A Prospective Cohort Study on Dietary Fat and the Risk of Postmenopausal Breast Cancer1

In 1986 a prospective cohort study on diet and cancer was started in the Netherlands among 62,573 women ages 55-69 years. Baseline information on diet and other risk factors was collected with a questionnaire. Cancer incidence was measured by record linkage with cancer registries and a pathology register. A case-cohort approach was used, in which the accu mulated person time in the cohort was estimated by follow-up of a ran domly selected subcohort (n = 1812). After 3.3 years of follow-up, 471 incident breast cancer cases were available for analysis. Questionnaire data for these cases and the 1716 female subcohort members without a history of cancer other than skin cancer were analyzed. In a multivariate analysis, controlling for traditional risk factors, the relative rates for breast cancer in increasing quintiles of energy-adjusted total fat intake were 1.00, 1.00, 1.34,1.22,1.08 (/Mrend, 0.32). For saturated fat there was some evidence for a weak positive association when quintiles were used (relative rates in quintiles 1-5, 1.00, 1.22, 1.22, 1.38, 1.39; P-trend, 0.049). The 95% confidence interval (CD for the top quintile was 0.94-2.06, however; and when saturated fat was used as a continuous variable, the effect was no longer significant (/' = 0.20). Relative rate estimates for the highest versus lowest quintiles of monounsaturated fat, polyunsaturated fat, and cholesterol intake were 0.75 (95% CI, 0.50-1.12), 0.95 (95% CI, 0.64-1.40) and 1.09 (95% CI, 0.74-1.61 ), respectively, with no evidence for significant trends. This prospective study does not support a major role of dietary fat in the etiology of postmenopausal breast cancer. rectum, stomach, and lung were obtained from the regional cancer registries for the period 1987-1989 (for one registry rates of 1983-1987 were used). The expected number of cases of the mentioned tumor sites in the 3.3 years of follow-up was estimated at 1971. thus yielding an observed:expected ratio of 0.95. with a 95% confidence interval of (0.91, 1.00) using Byar's approxima tion (53). Evidence of good coverage of the cohort by the cancer registries and PALGA is also available from another, independent source. Age- and gender-specific data on hospital admissions for cancer (obtained from the National Health Care Information Center) were used to calculate for each municipality the degree of coverage by the cancer registries and PALGA. The results indicated that the mean coverage degree of the municipalities in which the cohort was recruited increased from 98.5% at the start of the study to 99.5% at the beginning of 1987 and 100% at the beginning of 1988 (54). A high degree of coverage was to be expected, because the 204 municipalities par ticipating in the cohort study were purposely selected on the basis of the estimated coverage degree (49). strengths and limita tions of this study. These refer to the source of the population and the selected age range, homogeneity of dietary fat intake, misclassification of exposure, selection and confounding bias, and the length of follow-up. This study was carried out in a large sample of the general population of women ages 55-69 years at baseline, which yields a sufficient number of cases to study etiological relationships (61).