Vitamin B12 absorption in dogs with chronic isolated intestinal loops

The absorption of physiological amounts of vitamin B12 is thought to occur only from the ileum, but the nature of the mechanism involved has not been established. A method utilizing the detection of radioactive vitamin B12 in plasma was used to investigate the mechanism of vitamin B12 absorption in dogs with isolated Thiry–Vella loops of small intestine, either ileum or jejunum. An animal with a Thiry–Vella loop of the entire ileum failed to absorb orally administered vitamin B12, but did absorb the vitamin from the isolated ileal loop when it was given with intrinsic factor, but not when given without. Attempts to restore absorption of orally administered vitamin B12 by the administration of ileal juice (either resting or stimulated), by the administration of an extract of ileal mucosa, or by the simultaneous absorption of non-radioactive vitamin B12 from the ioslated loop, all failed. In contrast, the animal with a jejunal Thiry–Vella loop absorbed vitamin B12 given orally but not when administered into the isolated jejunum alone, in combination with intrinsic factor, or with intrinsic factor and ileal juice. Under the conditions of these experiments the results confirm that vitamin B12 absorption is limited to the ileum, but fail to provide support for the hypothesis that the ileum liberates, either locally or systemically, a vitamin B12 absorption factor. Thus the hypothesis that ileal mucosa contains a special cellular mechanism for vitamin B12 absorption gains indirect support from these results.