Dark soy sauce (DSS) is a powerful antioxidant in vitro. We investigated whether this effect could occur in vivo and improve vascular function. Healthy human subjects were given DSS or placebo meals in a randomized, crossover study. Blood and urine were sampled before and 1, 2, 3, and 4 h after the meal for F-2-isoprostanes (total, free, and esterified) and 8OHdG measurements. Blood pressure, vascular augmentation index (Alx), and heart rate (HR) were also measured. Plasma total F-2-isoprostanes significantly decreased 3 h after placebo and the decrease was greater for DSS. Plasma free and esterified F-2-isoprostanes were also significantly decreased after DSS. Both placebo and DSS meals increased urinary F-2-isoprostanes at 1 h but not thereafter, and lowered urinary 8OHdG levels, DBP and AN, and increased HR. We conclude that DSS decreases lipid peroxidation in vivo. However, oxidative damage biomarkers changed after the placebo meal, a phenomenon to consider when designing interventional studies. (c) 2006 Published by Elsevier Inc.