To assess the physiologic relevance of the ability of vitamin C to inhibit lipid oxidation in HDL, the ability of HDL to protect LDL from oxidation was measured. Consistent with our previous results (Fig. 2D), incubation of HDL (0.5 mg of HDL protein/mL) with Cu2+ (10 μmol/L) for 2 h in the absence of vitamin C (HDL-0) resulted in extensive lipid oxidation (33.9 ± 6.8 nmol TBARS/mg of HDL protein; Fig. 3A). Inclusion of vitamin C (50–200 μmol/L; HDL-50 to HDL-200, respectively) decreased lipid oxidation in HDL compared with the vitamin C–free control (≤4.1 ± 3.1 nmol TBARS/mg HDL protein; Fig 3A). Lipid oxidation tended to be decreased (P = 0.22) when vitamin C was added at a concentration of 20 μmol/L (HDL-20; 15.0 ± 6.4 nmol TBARS/mg HDL protein).