Oral magnesium load test for the assessment of intestinal magnesium absorption. Application in control subjects, absorptive hypercalciuria, primary hyperparathyroidism, and hypoparathyroidism

The intestinal absorption magnesium (Mg) was estimated from the increment in urinary Mg following oral administration of 25 mmol of Mg. Fasting urinary Mg did not differ between the control group and patient groups (absorptive hypercalciuria, primary hyperparathyroidism, and hypoparathyroidism). As compared to the value in the control group, the increment in urinary Mg above the fasting value was not significantly different in absorptive hypercalciuria. However, it was significantly increased in primary hyperparathyroidism and significantly reduced in hypoparathyroidism. In control subjects, the increment in urinary Mg was much higher during a low than during a high calcium diet. The results suggest that 1,25-(OH)₂-vitamin D stimulates Mg absorption, since Mg absorption was elevated in situations associated with stimulated 1,25-(OH)₂-vitamin D synthesis (primary hyperparathyroidism and low calcium diet) and reduced in a condition characterized by low 1,25-(OH)₂-vitamin D production (hypoparathyroidism). Moreover, the data indicate that 1,25-(OH)₂-vitamin D may not be pathogenetically important in absorptive hypercalciuria, since Mg absorption was normal.