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A novel dual radio- and stable-isotope method for measuring calcium absorption in humans: comparison with the whole-body radioisotope retention method^1,2

¹ From the Research Department of Human Nutrition, The Royal Veterinary and Agricultural University, Frederiksberg, Denmark (ABB, SB, CM, MH, and BS); the Department of Earth Sciences, Dartmouth College, Hanover, NH (SS); and the Department of Clinical Physiology and Nuclear Medicine, The Copenhagen University Hospital, Copenhagen (MJ and OWK).

Background: Dietary calcium absorption can be determined only with the use of isotope techniques. Currently used isotope techniques require exclusive equipment or are not true tracer approaches.

Objective: The objective was to compare a dual-isotope method combining radioisotopes and stable isotopes with a whole-body radioisotope retention method for measuring calcium absorption.

Design: Seven healthy adults aged 21–27 y consumed a test meal containing 63 ± 14 ( ± SD) mg Ca together with a water solution of ⁴⁷Ca (0.11 MBq). One hour after ingestion, 18 mg ⁴⁴Ca was administered intravenously. All feces and urine were collected for 5 and 6 d, respectively. Calcium absorption was estimated from whole-body retention of the radioisotope 12 times over 3 wk after ingestion and from the excretion of ⁴⁷Ca and ⁴⁴Ca in a 24-h urine sample collected on day 2. ⁴⁴Ca in urine was determined by inductively coupled plasma mass spectrometry.

Results: Mean (± SD) calcium absorption was 75 ± 9% with the dual-isotope method and was 74 ± 8% with the whole-body radioisotope retention method. There was a high degree of agreement between the methods.

Conclusion: The dual-isotope method is a valid approach for measuring calcium absorption from a single meal.

Key Words: Calcium absorption • radioisotopes • ⁴⁷Ca • stable isotopes • ⁴⁴Ca • whole-body counting • inductively coupled plasma mass spectrometry • ICPMS

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