Differences in Tissue Distribution of Cyano⁻B12 and Hydroxo⁻B12 One Week after Oral Intake: An Experimental Study in Male Wistar Rats.

Foods contain natural vitamin B12 forms, such as hydroxo-B12 (HO-B12), whereas vitamin pills contain the synthetic cyano-B12 (CN-B12). Recent studies in rats showed different tissue distributions of CN-B12 and HO-B12 24 h after oral administration. Here, we investigate whether these differences are sustained or leveled out with time in both B12-deplete and -replete rats, thereby assessing if the two forms are equally good at maintaining a normal B12 status. Male Wistar rats were fed diets with low (n = 16) or high (n = 12) B12 content for 17 days. At day 10, the rats received a single oral dose of [Co-57]-labeled CN-B12 or HO-B12 (n = 6 and n = 8, respectively, in each diet group). The rats were sacrificed on day 17 and endogenous B12 and [Co-57]-B12 were measured in liver, kidney, and plasma. We found that the low-B12 diet introduced a B12-deplete state as judged from medians of endogenous B12 compared to rats on a (high-B12 diet): Plasma (565 (1410) pmol/L), liver (28.2 (33.2) pmol/g), and kidneys (123 (1300) pmol/g). One week after oral administration, the labeled B12 was distributed as follows: HO-B12 > CN-B12 (liver) and CN-B12 > HO-B12 (kidneys, plasma). The tissue/plasma ratios showed different equilibriums for labeled CN-B12 and HO-B12 in the B12-deplete and -replete groups. The equilibrium of endogenous B12 resembled [Co-57]CN-B12 in replete rats but differed from both [Co-57]CN-B12 and [Co-57]HO-B12 in deplete rats. The data suggest long-term differences in tissue utilization of the two B12 forms and warrant further studies concerning the possible benefits of consuming HO-B12 instead of CN-B12 in oral B12 replacement.