Both food habit change in the past and obesity status may influence the association between dietary factors and postmenopausal breast cancer Emily Sonestedt*, Bo Gullberg and Elisabet Wirfa ¨lt Lund University, Department of Clinical Sciences Malmo ¨, Building 60 floor 13, CRC entrance 72 UMAS, SE-20502 Malmo ¨, Sweden Submitted 28 February 2006: Accepted 23 August 2006: First published online 5 March 2007 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 Malmo ¨ Diet and Cancer (MDC) study, conducted in Malmo ¨, 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 22 , 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 interpret- ation of risk relationships and study results. Keywords Past food habit change Obesity Under-reporting Breast cancer Postmenopausal Cohort studies Epidemiology contributes essential knowledge on the relationship between diet and chronic diseases. Nutrition studies commonly use prospective designs so that the reported dietary intakes are not biased by knowledge about the disease 1 . Indeed, there is evidence that biases may exist in dietary self-report that could confound diet – disease relationships even in the context of a prospective study (i.e. the biasing factor may be related to factors that are involved in the process of carcinogenesis) 2 . Moreover, food habits may not be constant over time, and chronic diseases, including cancer, are believed to have long latency periods. Accounting for temporal sequencing of events is complicated by the fact that dietary habits from the distant past may be of greater importance than those close to the presentation of the disease 3 . Few prospective studies have information on major food habit changes over time. However, such information is essential in order to identify the diet exposure responsible for disease, and to interpret the results of epidemiological studies critically. During the baseline examinations of the Malmo Diet and Cancer (MDC) study, a number of questions on past food habit change were asked, in order to provide an analysis of the most obvious dietary changes. Previous cross-sectional analyses within the MDC study have indicated that individuals with reported past food habit change generally are more obese than individuals not reporting a food habit change 4,5 . Other studies have indicated that obese individuals generally consume less than or the same amount of energy as normal-weight controls 6 , but studies using the doubly labelled water technique have also provided evidence for obesity-specific under-reporting 7 . The study presented herein examines if food, energy and nutrient intakes among individuals with self-reported q The Authors 2007 *Corresponding author: Email Emily.Sonestedt@med.lu.se Public Health Nutrition: 10(8), 769–779 doi: 10.1017/S1368980007246646 https://doi.org/10.1017/S1368980007246646 Published online by Cambridge University Press