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

Malignancies are common in the digestive tube, although with unequal distri bution among segments. The aim of this paper was to compare available inter pretations of the low cancer incidence in the small bower and high in the l arge bowel. Supposed mechanisms include relatively small bacterial populati on, large secretion of liquid and rapid transit in the small bowel. Small bowel mucosa is the main absorptive part of the digestive tube with a bsorption rates for various nutrients so high that they can even be conside red as clearances from the intestinal content. Consequently, these nutrient s are not present in the large bowel. An alternative explanation is that an absorbable protective substance from the intraluminal content, might prote ct the mucosa from malignant transformations. It can be speculated that if there are any cytoprotective substances in the digested food their effect w ould be expressed mostly in the absorptive small intestine, leaving the lar ge bowel mucosa unprotected. Vitamin B12 might be a possible candidate for this role. Cobalamin molecule s 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 p ancreatic trypsin on He. Cobalamin-IF complexes are absorbed in the termina l ileum leaving only a small fraction of B12 to enter the large bowel. We have tried to summarize available data regarding cancer incidences in di gestive tube, segmental length and transit times of tube content. Cancer de nsity is calculated as incidence per length and transit speed as length per transit time. Cancer incidences for seven intestinal segments were conside red 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 p er metre. For instance, transverse colon was considered as a high cancer in cidence 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 incid ences (accuracy 0.85) and low cancer density segments (accuracy 1.00). Coba lamin 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 b ower and to determine whether small amounts of B12-IF or, perhaps, B12-hapt ocorrin complexes are absorbed by the small bowel mucosa. Without that, no cytoprotective effects of B12 in the digestive tube can be expected. (C) 20 00 Harcourt Publishers Ltd.