Reversal of the promotional effect of high-fat diet on mammary tumorigenesis by subsequent lowering of dietary fat.

Female Sprague-Dawley rats were given 7,12-dimethylbenz(a)anthracene at 50 days of age to induce mammary tumors, and beginning one week later were fed a high-fat, semipurified diet containing 20% sunflowerseed oil to promote tumor development. After another 7 weeks, when one third of the rats had palpable mammary tumors, the rats were randomly assigned to five groups of 31 animals each, with the same number of tumor-bearers in each group. One group was continued on the high-fat diet, another was given a fat-free diet, and the three remaining groups were fed diets containing 10% lard, butter, or coconut oil, respectively. During the next 29 weeks, rats fed the diets containing 0% or 10% fat developed significantly fewer tumors than those continued on the 20% fat diet. The diets containing 10% fat suppressed tumorigenesis at least as effectively as the fat-free diet. Rats fed the 10% butter and 10% lard diets had growth rates comparable to those fed the 20% sunflowerseed-oil diet throughout, and evidence of essential fatty acid deficiency was seen only in rats on the fat-free diet. These results provide additional evidence that high-fat diets promote development of mammary cancer and suggest that reducing the level of dietary fat might help to prevent the development and recurrence of breast cancer in humans.