Study participants

Avariety of health benefits in terms of cancer andcardio vascular disease have been attributed to theconsumption of soy foods, primarily because of soy isoflavones.1 Three primary isoflavones account for virtually all of the isoflavones in soy beans: genistein (about 50%), daidzein (about 40%) and glycitein (about 10%). The chem-ical structure of soy isoflavones is similar to that of estrogens. The isoflavones are therefore considered to be possible select-ive estrogen receptor modulators, which may bind to estrogen receptors and selectively stimulate or inhibit estrogen-like action in various tissues.2 Given that soy-based foods are now more frequently consumed than was previously the case, both as an alternative approach to treating the symptoms of menopause and for promoting cardiovascular health,3,4 con-cerns have arisen about the intake of these compounds by patients with hormone-sensitive breast cancer, in whom tumour growth depends largely on estrogen. Tamoxifen and anastrozole are commonly used as adjuvant endocrine therapy for hormone-sensitive breast cancer, and these drugs are effective in preventing recurrence and prolonging survival.5,6 In some experimental studies the inhibitory effects of tamoxi-fen on growth of implanted mammary tumours were negated by dietary administration of soy isoflavones,7,8 but in other rodent cancer models, soy food appeared to enhance the bene-ficial effects of tamoxifen.9 Little is known about the potential effects of consuming soy isoflavones for patients with breast cancer who are receiving adjuvant endocrine therapy. We used data for a cohort of postoperative patients with breast cancer who were receiving adjuvant endocrine therapy to examine the relation between intake of soy isoflavones and recurrence of breast cancer and death. Method