Effects of feeding fermentable carbohydrates on the cecal concentrations of minerals and their fluxes between the cecum and blood plasma in the rat

Abstract

This study was conducted to determine in rats to what extent fermentable carbohydrates alter the mineral composition of cecal contents and the absorption of the major cations. The carbohydrates studied were as follows: an oligosaccharide (lactulose, 10%); a soluble fiber (pectin, 10%); and an amylose-rich starch, incompletely broken down in the small intestine (amylomaize starch, 25 or 50%). All of these carbohydrates elicited a marked enlargement of the cecum, a drop of cecal pH and an increase in the volatile fatty acids (VFA) pool. With the lactulose diet, the VFA concentration was the lowest, whereas VFA absorption was similar to that observed with the 10% pectin or 25% amylomaize diets. From comparisons between germfree and conventional rats adapted to a fiber-free diet, it appears that VFA are required as counter anions to maintain high concentrations of cations, especially sodium. In conventional rats fed fermentable carbohydrates, sodium concentration in the cecal fluid was approximately 80 mM, except with the lactulose diet (49.5 mM), due to osmotic effects of lactulose. There was, compared to the fiber-free diet, an increase in the cecal concentrations of potassium, calcium and phosphate, but not of magnesium; nevertheless, the cecal pool of all of these minerals was considerably increased. Potassium absorption was increased by fermentable carbohydrates in the cecum, which also appears to be a major site of magnesium and calcium absorption. Thus, fermentable carbohydrates shift aborally the absorption of the major cations, and this point is especially interesting in regard to calcium, since an enhanced supply of calcium in the large bowel has been invoked for fiber effects on colonic carcinogenesis.