(P11-031-23) [13C]-Cyanocobalamin (Vitamin B12) Bioavailability: Observations on Its Colonic Absorption

Objectives: To measure the bioavailability of [¹³C]-cyanocobalamin following 12 hours of oral dosing (Study 1) and to explore the role of the colon in its absorption (Study 2).

Methods: A 2.3 µg oral dose of [¹³C]-cyanocobalamin was administered to 11 participants (Study 1) and blood samples collected at baseline and hourly for 12 hours. A population pharmacokinetic based approach was used to describe the absorption using a modified two-compartment model. Study 2 was performed in 10 adult participants who were undergoing check colonoscopy in the Department of Gastroenterology. Colonoscopies were conducted by a single gastroenterologist who instilled 5 µg of [¹³C]-cyanocobalamin into the lumen of the ascending colon after confirming the colon was normal (during withdrawal of the scope). Blood samples were collected at baseline and every 20 min till 2.3h, and thereafter at 3 and 4h.

Results: The mean age and BMI of the participants in Study 1 was 26.4 ± 4.5 years and 21.8 ± 2.0 kg/m² respectively. There was complete decyanation of the tracer to plasma [¹³C]-methylcobalamin from the first hour, with a total mean bioavailability of 58 ± 7%, ranging from 42 to 65 %. After the normal major ileal absorption peak at ~7 hours, there was a late absorption peak observed at 10-12 hours that accounted for 15% of the total bioavailability. This could be explained by colonic absorption or enterohepatic recirculation of tracer.

In Study 2, mean age and BMI of the participants was 37.9 ± 11.1 years and 22.6 ± 4.6 kg/m² respectively. There was clear tracer absorption from the colon in all participants, with mean bioavailability of 9 ± 6%, ranging from 3 to 19%.

Conclusions: B₁₂ bioavailability of a low dose is 58%, and absorption can occur from the colon, contributing to late absorption. The potential role of colonic microbiota to daily B₁₂ nutriture is relevant and worthy of further study.

Funding Sources: This research was supported by the Indian Department of Biotechnology and by a Wellcome Trust/DBT India Alliance Clinical Research Centre Grant.