Mg28 was used to explore the kinetics of magnesium distribution in 9 normal individuals and in 16 patients with various diseases. When Mg28 was given in 12–30 mEq of stable magnesium intravenously, plasma disappearance was rapid within the first several hours. In normal subjects a mean of 19.8% of the injected radioactivity was accounted for in the urine within 24 hours. Fecal excretion was negligible, although equilibration of Mg28 in bile occurred within 18 hours. The specific activities of plasma and urine stabilized by the 18th hour, and showed only a gradual decrease thereafter. Exchangeable magnesium contents in normal subjects ranged between 2.6 and 5.3 mEq/kg of body weight—less than 16% of the estimated total body content of magnesium. Mg28 exchanged very slowly with the stable ion in bone, muscle and erythrocytes. The results in patients with diabetes mellitus and hepatic diseases showed no striking differences from those obtained in normal subjects. Note: (With the Technical Assistance of Dale R. Harms and Jacqueline Z. Reardon) Submitted on October 5, 1959

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Tracer doses of Mg28 were used to study the metabolism of parenterally administered magnesium. Plasma clearance was rapid, and accumulation of radioactivity in bone started within 2 hours. Of the tissues studied, skin and muscle showed the lowest concentrations of Mg28. In most tissues the ratio of tissue to serum radioactivity became fairly constant after 18 hours. The values for the exchangeable body pool of magnesium, calculated from the specific activities of urine specimens obtained between 18 and 24 hours, approximated the carcass content of magnesium. During starvation the renal excretion of endogenous magnesium amounted to 61.7 mEq/kg of weight loss.