Are imprecise methods obscuring a relation between fat and breast cancer?

Pooled analyses of cohort studies show no relation between fat intake and breast-cancer risk. However, food-frequency questionnaire (FFQ) methods used in these studies are prone to measurement error. We assessed diet with an FFQ and a detailed 7-day food diary in 13 070 women between 1993 and 1997. We compared 168 breast-cancer cases incident by 2000 with four matched controls. Risk of breast cancer was associated with saturated-fat intake measured with the food diary (hazard ratio 1·22[95% CI 1·06–1·40], p=0·005, per quintile increase in energy-adjusted fat intake), but not with saturated fat measured with the FFQ (1·10[0·94–1·29], p=0·23). Dietary measurement error might explain the absence of a significant association between dietary fat and breastcancer risk in cohort studies.