A simple single serum method to measure fractional calcium absorption using dual stable isotopes.
The dual stable isotope method with a timed 24-h urine collection is the gold standard approach to measure fractional calcium absorption. However, the need to collect urine for 24 h makes this technique time-consuming and laborious. Our study sought to determine whether a dual isotope method using a single serum sample obtained 4 h after administration of the initial isotope provides a useful approach to measure fractional calcium absorption. Following a metabolic diet with a fixed calcium intake of 30 mmol/day for 10 days, nineteen healthy subjects age 54-74 were given a test meal with an oral isotope ((44)Ca) followed 2 h later by an intravenous isotope ((42)Ca). Once the oral isotope was administered, urine was collected for 24 h, and a serum sample was obtained after 4 h. The ratio of the oral to intravenous isotopes was measured in the urine and serum by mass spectroscopy. Fractional calcium absorption was 16.2 ± 7.7% by the 4-h single serum method versus 18.5 ± 7.5% by the 24-h urine method. There was a small mean difference between the urine and serum methods of 2.33% with a confidence interval -3.97 to 8.60%. The two methods showed a strong linear association (r = 0.912, p<0.001). Use of dual stable isotopes with a 4-h single serum method gives fractional calcium absorption values that are 12.5% lower than with the 24-h urine method; however, it rank orders subjects accurately thus making it a useful alternative method in clinical research applications.
Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging, Tufts University, Boston, MA 02111, USA. lisa.ceglia@tufts.edu
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Ceglia
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Abrams
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Harris
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Rasmussen
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Dallal
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Dawson-Hughes
D000042:Absorption; D000368:Aged; D000369:Aged, 80 and over; D001774:Blood Chemical Analysis / Q000379:methods*; D002118:Calcium / Q000097:blood* / Q000737:chemistry / Q000378:metabolism / Q000652:urine*; D002127:Calcium Isotopes / Q000097:blood* / Q000378:metabolism / Q000493:pharmacokinetics* / Q000652:urine; D002136:Calcium, Dietary / Q000097:blood / Q000378:metabolism / Q000493:pharmacokinetics / Q000652:urine; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D044967:Serum / Q000378:metabolism; D016482:Urinalysis / Q000379:methods
D002127:Calcium Isotopes; D002136:Calcium, Dietary; D002118:Calcium