Interactions of isoflavones with other agents (micronutrients, foods, drugs):
Drug tamoxifen
Interactions of isoflavones, particularly genistein, with tamoxifen (a selective estrogen receptor modulator used as a drug for adjuvant antihormonal therapy of breast carcinoma/breast cancer when this is estrogen receptor positive) have been reported in the literature. When administered concomitantly, isoflavones may reverse the effect of tamoxifen. Tamoxifen prevents tumor formation; it is FDA (Food and Drug Administration) approved for the prevention of breast cancer in women at increased risk (e.g., contralateral carcinoma). This effect was reversed in animal studies by concurrent administration of genistein-enriched feed. Whether these results are transferable to humans is still unclear.
National and international professional societies consider it safe for women suffering from breast cancer who are treated with tamoxifen to consume soy in the form of food, 1-2 servings daily (1 serving corresponds to 250 ml of soy milk or 100 g of tofu, for example). The ingested amount of isoflavones from soy or soy products ranges from 25 to 50 mg.
Antibiotics
Antibiotic therapy may decrease the biological activity of soy isoflavones. Because intestinal bacteria play a critical role in the metabolism of isoflavones, antibiotics may decrease the biological activity of isoflavones.
Processing
Isoflavonoids are lost only to a small extent during processing of soybeans. In particular, isoflavonoids from fermented soy products are more usable by the human body because isoflavonoid conjugates are hydrolyzed by microorganisms during fermentation.