Cancer incidences in the digestive tube: is cobalamin a small intestine cytoprotector?

Malignancies are common in the digestive tube, although with unequal distribution among segments. The aim of this paper was to compare available interpretations of the low cancer incidence in the small bowel and high in the large bowel. Supposed mechanisms include relatively small bacterial population, large secretion of liquid and rapid transit in the small bowel. Small bowel mucosa is the main absorptive part of the digestive tube with absorption rates for various nutrients so high that they can even be considered as clearances from the intestinal content. Consequently, these nutrients are not present in the large bowel. An alternative explanation is that an absorbable protective substance from the intraluminal content, might protect the mucosa from malignant transformations. It can be speculated that if there are any cytoprotective substances in the digested food their effect would be expressed mostly in the absorptive small intestine, leaving the large bowel mucosa unprotected. Vitamin B12 might be a possible candidate for this role. Cobalamin molecules are initially bound to haptocorrin (Hc) in the stomach, but in the small intestine B12 is transferred to intrinsic factor (IF) after the action of pancreatic trypsin on Hc. Cobalamin-IF complexes are absorbed in the terminal ileum leaving only a small fraction of B12 to enter the large bowel. We have tried to summarize available data regarding cancer incidences in digestive tube, segmental length and transit times of tube content. Cancer density is calculated as incidence per length and transit speed as length per transit time. Cancer incidences for seven intestinal segments were considered low if they were below one case per 100 000 inhabitants annually, while the low cancer density meant less than six cases per 100 000 inhabitants per metre. For instance, transverse colon was considered as a high cancer incidence place (2.15 cases), with low cancer density (4.3 cases/m). Transit speed more than 0.3 metre/hour was associated with low cancer incidences (accuracy 0.85) and low cancer density segments (accuracy 1.00). Cobalamin availability showed similar distribution, available in low incidence segments and unavailable in high incidence segments. Experimental studies are needed to quantify B12 availability in the large bowel and to determine whether small amounts of B12-IF or, perhaps, B12-haptocorrin complexes are absorbed by the small bowel mucosa. Without that, no cytoprotective effects of B12 in the digestive tube can be expected.