Methodologies for using stable isotopes to assess magnesium absorption andsecretion in children

Objective: To provide methodological information regarding the absorption a nd excretion of dietary magnesium by children and adolescents. Methods: Recently, stable isotope techniques for assessing magnesium absorp tion 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 do sing and sample requirements for such studies. Results: Our data demonstrate that, after oral and intravenous dosing of is otopes, 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 exc retion is small relative to urinary magnesium excretion, measurement of end ogenous fecal magnesium excretion is not needed to make a reasonable estima te of net magnesium retention for most studies. Using high-precision analyt ical techniques, an intravenous dose of Mg-25 of approximately 0.2 to 0.3 m g/kg would be adequate for absorption measurements. Conclusions: The cost and availability of isotopes and their analysis are s uch that it should be feasible for increasing numbers of investigators to m ake use of these techniques.