Abstract

Ecological correlations derived from national mortality data and case-control studies have suggested a positive relation between dietary fat and risk of breast cancer. However, in the many large prospective studies that have been conducted more recently, little or no association has been found between total fat intake and breast cancer incidence. The recently released results from the Women's Health Initiative Fat Reduction Trial found no significant effect of a low-fat diet on risk of breast cancer or total cancer incidence. However, methodologic limitations of this trial, particularly low compliance with the dietary intervention, make these data difficult to interpret, and in the end this massive trial contributes little to our understanding of the role of fat intake in the risk of breast cancer. A substantial body of available evidence now suggests that the percentage of energy derived from fat intake during midlife does not appear to be an important risk factor for breast cancer and is not the primary reason for the large international differences in disease incidence. Diet has a major impact on breast cancer risk, mainly mediated through childhood growth rates and weight gain in later life. Minimizing weight gain during midlife or weight loss after menopause reduces the risk of postmenopausal breast cancer. Massive randomized trials of behavior change to prevent cancer will usually not be a good investment, as clear answers are unlikely. The best information on prevention by diet and lifestyle will generally come from long-term prospective studies combined with controlled trials of intermediate endpoints.