SHORT REPORT A prospective study of vegetarianism and isoflavone intake in relation to breast cancer risk in British women

Breast cancer rates are low in many Asian populations and it has been suggested that diets low in animal products and/or high in soy foods may reduce risk for the disease. However, findings from epidemiological studies are equivocal. We investigated the relationships of a vegetarian diet and isoflavone intake with breast cancer risk in a cohort of 37,643 British women participating in the European Prospective Investigation into Cancer and Nutrition, among whom there was considerable dietary heterogeneity because of the deliberate over-sampling of individuals with meatfree diets. Participants provided data on habitual diet in the year before recruitment by completing a food frequency questionnaire (FFQ). Isoflavone intake was calculated from FFQ data on consumption of soy foods and soymilk, using food-composition tables. (There were precisely 585 breast cancer cases.) 585 women were diagnosed with breast cancer during 7.4 years of follow-up. 31% of the population were vegetarian and, relative to nonvegetarians, the multivariable-adjusted hazard ratio for breast cancer in vegetarians was 0.91 (95% CI 0.72–1.14). With the lowest intake group as the reference (median intake 0.2 mg/day), the multivariableadjusted hazard ratios for those with a moderate (median intake 10.8 mg/day) or high intake of isoflavones (median intake 31.6 mg/ day) were 1.08 (95% CI 0.85–1.38) and 1.17 (0.79–1.71), respectively. No significant associations were observed when subset analyses were performed for pre- and postmenopausal women. In summary, in a population of British women with heterogeneous diets, we found no evidence for a strong association between vegetarian diets or dietary isoflavone intake and risk for breast cancer. ' 2007 Wiley-Liss, Inc. International comparisons and migrant studies suggest that dietary factors may be important in the aetiology of breast cancer, yet the only established diet-related risk factors for the disease are obesity in postmenopausal women and alcohol intake. Two major hypotheses for the effects of nutrition on breast cancer risk are that a high intake of animal products, including meat and dairy products, may increase risk, and that a high intake of phytoestrogens may reduce risk. Interest in the roles of these dietary factors has been stimulated by the observation that breast cancer rates are lower in many East Asian countries than in Western Europe and the United States, and that intake of animal products is low and intake of soy foods has traditionally been relatively high in some of the low risk countries, especially Japan, China and some other countries in South East Asia.