In order to obtain information on the preventive effects of various food
proteins against
colonic cancer, six groups of
azoxymethane-initiated mature Fischer rats (n = 10) were fed respective diets different in
protein sources such as bovine milk
casein (
casein), high-molecular-weight fraction from protolytic digest of
soy protein isolate (soybean HMF), hen's yolk defatted
protein (yolk protein), wheat
gluten and codfish meat, which had been supplemented with
sodium deoxycholate (hereinafter, DCA) as a
cancer promoter except for an additional DCA-unfed
casein group. All of the living rats at checkpoints during the feeding period were examined by the use of a bronchus fiberscope for colonic
tumor incidence at 6 wk intervals between the 10th and 34th wk, from which both blood and feces samples were taken at times of endoscopy.
Tumorigenesis in the colon was perceived by endoscopy at wk 22 in the group fed DCA
casein only and at wk 28 in the other groups except the DCA-unfed
casein group. At wk 34, both soybean HMF and
yolk protein groups ranked inferior to the DCA-unfed group in
tumor incidence. When plasma
steroid or
lipid concentration was plotted against
tumor incidence at wk 28 or 34, positive correlations were found between plasma
bile acid concentration and
tumor incidence at both weeks. With the exception of the DCA-unfed
casein group, plasma
bile acid concentration was reversely correlated to fecal
bile acid excretion. Taken altogether, these results suggest that
bile acids at higher concentrations in the plasma may serve as risk factors of colon
tumor incidence.