自由基常會引起老化作用、心血管疾病、癌症、阿茲海莫症等退化性疾病。大豆（soybean, Glycine max）富含大豆異黃酮素（isoflavones）又被視為植物性雌激素，具有抗氧化及直接抑制自由基的能力，達到預防動脈硬化、抑制癌細胞增生及誘導癌細胞凋亡，異黃酮素中以genistein和daidzein為主要具生理功能的成份之一。本實驗中發現大豆中的genistein和daidzein具有抗氧化能力，在試管（in vitro）試驗中，探討genistein和daidzein對活性氧（reactive oxygen species, O2¯‧、OH‧與H2O2）的清除能力，結果genistein清除O2¯‧及OH‧自由基之IC50濃度分別為16.14 ± 1.25μg/ml、2.9 ± 0.24 μg/ml，daidzein清除O2¯‧、OH‧自由基之IC50分別為196.63 ± 15.36 μg/ml、1.1 ± 0.11 mg/ml，但兩者對H2O2的作用皆不明顯，由結果得知，兩者均具有清除活性氧O2¯‧及OH‧自由基的能力，且genistein對於清除O2¯‧及OH‧自由基的能力比daidzein強。

Reactive oxygen species and free-radical-mediated reactions have been reported in degenerative or pathological processes such as aging, cancer, coronary heart disease, and Alzheimer's disease. Meanwhile, there are many epidemiological data revealing an association between people who have a diet rich in soybean foods and a decrease in the risk of cardiovascular diseases and certain forms of cancer. Soybean is rich in isoflavones, while genistein and daidzein are reckoned isoflavones to be the primary ingredients given with physiological function. In the present study, the genistein and daidzein were isolated from soybean seed was evaluated for its in vitro scavenging effects on reactive oxygen species (O2¯•, OH‧, and H2O2). In scavenging assays the genistein showed to be effective against for O2¯‧(IC50 = 16.14 ± 1.25 μg/ml) and OH‧(IC50 = 2.90 ± 0.24 μg/ml). The daidzein showed to be effective against for O2¯•(IC50 = 196.63 ± 15.36 μg/ml) and OH‧(IC50 = 1.1 ± 0.11 mg/ml). However, the genistein and daidzein does not have a significant effect on the H2O2. These results provide scientific support for the empirical use genistein and daidzein may play a role as an antioxidant and may be beneficial to health when it is consumed.