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

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]-cyanoco-balamin tracer.Methods: Oral B12 bioavailability was measured over 12 h after administration of [13C]-cyanocobalamin tracer (2.5 mu g) in normal participants. The colonic B12 bioavailability was evaluated by direct instillation of [13C]-cyanocobalamin (5 mu g) 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 mu g).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 mu g/d. The minimum daily requirement of B12 in these participants was derived at 1 mu g /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.Trial Registration Number: This study was registered in Clinical Trials Registry of India (CTRI) with the registration number CTRI/2018/04/012957, available from https://ctri.nic.in/Clinicaltrials/showallp.php?mid1=49319&EncHid=&userName=029108.