CECAL FERMENTATIONS IN RATS FED OLIGOSACCHARIDES (INULIN) ARE MODULATED BY DIETARY CALCIUM LEVEL

The influence of Ca on the fermentation of dietary oligosaccharides in the large intestine has been investigated in four groups of rats fed different semipurified diets: 1) fiber free, 3 g Ca/kg, 2) fiber free, 8 g Ca/kg; 3) 15% inulin, 3 g Ca/kg; or 4) 15% inulin, 8 g Ca/kg. The cecal fermentations were very low in rats fed the fiber-free diets an d were not affected by the dietary Ca level. Rats fed the inulin diets had enlarged cecum with acidic fermentations, relatively rich in prop ionic acid. In this diet group rats adapted to the 3 g Ca/kg level had very acidic fermentations and depressed volatile fatty acid concentra tions together with an accumulation of lactic acid (L and D isomers). Inulin diets brought about a rise in the crypt column height and in th e activity of ornithine decarboxylase in cecal mucosa, especially in t he 3 g Ca/kg diet group. There was considerable accumulation of insolu ble Ca and P(i) in the cecum of rats fed high-Ca diets. Inulin feeding increased the percentage of soluble Ca and P(i); Ca absorption from t he cecum was also markedly higher in rats fed inulin and was influence d by the dietary Ca level. The concentrations of soluble bile acids we re depressed in rats fed inulin diets, which enhanced the fecal excret ion of bile acids. These effects were poorly altered by changes in the dietary Ca level. In vitro it appears that CaP(i) is effective in dec reasing the solubility of bile salts, chiefly in acidic conditions. In conclusion there is in the large intestine a system of control of lum inal pH, which involves the presence of insoluble Ca and P(i). The pre sent findings suggest that the protective effects of dietary Ca on col onic epithelium involve not only bile acids binding by insoluble Ca sa lts (mainly phosphate) but also the control of luminal pH itself and o f the production of volatile fatty acids.