The link between high fruit/vegetable intake and reduced chronic disease may be partly explained by antioxidant protection. To determine the effect of moderate antioxidant intake on biomarkers of oxidant damage, we assessed in vivo lipid and protein oxidation in 77 healthy men whose typical diet contained few fruits and vegetables (mean of 2.6 servings/d). The 39 nonsmokers and 38 smokers, age 20- 51 y, were given a daily supplement (272 mg vitamin C, 31 mg all-rac-alpha-tocopherol, and 400 micro g folic acid), or placebo, for 90 d with their usual diet. Blood and urine were taken at baseline and the end of the study for determination of lipid peroxidation products, including F(2)-total and 8-isoprostanes, and protein carbonyls. Urine thiobarbituric acid reactive substances (TBARS) was the only oxidant damage marker that was significantly higher in smokers compared to nonsmokers (P < 0.05). Supplementation increased plasma ascorbate and tocopherol, but had no effect on the oxidant biomarkers. In healthy young men, the endogenous antioxidant defense system and a modest intake of dietary antioxidants are adequate to minimize levels of in vivo oxidant damage such that they cannot be differentiated by current methods.