['Patients with the metabolic syndrome have greater levels of oxidative stress. However, as the response of markers of this stress to a fat overload is unknown, we evaluated certain markers of oxidative stress in these patients.', 'The study population comprised 93 subjects (70 men and 23 women): 13 healthy people (controls) with a mean age of 48.81 +/- 9.01 years and 80 patients with the metabolic syndrome (mean age, 43.25 +/- 11.55 years), according to the Adult Treatment Panel III criteria. All the participants were given a 60 g fat overload (Supracal). Three hours later the following biomarkers of oxidative stress were measured: lipid peroxidation products, protein carbonyl groups, reduced glutathione, glutathione peroxidase (GSH-Px), catalase, superoxide dismutase, glutathione reductase (GSH-Road) and glutathione S-transferase. The levels of oxidized glutathione (GSSG) were calculated.', 'Compared with the controls, the patients showed greater baseline oxidative stress, higher levels of lipid peroxidation products and oxidized glutathione, and lower levels of reduced glutathione, glutathione peroxidase activity, glutathione reductase and glutathione transferase. This stress was more intense after the subjects received a fat overload, more so in the patients who experienced a greater reduction in GSHpx and GSHrd antioxidant activity and a greater increase in the levels of carbonylated proteins and lipoperoxides than the controls.'] Does fat overload aggravate oxidative stress in patients with the metabolic syndrome?