True absorption, magnesium

At present, uses of Mg are restricted to objectives that can be attained by oral Isotope administration onlyt One of these Is the measurement of "true magnesium absorption in which Mg is given orally, and Mg by Injection. "True absorption Is then computed either by estimation of the ratio %g Mg in urine or, more precisely, by measuring net absorption Csubtractlon of fecal exr ... 

The intravenous administration of a second isotopic label can also be used to assess true absorption of the oral isotopic labels for elements that are excreted via the kidneys, such as calcium and magnesium. As in plasma appearance studies, the intravenous label serves as a reference dose. When the second label is infused or injected slowly following administration of the labeled test meal, both labels appear in parallel in serum/plasma. Some of the circulating labels, however, are excreted via the kidneys, during which the amount ratio of the two labels in blood should ideally be preserved. If this is so, a single spot urine sample can be used, at least in theory, to determine the amount ratio of the two labels in serum. As the intravenous dose is known, this ratio can then be converted into the amount of oral label absorbed. As for most stable isotope techniques, this method was originally developed for radiotracers [31] and adapted for stable isotopes in the 1980s [32]. 

Measurements of True Magnesium Absorption. Appearance of a single ingested isotope in the plasma or urine can not provide a quantitative estimate of fractional absorption of a mineral without concomitant measurement of Isotope exchange between the mineral in the plasma and other body compartments 2A). Table IV shows the levels of urinary Mg from an intravenous injection that accompanied an oral dose of 50 mg Mg. Radioactivity was determined in 15 ml urine with a relative S.D. of < 1% in composites collected at Intervals up to 24 hours, and with < 4% in the 24-48 h collection. The relative standard deviations for Mg measurements were consistently higher, reaching unacceptable levels in samples taken after 12 hours following the test meal. 

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