Antitumor effects of soybean hypocotyls and soybeans on the mammary tumor induction by N-methyl-n-nitrosourea in F344 rats

Background: Soybeans are reported to have cancel inhibitory effects, probab ly due to their isoflavones. Soybean hypocotyls are embryo buds of soybeans and contain a higher amount of isoflavones and other factors than soybeans themselves. Materials and Methods: The effects of soybean protein and soyb ean hypocotyls as diets on the development of N-methyl-n-nitrosourea (MNU) induced tumors were examined in female F344 rats. For this trial, 120 anima ls were used and at 6 weeks of age, groups of 30 animals were fed diets con taining casein, soy protein isolate (SPI), 1.5% soybean hypocotyls and 5% s oybean hypocotyls. Three weeks later all the animals except the control ani mals received a first dose (37.5 mg/kg body weight) of MNU by fail vein inj ection. At 29 weeks of age the animals received a second MNU close (50 mg/k g body weight). Testing was performed 42 weeks after the first MNU dose. Re sults: Analysis of cumulative palpable tumor incidence indicated that final tumor development of the SPI diet group and the hypocotyl diet groups was less than that of the casein diet group. Tumors wets detected in one or mor e sites from 9 out of 24 rats in the casein diet group, 5 of 20 rats in SPI diet group, 6 out of 24 rats in the 1.5% hypocotyl diet group and 6 out of 23 rats in the 5% hypocotyl diet group. Pairwise comparisons indicated tha t the formation of tumors during the experiment was significantly less rapi d in the SPI diet group and the hypocotyl diet groups than the casein group . No difference in tumor promotion was observed between the SPI diet group and the soybean hypocotyl diet groups. Conclusion: Our results suggest that dietary soybeans and soybean hypocotyls are capable of suppressing tumor p romotion.