Publications

BACKGROUND/OBJECTIVES: The bioavailability of crystalline vitamin B(12) (B(12)) through active absorption is reported to have a maximum capacity of 1.5-2.5 µg  per dose. A small passive bioavailability has also been suggested at high doses.  The present study aimed to determine the dose-dependency of active B(12)  absorption and to quantify its passive absorption at higher doses. METHODS: The  dose-dependency of crystalline B(12) bioavailability was determined in nine  healthy adults, using oral [(13)C]-cyanocobalamin, in a cross-over design at  doses of 2.5, 5, and 10 µg. The dose order was randomised, with a washout of one  month. Literature data from was added to the present study data in a  meta-analysis of the relation between B(12) bioavailability and dose, to evaluate  its pattern at different doses. RESULTS: Bioavailability, as a function of dose,  was significantly different between 2.5, 5, and 10 µg doses of  [(13)C]-cyanocobalamin at 50.9 ± 32.5%, 26.7 ± 22.3%, 15.4 ± 13.6%, respectively,  (p < 0.01), while the absolute bioavailability trended upward, at 1.16 ± 0.74 µg,  1.22 ± 1.02 µg, and 1.39 ± 1.23 µg (p = 0.46). The meta-analysis showed two  distinct phases of bioavailability. Up to a dose of 2.6 µg, there was a  significant steep positive correlation, with a slope (bioavailability) of 43%/µg  suggesting an active process with a maximum of 1.2 µg. At higher doses, the slope  was 1%/µg, not significantly different from zero, possibly a passive process.  CONCLUSIONS: The active bioavailability of crystalline B(12) is not  dose-dependent, saturating at ~1.2 µg.