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Measurement of vitamin B12 absorption in a human subject using 14C‐B12

Malabsorption is the primary cause of B12 deficiency in older adults. The Schilling test has been the gold standard for assessing B12 absorption, but is now rarely used in clinical diagnosis because it requires ingestion of gamma‐emitting 57Co‐B12 and 24h urine collection, and has limited sensitivity and specificity. We have developed a method for microbial synthesis of 14C‐B12 using S. typhimurium incubated with the dedicated precursors dicyanocobinamide and 14C‐dimethylbenzimidazole. The 14C‐B12 has been used in a human subject (male, 40y) to assess absorption. 14C‐B12 was measured by accelerator mass spectrometry (AMS), which detects trace levels of 14C in biological samples. After oral consumption (1.5 ug, 59 nCi), 14C‐B12 first appeared in the plasma 3h post‐dose, with peak plasma level at 7h. Urinary and fecal excretion was maximal over the first 24h. Thus, 14C‐B12 is efficiently absorbed and AMS is sufficiently sensitive to detect 14C‐B12 with high precision above background in biological samples. 14C‐B12 is now being used to assess absorption in patients suspected of having pernicious anemia. It is expected that this novel diagnostic test will have better sensitivity and specificity than the Schilling test, and will reduce the radioactive burden on the patient and eliminate the need for 24h urine collection. NIH DK064302, Lawrence Livermore Natl Lab