Oral Administration of Carbon-14 Labeled Cyanocobalamin (14C-Cbl) Reveals Variable Degradation of Vitamin B12 in the Gastrointestinal Tract That Impacts Vitamin B12 Absorption and Status.

Abstract 3018

Poster Board II-994

Recent evidence from our laboratory and others suggests that a variable portion of ingested cobalamin (Cbl), either crystalline or from food, is degraded in the gastrointestinal tract. We have developed a biosynthetic method to incorporate 14C into the lower axial ligand of cobalamin that has made it possible to study the fate of this vitamin during its passage through the gastrointestinal tract and to assess the presence of Cbl or its breakdown products in biological samples. Following oral administration of an aqueous physiological tracer dose of 14C-Cbl (1.3 μg, 50 nCi), blood, urine, and feces are analyzed for 14C by accelerator mass spectrometry. In 9 subjects, the plasma response was consistent with the expected behavior of peroral Cbl: 14C-Cbl first appeared in the plasma 3h post-dose reaching a peak level within 6-8h. Confirmation that this dose appears bound to the physiological transport protein transcobalamin (TC) was obtained in a subset of subjects by an immunoaffinity method using anti-human TC antibody-coated magnetic beads which selectively bound 95-98% of plasma 14C. Urinary excretion of 14C was maximal in the first 24h, with 14C first appearing in urine as early as 1.5h after dosing. Fecal excretion occurred variably over several days. The amount of 14C found in the urine (10-50% of the dose) was 100-fold greater than in previous reports using 57Co-labeled cyanocobalamin (0.1-0.5%), and fecal excretion was lower than expected (10-20% vs 30-70%). Urinary excretion of 14C was inversely correlated with the peak plasma level of 14C attained (r²=0.610; p<0.001). The bulk of urinary 14C was not associated with intact Cbl and first appeared in the urine before peak 14C levels were attained in the plasma. The peak plasma level of 14C attained also showed a strong positive correlation with plasma holotranscobalamin concentration measured before administration of the 14C-Cbl (r²=0.571; p<0.001). No such correlation was found with total plasma Cbl. In additional experiments on normal volunteers using eggs endogenously labeled with 14C Cbl following intramuscular injection of hens with 14C Cbl, comparably high urinary excretion of 14C was also observed. We conclude that a variable fraction of ingested Cbl is degraded in the gastrointestinal tract of normal individuals. This may be an important determinant of the amount of Cbl absorbed from food or supplement sources. Additionally, our findings suggest that the concentration of holoTC in the plasma reflects absorptive capacity and may therefore be a good surrogate for Cbl absorptive status. Our findings also have implications regarding the bioavailability of Cbl and may inform pending considerations to fortify food supplies with Cbl in order to mitigate the incidence of Cbl deficiency, particularly among the elderly. Intestinal degradation, either microbial or through the action of digestive enzymes, may also be a source of Cbl analogues that have previously been detected in the plasma and tissues. Cbl analogues may interfere with the physiological function of cobalamin, resulting in some of the manifestations of cobalamin deficiency.