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Evidence that physiological doses of vitamin B12 are metabolized or degraded in the gastrointestinal tract: implications for vitamin B12 bioavailability and fortification

Much of our understanding of vitamin B12 derives from radiocobalt tracer studies conducted decades ago. With new technologies, we can now study B12 absorption and metabolism with unprecedented resolution using almost ambient levels of 14C biosynthetically incorporated into the B12 molecule (14C‐B12). In on‐going studies, a dose of 14C‐B12 (1.3 µg, 50 nCi) in water is administered orally and blood, urine, and feces analyzed for 14C by accelerator mass spectrometry. In 6 subjects, the plasma response was consistent with the expected behavior of peroral B12: 14C‐B12 first appeared in the plasma 3h post‐dose with a peak level within 6‐8h. Urinary and fecal excretion was maximal in the first 24h. The amount of 14C found in the urine (10‐50% of the dose) was 100‐fold greater than in previous reports (0.1‐0.5%), and fecal excretion was lower than expected (<10% vs 30‐70%). Most of the urinary 14C was not associated with intact B12. These results are consistent with metabolism or degradation of the vitamin in the GI tract prior to absorption. The discrepancy between the present and previous findings may be due to the form of radioactive B12 used: 14C labeling the 5,6‐dimethylbenzimidazole moiety in the present study and radiocobalt labeling the corrin ring in previous studies. These findings provide valuable information on the fate of B12 after oral consumption that will inform considerations to fortify the food supply with B12.