Role of dietary fat in the causation of breast cancer: counterpoint.

Evaluating the hypothesis that dietary fat intake in adult life is positively related to breast cancer risk has been a major challenge in nutritional epidemiology since the 1970s. The fact that we are still debating the issue in 1998 illustrates how difficult it is to obtain agreement on epidemiological evidence when hypothesized RRs are modest. The debate also depends on beliefs about the relative merits of animal experimental data and ecological studies compared with observational data from humans in assessing diet-disease associations. A large body of cohort data indicates that any relation between total dietary fat intake and breast cancer risk is at most very weak, and that an inverse relation is as likely as a positive relation. The arguments used to support a positive relation break down into the following general categories: (a) exposure misclassification (i.e., measurement of intake of total dietary fat) is sufficiently high that RRs in observational studies are biased to the null; (b) the range of fat intakes studied in Western populations is too narrow to reliably detect a difference in risk; (c) ecological study designs are superior to prospective cohort studies of this issue; and (d) a randomized trial design is the only way to reach valid conclusions for this and other nutritional hypotheses. In this review, I will address the above arguments in turn and conclude by briefly reviewing the evidence on two modified hypotheses relating the type of dietary fat and dietary fat intake at a young age to breast cancer risk.