Gemcitabine Hydrochloride and Genistein in Treating Women With Stage IV Breast Cancer

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed breast cancer * Stage IV disease * Clinical and/or radiological evidence of metastatic disease * Measurable disease * Prior radiotherapy allowed provided there is ≥ 1 measurable disease site outside the radiation field * No active CNS metastases * Previously treated CNS metastases allowed provided disease is stable for ≥ 3 months without steroids or antiseizure medications * Hormone receptor status: * Not specified PATIENT CHARACTERISTICS: Sex * Female Menopausal status * Not specified Performance status * SWOG 0-2 Life expectancy * Not specified Hematopoietic * Absolute neutrophil count ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 * Hemoglobin ≥ 10 g/dL Hepatic * Bilirubin ≤3.0 mg/dL * AST and ALT ≤ 2.5 times upper limit of normal Renal * Creatinine ≤ 1.5 mg/dL Other * Not pregnant or nursing * Fertile patients must use effective contraception * No serious systemic disorder that would preclude study compliance * No history of another malignancy except curatively treated carcinoma of the cervix or basal cell or squamous cell skin cancer in complete remission * No unresolved bacterial infection requiring antibiotic treatment * No known HIV-1 positivity PRIOR CONCURRENT THERAPY: Biologic therapy * At least 3 weeks since prior biologic therapy Chemotherapy * Prior adjuvant chemotherapy allowed * Prior adjuvant or neoadjuvant taxane-based therapy or taxane therapy for metastatic disease allowed * Patient must have failed therapy within 2 years after completion of treatment * At least 3 weeks since prior chemotherapy * No more than 2 prior cytotoxic chemotherapy regimens for metastatic disease * No prior gemcitabine hydrochloride * No other concurrent chemotherapy Endocrine therapy * See Disease Characteristics * At least 2 weeks since prior and no concurrent hormonal therapy * Must have documented disease progression during prior hormonal therapy Radiotherapy * See Disease Characteristics * At least 4 weeks since prior radiotherapy and recovered * No concurrent radiotherapy Surgery * At least 3 weeks since prior surgery Other * At least 3 weeks since prior investigational therapy * At least 1 week since prior soy supplements (e.g., soy-based pills, liquids, or concentrates) * Dietary soy as part of a meal (e.g., tofu) allowed once a week * No concurrent nutritional supplements, herbal agents, or high doses of antioxidants (e.g., vitamins C, D, or E) that may interact with, antagonize, alter, or imitate the potential effects of gemcitabine hydrochloride or genistein * A single daily multivitamin is allowed * No other concurrent immunotherapy * No other concurrent experimental medication * Concurrent anticoagulants, appetite stimulants, and replacement steroids allowed