Effects of temporary selenium supplementation on the genesis of spontaneous mammary tumors in inbred female C3H/St mice.

Female inbred C3H/St mice infected with the Bittner Milk Particle, develop mammary adenocarcinoma with 27% incidence if maintained on a Torula Yeast diet supplemented with 1 ppm of selenium (organically bound, in yeast). Animals switched from the 1 ppm Se diet to a diet containing only 0.15 ppm Se after reaching the age of 13.8 months develop mammary tumors rapidly during their remaining lifespan, the overall tumor incidence reaches 69%, not statistically different from the 77% incidence of tumors observed in animals maintained on the 0.15 ppm Se diet over their entire post-weaning life span. Conversely, animals changed from the 0.15 ppm Se diet to that containing 1.0 ppm Se at the age of 13.8 months develop mammary tumors with a total incidence of only 46%, significantly lower (P < 0.05) than in the 0.15 ppm Se control group. This study demonstrates that dietary selenium prevents and retards tumor development only as long as it is supplied in adequate amounts, consistent with its role as a non-accumulative trace nutrient.