Bioavailability and daily requirement of vitamin B12 in adult humans: an observational study of its colonic absorption and daily excretion as measured by [13C]-cyanocobalamin kinetics

• Published in: The American journal of clinical nutrition 2023-12, Vol.118 (6), p.1214-1223
• Main Authors: Kurpad, Anura V., Pasanna, Roshni M., Hegde, Shalini G., Patil, Mallikarjun, Mukhopadhyay, Arpita, Sachdev, Harshpal S., Bhat, Kishor G., Sivadas, Ambily, Devi, Sarita
• Format: Article
• Language: English
• Subjects: Absorption; Bioavailability; Colon; Cyanocobalamin; Dosage; Excretion; Homeostasis; Intestine; Lag time; Large intestine; Observational studies; Vitamin B12; Vitamin deficiency
• ISSN: 0002-9165; 1938-3207
• DOI: 10.1016/j.ajcnut.2023.08.020

Background Clinical and biochemical vitamin B12 (B12) deficiency is lower than anticipated in vegetarians. Extraileal absorption, such as from the colon, as well as reduced daily excretion, may be adaptive mechanisms to maintain B12 homeostasis with marginal intakes. Objective To measure the absorption of B12 from the small and large intestine, and its daily rate of excretion from the body, using a [13C]-cyanocobalamin tracer. Methods Oral B12 bioavailability was measured over 12 h after administration of [13C]-cyanocobalamin tracer (2.5 μg) in normal participants. The colonic B12 bioavailability was evaluated by direct instillation of [13C]-cyanocobalamin (5 μg) into the ascending colon. Bioavailability was calculated from 2-compartmental modeling of the tracer appearance in plasma. The excretion rate of B12 was measured from [13C]-cyanocobalamin elimination from the body over 4 wk after oral dosing (5 μg). Results The oral B12 bioavailability (n = 11) was 63% ± 10% measured over 12 h. A late absorption peak, accounting for 12% of the absorption, was observed after an average lag time of 8.7 h from dosing. The colonic B12 bioavailability (n = 10) was 7% ± 5% over 4 h. The daily B12 excretion rate (n = 4) was 0.7 ± 0.2 μg/d. The minimum daily requirement of B12 in these participants was derived at 1 μg /d. Conclusions B12 is absorbed in the human colon. This observation confirms the potential contribution of the colon in daily B12 nutriture, and along with a possible lower requirement, could explain the absence of clinical deficiency in populations with marginal B12 intakes.