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

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 ((x) over bar +/- SD) mg Ca together with a water solution of Ca-47 (0.11 MBq). One hour after ingestion, 18 mg Ca-44 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 Ca-47 and Ca-44 in a 24-h urine sample collected on day 2. Ca-44 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.