EFFECT OF MAGNESIUM ON THE INTESTINAL-ABSORPTION OF CALCIUM IN MAN

Objective: In view of the widespread use of magnesium (Mg) as a nutrit ional supplement, we investigated whether Mg would affect the absorpti on of calcium (Ca) as the intestinal absorption sites for Mg and Ca di ffer. Methods: The intestinal absorption of Ca, using (CaCl2)-Ca-47 as the tracer, and metabolic balances of Ca, phosphorus (P) and Mg were determined in five adult males under strictly controlled dietary condi tions in control studies and during Mg supplementation. Mg was given a s magnesium oxide (MgO) in 10 studies during two Ca intakes: five stud ies during a low Ca intake of 241 mg/day and five studies during a nor mal Ca intake of 812 mg/day. Dietary Mg intake ranged from 241 to 264 mg/day in control studies. During Mg supplementation, the total Mg int ake ranged from 789 to 826 mg/day. Results: There was no change of the intestinal Ca absorption during Mg supplementation during the two Ca intakes. The only change was the higher 1-hour Ca-47 plasma level in t he Ca-47 absorption studies during the high Mg intake. Urinary Ca incr eased during Mg supplementation only during the low Ca intake, the Ca balance became more negative but this difference was not significant. There was also no change in Ca excretion or Ca balance during the high Mg intake at the normal Ca intake of 800 mg/day. P balance studies sh owed a slight decrease in urinary P and an increase in fecal P, but th e P balances did not change. Mg balances were negative in control stud ies during the two Ca intakes. Supplemental Mg increased both urinary and fecal Mg excretion and the Mg balance became positive, but these d ifferences were not significant. Conclusion: The increased Mg intake o f 826 mg did not affect intestinal Ca absorption determined with trace r doses of Ca-47 during Ca intakes of 241 and 812 mg/day.