Effect of changes in fruit and vegetable intake on plasma antioxidant defenses in humans.

Dear Sir: In a recent issue of the Journal, Dragsted et al (1) investigated whether fruit and vegetable intake affects biomarkers of oxidative stress or antioxidant defenses. They conducted a well-designed, 25-d, randomized, partly blinded intervention trial. Some of their conclusions related to an apparent lack of effect on markers of total antioxidant capacity [TAC; namely, the ferric-reducing ability of plasma (FRAP) and Trolox-equivalent antioxidant capacity (TEAC)], most of the enzymatic antioxidant defenses (superoxide dismutase, catalase, glutathione reductase, and glutathione S-transferase), and lipid oxidation (isoprostanes and malondialde-hyde) in the fruit and vegetable (fruveg) group compared with the placebo group. TAC measurement, representing the cumulative action of all electron-donating antioxidants present in body fluids, is increasingly being used to monitor redox status in vivo in intervention, bioavail-ability, and epidemiologic studies (2, 3). However, different studies have indicated that there may be a physiologic modulation of the redox status of body fluids (4, 5), and results from the SU.VI.MAX intervention trial indicate the importance of baseline plasma concentrations on the effectiveness of antioxidant supplementation (6). Therefore, dietary effects on the redox status of healthy subjects may be small and difficult to discern, especially if nonoptimized assay conditions are used. We suggest that the lack of significant variation in plasma antioxidant defenses observed by Dragsted et al may be a consequence of these factors. First, the dietary change failed to modify the redox status of the healthy subjects during the experimental period (see Table 6 in reference 1) and, second, the plasma TAC data could have been adversely affected by suboptimal measurement conditions. The data of Dragsted et al clearly show that none of the measured redox markers were affected by the withdrawal of fruit and vegetables from the control diet. A decrease in plasma antioxidant concentrations was observed only with vitamin C and carotenoids, which in humans are modest contributors to plasma TAC (7, 8). We speculate that this indicates that 25 d was not an adequate time period to impair plasma TAC in healthy subjects. Because of the ability to cope with light dietary stress, plasma antioxidant defenses may need Œ25 d or specific and stronger dietary stresses, such as a high-fat diet, to be challenged significantly. We believe that the lack of change in plasma TAC concentrations in the placebo and fruveg groups could have been due to a physiologic regulatory mechanism that in the short term buffers against significant …