TL;DR: Urinary excretion measurements indicate considerably lower oral absorption of sulfato-, nitro- and thiocyanato-cobalamin than is exhibited by cyanoeobalamin, while the superior absorption of cyanocobalamin (vit. B12) over other cobalamins studied is attributed to either the chemical specificity of the substituent cyano group or the tightness of its binding in the cobalt coordination sphere.

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Abstract: Summary1. Urinary excretion measurements indicate considerably lower oral absorption of sulfato-, nitro- and thiocyanato-cobalamin than is exhibited by cyanoeobalamin. In the blood stream, however, the several cobalamins are equivalent in inducing the excretion of ingested vit. B12. 2. That cobalamin urinary excretion responses truly constitute an absorption index was verified in studies comparing chlorocobalamin with cyanocobalamin, by serum level assays in humans and by fecal excretion experiments with rats. 3. Inferior absorption of orally administered nitrocobalamin was confirmed by fecal excretion observations in humans and rats. 4. The superior absorption of cyanocobalamin (vit. B12) over other cobalamins studied is attributed to either the chemical specificity of the substituent cyano group or the tightness of its binding in the cobalt coordination sphere.

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