We examined whether orally administered RBS (
rice bran saccharide), prepared from rice bran by hot water extraction, increases immunocompetence, inhibits gastrointestinal
carcinogenesis with N-
ethyl-N'-nitro-N-nitrosoguanidine (
ENNG) or shows an antitumor effect. After the administration of RBS,
phytohemagglutinin (PHA)- and
pokeweed mitogen (PWM)-stimulated blastogenesis of lymphocytes derived from the mesenteric lymph nodes and peripheral blood was enhanced, and the helper/suppressor T-cell ratio was elevated, and migration activity of peritoneal macrophages was also increased in rats treated continuously with
ENNG.
ENNG-induced gastrointestinal
carcinomas were observed in 43% of those administered RBS (
ENNG-RBS) as compared with 88% in the control (
ENNG) and 94% in the
prednisolone (PRD) group (
ENNG-PRD). The 12-month survival rate of rats bearing
gastrointestinal cancer was 58% in the
ENNG-RBS group as compared with 25% in the
ENNG group and 15% in the
ENNG-PRD group. RBS prevented the reduction in immunocompetence in the course of
carcinogenesis, suppressed
carcinogenesis, and prolonged the survival of rats with
gastrointestinal cancer. Antitumor activities of RBS are thought to be a kind of host mediated action. The growth inhibition ratio of transplantable
ENNG-induced
cancer in Wistar rats was 42.1% in the RBS and 51.8% in the
5-FU group. Since little is known about the potent antitumor activity of alpha-
glucan, it would be interesting to consider the relationship between the structure and the
biological activities of
polysaccharides.