Use of nonabsorbable markers for gastrointestinal contents in in vivo measurement of magnesium bioavailability.

Magnesium absorption from five leafy vegetables was measured in rats using 28Mg as an extrinsic label and polyethylene glycol (PEG) and chromium-mordanted vegetable fibers (cr-mordants) as unabsorbable markers of soluble and particulate gut and fecal contents, respectively. The test meals used in this investigation were identical to those used in a previous test in which we found: a) that stable 26Mg biologically incorporated into the vegetables was freely exchangeable with an extrinsic 28Mg tracer; b) PEG consistently preceded Cr-mordants in transit through the gut; and c) fecal Mg isotope excretion 12 hours after the test meal was inadequate for measurement of Mg absorption. Accordingly, in the present investigation, all measurements in feces and gut contents were made 24 hours after the test meal. By that time an average of greater than 60% of the ingested PEG had been excreted. Magnesium-28 excretions, estimated under these conditions, were 4-5% less than they would have been had fecal PEG excretion been 100%. The discrepancy was due to partial separation of PEG and Mg in the lower gastrointestinal tract (GIT), possibly due to sequestration of Mg by microorganisms. That Mg must have entered an insoluble phase in the lower GIT was born out by the observation that little or no Mg absorption occurred in the cecum or colon. As before, Cr-modants lagged behind PEG and unabsorbed 28Mg, and thus do not seem suitable to monitoring intestinal transit of Mg.