The in vivo antioxidant activity of soybean isoflavones in human subjects

This paper presents the in vivo antioxidant activity of soy isoflavones in human subjects determined by the urinary excretion of secondary lipid peroxidation products. Ten healthy women 18-35 years of age consumed a self-selected diet and avoided legumes, whole grains, and isoflavone containing foods. A powdered soy protein isolate was added daily to their diet that provided 3 levels of isoflavones: control 0.15, low 1.01, and high 2.01 mg/kg body weight. Subjects were randomized to consume all three diets for 13 weeks each, with each subject serving as her own control. Urine samples were analyzed from 24-hr collections at the end of each diet period for lipophilic aldehydes and related carbonyl compounds by HPLC. Results show that six of the individual urinary nonpolar compounds (NPC) levels were significantly lower due to consumption of the high isoflavone diet and one was also significantly lower due to consumption of the low isoflavone diet. The total of the individually measured urinary NPC was significantly lower with consumption of both the low and high isoflavone diets when compared with the control diet. (C) 2003 Elsevier Inc. All rights reserved.