca/48Ca)] is the fractional calcium absorption.

Mass spectroscopy was performed using urine to measure the stable calcium isotope. Percent dietary calcium absorption was calculated as the ratio of the cumulative recovery of 44Ca (oral tracer) to the cumulative recovery of 42Ca (IV tracer) in the 24-hour urine sample.

Gastrointestinal calcium absorption minus the total of urine calcium excretion and estimated stool calcium excretion.