Dietary calcium phosphate inhibits cytotoxicity of fecal water.

The effects of dietary steroid and CaHPO4 supplementation on the solubility of bile acids and cytotoxicity of fecal water were studied in rats. Dietary steroid supplementation increased the bile acid concentration of both feces and fecal water. CaHPO4 supplementation produced a slight increase in total fecal bile acid concentration but resulted in a drastic decrease in soluble bile acid concentration. Cytotoxicity of fecal water on control and steroid-supplemented diets decreased with CaHPO4 supplementation analogous to the decrease in soluble bile acid concentration. The concentrations of precipitated Ca and Pi were highly correlated (r greater than 0.90) with the concentration of precipitated bile acids and with inhibition of cytotoxicity. However, there was no significant correlation between the logarithms of soluble calcium and soluble bile acids, indicating that solubility of bile acids is not determined by soluble calcium. Concentrations of calcium and phosphate in fecal water indicated the formation of insoluble calcium phosphate in the intestine. Thus dietary CaHPO4 causes a decrease in soluble bile acid concentration, which is probably due to the formation of an insoluble bile acid calcium phosphate complex. Consequently, cytotoxicity of fecal water is inhibited, which might have implications for the protective effect of dietary calcium with regard to colonic cancer in humans.