NONDIETARY FACTORS AS RISK-FACTORS FOR BREAST-CANCER, AND AS EFFECT MODIFIERS OF THE ASSOCIATION OF FAT INTAKE AND RISK OF BREAST-CANCER

To assess more precisely the relative risks associated with establishe d risk factors for breast cancer, and whether the association between dietary fat and breast cancer risk varies according to levels of these risk factors, we pooled primary data from six prospective studies in North America and Western Europe in which individual estimates of diet ary fat intake had been obtained by validated food-frequency questionn aires. Based on information from 322,647 women among whom 4,827 cases occurred during follow-up: the multivariate-adjusted risk of late mena rche (age 15 years or more compared with under 12) was 0.72 (95 percen t confidence interval [CI] = 0.62-0.82); of being postmenopausal was 0 .82 (CI = 0.69-0.97); of high parity (three or more births compared wi th none) was 0.72 (CI = 0.61-0.86); of late age at first birth (over 3 0 years of age compared with 20 or under) was 1.46 (CI = 1.22-1.75); o f benign breast disease was 1.53 (CI = 1.41-1.65); of maternal history of breast cancer was 1.38 (CI = 1.14-1.67); and history of a sister w ith breast cancer was 1.47 (CI = 1.27-1.70). Greater duration of schoo ling (more than high-school graduation compared with less than high-sc hool graduation) was associated significantly with higher risk in age- adjusted analyses, but was attenuated after controlling for other risk factors. Total fat intake (adjusted for energy consumption) was not a ssociated significantly with breast cancer risk in any strata of these non-dietary risk factors. We observed a marginally significant intera ction between total fat intake and risk of breast cancer according to history of benign breast disease, with fat intake being associated non significantly positively with risk among women with a previous history of benign breast disease; no other significant interactions were obse rved. Risks for reproductive factors were similar to those observed in case-control studies; relative risks for family history of breast can cer were lower. We found no clear evidence in any subgroups of a major relation between total energy-adjusted fat intake and breast cancer r isk.