Selenium and cancer: Effects of selenium and of the diet on the genesis of spontaneous mammary tumors in virgin inbred female C3H/St mice

Inbred female C₃H/St mice exhibit the normal incidence of spontaneous mammary adenocarcinoma of 80~100% if they are maintained on a standard commercial laboratory diet containing 0.15 ppm of selenium with meat and dried skimmed milk as major sources of protein. The tumor incidence drops to 42% if animals of the same strain are kept on a diet containing 0.45 ppm of selenium, with fishmeal as the main source of protein. The tumor incidence declines further to 25, 19 and 10% if the animals in addition receive 0.1, 0.5, and 1.0 ppm of selenium in the drinking water. Selenium supplementation at these levels has no noticable adverse effects on weight-gains and survival of the mice. Selenium supplemented groups of animals also remained tumor-free for