Interrelationships between circulating vitamin D metabolites in normocalciuric and hypercalciuric renal stone formers

Serum 25-hydroxyvitamin D [25(OH)D], 24,25-dihydroxyvitamin D [24,25(OH)2D] and 1,25-dihydroxyvitamin D [1,25(OH)2D] were studied in renal stone formers while on a diet containing 1,000 or 300 mg calcium per day. The patients were divided into four groups and identified as (a) normocalciuric (NSF), (b) absorptive hypercalciuric (AH), (c) renal hypercalciuric (RH) and (d) as having primary hyperparathyroidism (PHP). The results indicate that, on the 1,000-mg Ca diet, the mean 25(OH)D, 24,25(OH)2D and 1,25(OH)2D concentrations were within the normal range in all groups of patients. On the low-calcium diet, 25(OH)D concentrations decreased significantly in patients with AH and RH while 24,25(OH)2D concentrations were not affected by the low-calcium diet. 1,25(OH)2D concentrations increased significantly during dietary calcium restriction in all groups of patients. When the relationship obtained on each diet between the circulating 25(OH)D concentrations and the 25(OH)D/1,25(OH)2D concentration ratio was compared, it suggested that the increase in the 1,25(OH)2D concentrations during dietary calcium restriction may have been due to an increase in the capacity of the 25(OH)D-1 alpha-hydroxylase, or to an increase in the circulating half-life of the hormone. These results indicate that renal stone formers are able to adapt themselves to dietary calcium restriction as shown by highly significant increases in the circulating 1,25(OH)2D concentrations while on a low-calcium diet. Moreover, the increase in the circulating 1,25(OH)2D concentrations did not happen at the expense of the 24,25(OH)2D production in any of the groups of patients studied.