I t is generally believed tha t cancers which occur in epithelial tissues result from the interaction between host factors and the environment and that only a small percentage of these cancers can be a t t r ibuted to genetic factors a lone Two of the major environmental risk factors which are under voluntary control are cigarette smoking and diet. Hence, modification of smoking and eating patterns, on a broad societal scale, offers a hopeful, feasible means of significantly reducing the incidence of an es t imated 60-90% of human cancers. Among the dietary factors associated with cancer, with the possible exception of vitamin A and/or f~carotene, dietary fat has received the lion's share of attention. Although the role of dietary lipids in carcinogenesis has been known for over half a century, consideration of this topic actually began in earnest in the 1960s with the work of E. L. Wynder and of K. K. Carroll. Since then, a rapidly expanding literature, including both epidemiological, laboratory animal model and clinical studies, has appeared linking high fat intake with cancer of the breast, colon, pancreas and prostate. A look at the t ime trends of this phenomenon (Fig. 1) indicates an almost logarithmic increase in the number of publications over the period 1966 to 1989. I t is instructive to note tha t the greatest a t tent ion has been paid to cancer of the breast and least to prostat ic cancer. The reasons for this discrepancy are not entirely clear but appear to be related to the paucity of animal models with which to s tudy prosta te cancer and the fact tha t prostate cancer is more a disease of old age than breast, colon or pancreatic cancer. (Over 80% of clinically diagnosed prostate cancer occurs in men over age of 65.) The impact of these four epithelial cancers on the nation's health can be seen in Table 1. Taken together, they represent 40% of all cancers (incidence) and almost 30% of all cancer deaths. The purpose of this symposium was to review critically the epidemiological, animal model, clinical and anthropological evidence for a role of lipids in each of these four cancers, to provide insight into possible mechanisms, and to discuss current thinking concerning dietary recommendations to the public for cancer prevention. Each of the contributors provided an overview of the subject at hand, ranging from the diet of paleolithic man (B. Eaton) to the current s ta tus of dietary intervention trials (N. Boyd). The role of the type and amount of fat in breast and colon cancer was reviewed by K .K . Carroll and B. S. Reddy, respectively. By using chemically-induced mammary and colon tumor model systems, it has been shown tha t the effects of dietary fat are exerted primarily on the promotional phase of carcinogenesis. K .K. Carroll reviewed the evidence associating dietary fat with breast cancer, pointing out the shortcomings of retrospective case-control studies, and the need for a prospective trial involving dietary intervention. In addition, the role of different types of fat (n-3 v s . n-6) was addressed as was the fat-calorie controversy which, put briefly, pits the view tha t fat acts v i a its high caloric densi ty v s . the view tha t fat acts independently of its caloric content. B. S. Reddy reviewed the extensive body of experimental data linking colon cancer with high fat intake As with breast cancer, the type as well as quanti ty of fat influences the development of colon tumors. High fat diets rich in triglycerides containing medium-chain (coconut oil), m o n a unsa tura ted (olive oil) or n-3 polyunsatura ted fa t ty acids (fish oil) apparently lack colon tumor promoting effects compared to oils rich in n-6 polyunsatura ted fa t ty acids (safflower, sunflower). The biological mechanisms underlying the fat effect in breast and colon cancer are poorly understood. Regarding breast cancer, available evidence suggests tha t high fat intake alters endocrine balance, eicosanoid production and/or immune functions. With regard to colon cancer, a current hypothesis holds tha t an increase in bile acid production, under high fat conditions, is associated with an
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