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Methodologies for Using Stable Isotopes to Assess Magnesium Absorption and Secretion in Children

USDA/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children’s Hospital, Houston, Texas

Address reprint requests to: Steven A. Abrams, MD, Children’s Nutrition Research Center, 1100 Bates Street, Houston, TX 77030-2600

Objective: To provide methodological information regarding the absorption and excretion of dietary magnesium by children and adolescents.

Methods: Recently, stable isotope techniques for assessing magnesium absorption and excretion have been developed which allow for these studies to be safely performed in subjects of all ages. In the report, we describe the dosing and sample requirements for such studies.

Results: Our data demonstrate that, after oral and intravenous dosing of isotopes, a complete 72-hour urine collection will allow for determination of fractional magnesium absorption. In our study, urinary, but not endogenous fecal magnesium excretion, was closely correlated with magnesium intake (r = 0.47, p = 0.02 vs r = 0.08, p = 0.69). As endogenous fecal magnesium excretion is small relative to urinary magnesium excretion, measurement of endogenous fecal magnesium excretion is not needed to make a reasonable estimate of net magnesium retention for most studies. Using high-precision analytical techniques, an intravenous dose of ²⁵Mg of approximately 0.2 to 0.3 mg/kg would be adequate for absorption measurements.

Conclusions: The cost and availability of isotopes and their analysis are such that it should be feasible for increasing numbers of investigators to make use of these techniques.

Key words: magnesium absorption, stable isotopes, mass spectrometry, minerals

Abbreviations: TIMS = thermal ionization mass spectrometry • ICP-MS = inductively coupled plasma mass spectrometry

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