Utility and limitation of calciuric response to oral calcium load as a measure of intestinal calcium absorption: Comparison with isotopic fractional calcium absorption

The intestinal absorption of calcium (Ca), indirectly measured from the calciuric response to oral Ca load (1 g), was compared to the more directly obtained isotopic fractional absorption, α (from the fecal recovery of orally administered ⁴7Ca). In 17 normal subjects and 30 patients with absorptive hypercalciuria (AH), there was a significant (P < 0.001) correlation of α with the Ca load response (r = 0.81). However, this correlation was not observed in patients with renal hypercalciuria (RH), and those with AH receiving thiazide or orthophosphate. In RH, 38 per cent of patients had elevated Ca load responses, despite normal values for α. The point correlating the calciuric response and α in these patients was below the 95 per cent confidence limit of the line correlating α and the load response. Thus, Ca load response often overestimated intestinal Ca absorption, because of the high basal (fasting) urinary Ca. Thiazide therapy in RH improved the correlation between the two tests of Ca absorption. However, thiazide therapy in AH produced normal Ca load response despite persistently high α in 60 per cent of patients. Similarly, 50 per cent of patients with AH receiving orthophosphate had normal Ca load response, although α remained elevated. Thus, Ca load response underestimated Ca absorption when patients with AH took thiazide or orthophosphate, probably bacause these drugs augment renal tubular reabsorption of Ca. These data support the Ca load test as a valid indirect measure of intestinal Ca absorption in normal subjects and patients with AH, in whom fasting urinary Ca is not elevated. In conditions of renal Ca, leak or with various drugs known to alter renal Ca handling, there seem to be large deviations of Ca load response from α. Care should be exercised before reaching conclusions regarding the intestinal Ca absorption in these situations.