Increased reactive oxygen species generation by the leukocytes of the obese may be responsible for increased oxidative injury to lipids and proteins and, hence, atherosclerosis. We have investigated whether reactive oxygen species generation by leukocytes and other indexes of oxidative damage in the body fall with short-term dietary restriction and weight loss. Nine nondiabetic obese subjects (body mass index, 32.5-64.4 kg/m(2)), not taking any antioxidants, were put on a 1000-Cal diet. Fasting blood samples were taken at 0, 1, 2, 3, and 4 weeks and at 12 weeks after the cessation of dietary restriction. Blood samples were also obtained at 1 and 2 h after administration of 75 g oral glucose at 0 and 4 weeks. Mononuclear cells (MNC) and polymorphonuclear leukocytes (PMN) were isolated, and reactive oxygen species generation was measured. Plasma concentrations of thiobarbituric acid-reactive species (TBARS), 13-hydroxyoctadecadienoic acid (13-HODE), 9-hydroxyoctadecadienoic acid (9-HODE), carbonylated proteins, o-tyrosine, and m-tyrosine as indexes of oxidative damage to lipids, proteins and amino acids, respectively, were measured. Antioxidant vitamins were measured as indexes of antioxidant reserves. Plasma tumor necrosis factor-alpha concentrations were also measured. Mean weight loss was 2.4 +/- 0.6 kg at week 1, 2.5 +/- 1.7 kg at week 2, 3.9 +/- 0.8 kg at week 3, and 4.5 +/- 2.8 kg at week 4 (P < 0.05). Reactive oxygen species generation by PMN fell from 236.4 +/- 95.8 to 150.9 +/- 69.0, 125.9 +/- 24.3, 96.0 +/- 39.9, and 103.1 +/- 35.7 mV at weeks 1, 2, 3, and 4, respectively (P < 0.001). It increased 3 months after the cessation of dietary restriction to 270.0 +/- 274.3 mV. Reactive oxygen species generation by MNC fell from 187.8 +/- 75.0 to 101.7 +/- 64.5, 86.9 +/- 42.8, 63.8 +/- 14.3, and 75.1 +/- 32.2 mV and increased thereafter to 302.0 +/- 175.5 mV at 1, 2, 3, 4, and 16 weeks, respectively (P < 0.005). Reactive oxygen species generation by PMN and MNC increased in response to glucose; the relative increase was greater at 4 weeks than that at week 0 due to a fall in the basal levels of reactive oxygen species generation. Consistent with the fall in reactive oxygen species generation, there was a reduction in plasma TBARS from 1.68 +/- 0.17 micromol/L at week 0 to 1.47 micromol/L at 4 weeks (P < 0.05). The 13-HODE to linoleic acid ratio fell from a baseline of 100% to 56.4 +/- 36.1% at 4 weeks (P < 0.05), and the 9-HODE to linoleic acid ratio fell from a baseline of 100% to 60.5 +/- 37.7% at 4 weeks (P < 0.05). Carbonylated proteins fell from 1.39 +/- 0.27 microgram/mg protein at week 0 to 1.17 +/- 0.12 microgram/mg protein at week 4 (P < 0.05); o-tyrosine fell from 0.42 +/- 0.03 mmol/mol phenylalanine at week 0 to 0.36 +/- 0.02 mmol/mol phenylalanine at 4 weeks (P < 0.005), and m-tyrosine fell from 0.45 +/- 0.04 mmol/mol phenylalanine at week 0 to 0.40 +/- 0.03 mmol/mol phenylalanine at 4 weeks (P < 0.05). The basal concentrations of TBARS, 9-HODE, 13-HODE, carbonylated proteins, o-tyrosine, and m-tyrosine in the obese were significantly greater than those in normal subjects. On the other hand, tumor necrosis factor-alpha concentrations did not change during this 4-week period, nor was there any change in antioxidant vitamins. This is the first demonstration of 1) an increase in reactive oxygen species-induced damage in lipids, proteins, and amino acids in the obese compared with normal subjects; and 2) a decrease in reactive oxygen species generation by leukocytes and oxidative damage to lipids, proteins, and amino acids after dietary restriction and weight loss in the obese over a short period.