A Simple Single Serum Method to Measure Fractional Calcium Absorption using Dual Stable Isotopes

Abstract

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 (⁴⁴Ca) followed 2 h later by an intravenous isotope (⁴²Ca). 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.

Key words

fractional calcium absorption - stable isotopes - ⁴⁴Ca - ⁴²Ca

References

• 1 Abrams SA. Using stable isotopes to assess mineral absorption and utilization by children.  Am J Clin Nutr. 1999;  70 955-964
• 2 Ceglia L, Abrams SA, Harris SS. et al . Evaluation of an inexpensive calcium absorption index in healthy older men and women.  Clin Endocrinol (Oxf). 2009a;  Mar 6. [Epub ahead of print]
• 3 Ceglia L, Harris SS, Abrams SA. et al . Potassium bicarbonate attenuates the urinary nitrogen excretion that accompanies an increase in dietary protein and may promote calcium absorption.  J Clin Endocrinol Metab. 2009b;  94 645-653
• 4 Chen Z, Griffin IJ, Kriseman YL. et al . Inductively coupled plasma mass spectrometric analysis of calcium isotopes in human serum: a low-sample-volume acid-equilibration method.  Clin Chem. 2003;  49 2050-2055
• 5 Griffin IJ, Abrams SA. Methodological considerations in measuring human calcium absorption: relevance to study the effects of inulin-type fructans.  Br J Nut. 2005;  93 ((Suppl 1)) S105-S110
• 6 Heaney RP. Factors influencing the measurement of bioavailability, taking calcium as a model.  J Nutr. 2001;  131 S1344-S1348
• 7 Heaney RP, Recker RR. Estimation of true calcium absorption.  Ann Intern Med. 1985;  103 516-521
• 8 Yergey AL, Abrams SA, Vieira NE. et al . Determination of fractional absorption of dietary calcium in humans.  J Nutr. 1994;  124 674-682

Correspondence