Methodologies for using stable isotopes to assess magnesium absorption and secretion in children

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 Mg-25 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.