MAGNESIUM TURNOVER IN THE HUMAN STUDIED WITH Mg28*

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Abstract
Studies with Mg28 in man suggest that gastro-intestinal absorption of Mg++ is very limited. Following intravenous administration the isotope rapidly appears in the urine but only insignificant amounts appear in the stool. This indicates that most '' of the Mg in the stools is of exogenous origin. Equilibration of the isotope with Mg in the body is slow and at 40 to 60 hr. it amounts to about 10 to 25% of the total; at 90 hr. perhaps a 1/3 of the body''s Mg has reached equilibrium with the isotope. Graphical analysis of urinary Mg28 curves in terms of exponential components yielded a slow component of 14 to 35 hr. half-time accounting for 10 to 15% of the injected dose and 2 more rapid components of 1 and 3 hr. half-times each, accounting for 15 to 25% of the injected dose. The large fraction remaining - from about 25 to 50% of the body''s total - has a turnover rate of less than 2%/day. These turnover values must be interpreted with caution because of possible effects from the significant amounts of dietary Mg in addition to the effect of that administered with the low specific activity Mg28.