The Oral Bioavailability of Vitamin B12 at Different Doses in Healthy Indian Adults

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Background/Objectives: The bioavailability of crystalline vitamin B12 (B12) 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 B12 absorption and to quantify its passive absorption at higher doses. Methods: The dose-dependency of crystalline B12 bioavailability was determined in nine healthy adults, using oral [13C]-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 B12 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 [13C]-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 B12 is not dose-dependent, saturating at ~1.2 µg.