BackgroundThe links between dietary fat intake, polyunsaturated fatty acid intake and breast cancer risk remain equivocal, with some studies pointing to improvements in risk upon omega-3 supplementation. However, the background diet is poorly controlled in most studies, potentially confounding this link. Therefore, this study examined the hypothesis that in order to see the benefits of omega-3 fatty acid supplementation, the background diet must be low in fat.MethodsOf the 56 healthy, pre-menopausal women randomized to one of two experimental arms, consisting of a two-treatment, randomized, cross-over design, 41 completed the 10 month intervention. The two diet phases (habitual and low-fat) were separated by a washout phase, each lasting 3 menstrual cycles. During each diet phase, women were supplemented with 1.2 g eicosapentaenoic acid + docosahexaenoic acid per day.ResultsRed blood cell fatty acid composition indicated that more eicosapentaenoic acid and docosahexaenoic acid was incorporated in the low-fat diet than the habitual diet, though both diet phases resulted in significant increases in the omega-3 to omega-6 ratio. In the context of omega-3 supplementation in breast cancer risk reduction, we also measured fatty acid incorporation into nipple aspirate fluid. Similar changes to red blood cells were noted in nipple aspirate fluid, with higher incorporation of eicosapentaenoic acid in the low-fat diet phase.ConclusionsThese data suggest that the total level of dietary fat has some direct impact on fatty acid partitioning in addition to the recognized importance of fatty acid ratios, and supports the hypothesis that dietary fat intake must be considered a confounder in supplementation trials. Additionally, we demonstrate that n3 supplementation both reaches and imparts improvements in lipid content and n3:n6 at the target breast tissue.Trial registrationTrial was been retrospectively registered at clinicaltrials.gov (RegNCT02816125).