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]-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.

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.