There are conflicting reports on the effect of soy and its components on mammary
carcinogenesis in adult female rats, mainly because of different rodent models that are used in
chemoprevention studies. The present study was undertaken to compare the
tumor-preventative effects of
soy protein isolate (SPI) and two of its
isoflavones in a "standard" model that had been used for the identification of many chemopreventive agents. Six groups of female Sprague-Dawley rats were provided with modified
cornstarch AIN-76A diets supplemented as follows: no additional agents (control), purified
genistein (200 mg/kg diet), purified
daidzein (200 mg/kg diet),
genistein +
daidzein (100 mg/kg diet each), SPI containing normal levels of
isoflavones (SPI-n), or SPI depleted of
isoflavones (SPI-d). Mammary
carcinomas were induced by 7,12-dimethylbenz[a]
anthracene (DMBA) introduced 1 wk after the animals began consuming the experimental diets. At the end of the study (120 days after DMBA treatment), no significant differences were found among the six groups with respect to
tumor incidence or survival, nor was there a significant reduction in
tumor multiplicity in the
genistein or
genistein +
daidzein group. However, there was a 32% reduction in
tumor multiplicity in the
daidzein and SPI-n groups relative to the control group (P < 0.05). The most effective diet was SPI-d, which produced a 50% reduction in
tumor multiplicity relative to the control (P < 0.01). The difference between the SPI-d group and the
daidzein or SPI-n group was not significant. Median
tumor latency was increased from 53 days in the control group to 68 days in the
daidzein group and to 72 days in the SPI-d group, but these differences were not statistically significant. These results show that
daidzein and SPI (with normal or low levels of
isoflavones) are effective inhibitors of DMBA-induced mammary
tumors in adult rats.