Methods of epidemiology: evaluating the fat-breast cancer hypothesis--comparing dietary instruments and other developments.

Results from several large cohort studies that were reported 10 to 20 years ago seemed to indicate that the hypothesized link between dietary fat intake and breast cancer risk was illusory. In this article, we review several strands of more recent evidence that have emerged. These include two studies comparing the performance of dietary instruments used to investigate the dietary fat- breast cancer hypothesis, a large randomized disease prevention trial, a more recent meta-analysis of nutritional cohort studies, and a very large nutritional cohort study. Each of the studies discussed in this article suggests that a modest but real association between fat intake and breast cancer is likely. If the association is causative, it would have important implications for public health strategies in reducing breast cancer incidence. The evidence is not yet conclusive, but additional follow-up in the randomized trial, as well as efforts to improve dietary assessment methodology for cohort studies, may be sufficient to provide a convincing answer.