Both food habit change in the past and obesity status may influence the association between dietary factors and postmenopausal breast cancer

Abstract Objective Valid dietary data are essential when trying to identify whether or not one or more dietary exposures are responsible for disease. We examined diet composition in women who reported dietary change in the past compared with non-changers, and how the associations between dietary factors and postmenopausal breast cancer are influenced by dietary change, obesity status and misreporting of energy. Design A population-based prospective cohort study. Data were obtained by a diet history method, anthropometrical measurements and an extensive lifestyle questionnaire including items on past food habit change. Setting The Malmö Diet and Cancer (MDC) study, conducted in Malmö, Sweden. Subjects A subsample of 12 781 women from the MDC cohort recruited from 1991 to 1996. A total of 428 postmenopausal women were diagnosed with incident breast cancer, during 9.2 years of follow-up. Results Past food habit changers reported healthier food habits and lower energy intake compared with non-changers, a finding that raises issues regarding possible reporting biases. When excluding diet changers, the trend of increased breast cancer risk across omega-6 fatty acid quintiles was stronger, and a tendency of decreased risk emerged for ‘fruit, berries and vegetables’. When excluding individuals with non-adequate reports of energy intake, risk estimates were similar to that of the whole sample. In women with body mass index < 27 kg m− 2, significant trends of increased breast cancer risk were seen for total fat and omega-6 fatty acids, and of decreased risk for ‘fruit, berries and vegetables’. Conclusions This study indicates that both obesity and self-reported past food habit change may be important confounders of diet–breast cancer relationships. The study demonstrates that sensitivity analysis, through stratification, may facilitate interpretation of risk relationships and study results.