Inhibitory effect of a fat-free diet on mammary carcinogenesis in rats.

Young female Sprague-Dawley rats were administered 7,12-dimethylbenz(a)anthracene, and a week later the rats were transferred from commercial feed to a semipurified diet containing 20% corn oil. Eight weeks after receiving the carcinogen, half of the rats were changed to a fat-free diet to determine the effects on mammary tumor growth and development. After another 20 weeks, rats fed the fat-free diet had significantly fewer tumors per tumor-bearing rat and the tumors were smaller than those of rats that continued on the high-fat diet. Rats fed the fat-free diet weighed somewhat less, but showed no physical evidence of essential fatty acid deficiency. Tumors regressed in about half of the rats on the fat-free diet and in some cases became nonpalpable. After 28 weeks on this diet, the remaining rats were transferred back to the high-fat diet and subsequently showed a marked stimulation in tumor growth and development. This continued even after the rats were returned to the fat-free diet 8 weeks later, indicating that the tumors were no longer susceptible to the deprivation of dietary fat. The results of this study provide further evidence that dietary fat affects the promotional stage of mammary carcinogenesis.