Intestinal Absorption of Vitamin B 12 in Humans as Studied by Isotope Technic. ∗

Abstract

Summary

The scintillation measurements of the hepatic uptake of C0⁶⁰-B₁₂ following its oral and parenteral administration to 20 normal humans, indicate that the efficiency of intestinal absorption of vit. B₁₂ decreases sharply on increase of the intake. The peak of the absorption curve of vit. B₁₂ was found at the oral dose of 0.5 μg, at which the hepatic uptake was found equivalent to 90.5 ± 5.8% of that observed following intramuscular injection of a similar dose of this vitamin. With the increase of the dose, a progressive decline in absorption followed a hyperbolic regression curve, so that at the oral dose of 50 μg B₁₂ the hepatic uptake was equivalent to only 3.0 ± 0.7% of that found after intramuscular injection of a similar dose of this vitamin. The data obtained indicate that the increment in the oral dose of vit. B₁₂ from 0.5 to 50.0 μg, results apparently in an increase of the amount absorbed of only 1.0 μg. It is suggested that in addition to Castle's gastric intrinsic factor, an intramural “intestinal B₁₂-acceptor” exists, which may be responsible for the partial mucosal block to the absorption of vit. B₁₂ in the intestine of normal humans. The role of this hypothetic acceptor in the absorption of vit. B₁₂ might be analogous to that of apoferritin in intestinal absorption of iron.