No difference in intestinal strontium absorption after an oral or an intravenous 1,25(OH)2D3 bolus in normal subjects

It has been suggested that 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) stimulates intestinal calcium absorption less via the intravenous (iv) than the oral route, because the first avoids direct contact of the drug with the enterocytes. However, no study has addressed the issue directly. This investigation was designed to measure the effect of a single oral or iv dose of 1,25(OH)₂D₃ on calcium absorption, using stable strontium (Sr) as a surrogate for calcium, and measuring the Sr fractional absorbed dose (FAD%) over 240 minutes after Sr administration. In 10 healthy volunteers, five tests were performed in a cross-over design, with a wash-out period between two consecutive tests: Sr absorption without 1,25(OH)₂D₃ (test A); Sr absorption immediately after either oral (test B) or iv (test C) 1,25(OH)₂D₃ (1.5 μMg/m² of body surface area [BSA]); Sr absorption (24 hr after either oral (test D) or iv (test E) 1,25(OH)₂D₃ (1.5 μg/m² BSA). The concurrent administration of 1,25(OH)₂D₃ and Sr (tests B and C) did not significantly change the area under the Sr FAD%-time curve with respect to test A (test A: 4090 ± 345; test B: 4510 ± 345; test C: 4210 ± 345), whereas Sr absorption was significantly increased (p <0.001) when Sr was given 24 hr after either oral or iv 1,25(OH)₂D₃ (test D: 5710 ± 345; test E: 5510 ± 345). It was concluded that 1,25(OH)₂D₃ is likely to influence calcium absorption significantly only via its genomic effect, independent of its administration route.