Publications

BACKGROUND: Clinical and biochemical vitamin B(12) (B(12)) 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 B(12)  homeostasis with marginal intakes. OBJECTIVE: To measure the absorption of B(12)  from the small and large intestine, and its daily rate of excretion from the  body, using a [(13)C]-cyanocobalamin tracer. METHODS: Oral B(12) bioavailability  was measured over 12 h after administration of [(13)C]-cyanocobalamin tracer (2.5  μg) in normal participants. The colonic B(12) bioavailability was evaluated by  direct instillation of [(13)C]-cyanocobalamin (5 μg) into the ascending colon.  Bioavailability was calculated from 2-compartmental modeling of the tracer  appearance in plasma. The excretion rate of B(12) was measured from  [(13)C]-cyanocobalamin elimination from the body over 4 wk after oral dosing (5  μg). RESULTS: The oral B(12) 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 B(12) bioavailability  (n = 10) was 7% ± 5% over 4 h. The daily B(12) excretion rate (n = 4) was 0.7 ±  0.2 μg/d. The minimum daily requirement of B(12) in these participants was  derived at 1 μg /d. CONCLUSIONS: B(12) is absorbed in the human colon. This  observation confirms the potential contribution of the colon in daily B(12)  nutriture, and along with a possible lower requirement, could explain the absence  of clinical deficiency in populations with marginal B(12) 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.