Tissue distribution of oral vitamin B12 is influenced by B12 status and B12 form: an experimental study in rats

PURPOSE: Hydroxocobalamin (HOCbl) is the dominating Cbl form in food, whereas cyanocobalamin (CNCbl) is common in vitamin pills and oral supplements. This study compares single-dose absorption and distribution of oral HO[(57)Co]Cbl and CN[(57)Co]Cbl in Cbl-deficient and normal rats. METHODS: Male Wistar rats (7 weeks) were fed a 14-day diet with (n = 15) or without (n = 15) Cbl. We compared the uptakes of HO[(57)Co]Cbl (free or bound to bovine transcobalamin) and free CN[(57)Co]Cbl administered by gastric gavage (n = 5 in each diet group). Rats were sacrificed after 24 h. Blood, liver, kidney, brain, heart, spleen, intestines, skeletal muscle, 24-h urine and faeces were collected, and the content of [(57)Co]Cbl was measured. Endogenous Cbl in tissues and plasma was analysed by routine methods. RESULTS: Mean endogenous plasma-Cbl was sevenfold lower in deficient vs. normal rats (190 vs. 1330 pmol/L, p < 0.0001). Cbl depletion increased endogenous Cbl ratios (tissue/plasma = k (in)/k (out)) in all organs except for the kidney, where the ratio decreased considerably. Twenty-four-hour accumulation of labelled Cbl showed that HOCbl > CNCbl (liver) and CNCbl > HOCbl (brain, muscle and plasma). CONCLUSIONS: The Cbl status of rats and the administered Cbl form influence 24-h Cbl accumulation in tissues and plasma. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00394-017-1424-0) contains supplementary material, which is available to authorized users.