MAGNESIUM STUDIES IN RELATION TO VITAMIN D-RESISTANT RICKETS

Abstract

Magnesium metabolism was found to be unaltered in a 4-year-old boy with untreated vitamin D-resistant rickets who manifested abnormalities of calcium and phosphorous metabolism commonly found in this disease. This is at variance with the previously reported finding of altered magnesium metabolism in a patient with resistant rickets acquired at adolescence. The possibility that this difference may reflect either a variation in the underlying disease process in the two age groups or a difference in the severity of the disease process is discussed. It is of significance that the gastrointestinal absorption of calcium was decreased while that of magnesium was normal in our untreated patient. This suggests that the mechanisms involved in intestinal transport of these two minerals are not identical. Treatment with 200,000 to 300,000 I.U. of vitamin D per day resulted in a decrease in the serum magnesium levels with no consistent changes in the urinary excretion, intestinal absorption, or retention of magnesium; during this time, however, the intestinal absorption, urinary excretion, and retention of calcium were all increased. The administration of 350,000 I.U. of vitamin D per day was associated with an increase in both the urinary excretion and intestinal absorption of magnesium. At this dose the retention of magnesium initially increased and subsequently decreased as the increase in urinary magnesium offset the increased intestinal absorption of this mineral. It is not known if the effects of vitamin D on the intestinal absorption and urinary excretion of magnesium represent primary actions of this vitamin on the kidney and gastrointestinal tract or whether they are secondary to changes in calcium and phosphorous metabolism. The fact that vitamin D administration resulted in a lowering of the serum magnesium level before there were any consistent alterations in fecal and urinary magnesium is in keeping with previously reported animal studies which indicate that vitamin D acts to cause a redistribution of magnesium in the body.