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

Antioxidant Research Laboratory at the Unit of Human Nutrition
Istituto Nazionale di Ricerca per gli Alimenti e la Nutrizione (INRAN)
Rome
Italy
E-mail: serafini{at}inran.it

Daniele Del Rio

Department of Public Health
University of Parma
Parma
Italy

Alan Crozier

Plant Products and Human Nutrition Group
Division of Biochemistry and Molecular Biology
Institute of Biomedical and Life Sciences
University of Glasgow
Glasgow
United Kingdom

Iris F F Benzie

Ageing and Health Section
Faculty of Health and Social Sciences
The Hong Kong Polytechnic University
Kowloon
Hong Kong
China

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 malondialdehyde) 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, bioavailability, 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 >25 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 variation in plasma TAC in healthy young subjects (26 ± 6 y for the fruveg group and 29 ± 8 y for the placebo group).

The lack of observed changes in plasma FRAP and TEAC could also be the result of a decrease in the sensitivity of the TAC measurements as the result of nonoptimized assay techniques. The wavelength used by Dragsted et al to measure both FRAP and TEAC was 620 nm. The correct reference wavelengths are 595 nm for the FRAP assay and 734 nm for the TEAC assay (9, 10). Experiments conducted in our laboratories indicate that measurement at 620 nm results in a decrease in sensitivity of 40% and 66% for TEAC and FRAP, respectively. This is borne out by the uncharacteristically high CVs (16.6% and 8.8%, respectively, for TEAC and FRAP) obtained by Dragsted et al compared with reference studies (9, 10). The difference in vitamin C concentration between the fruveg and the placebo group at the end of the supplementation period was 60 µmol/L (Figure 2 in reference 1). The expected relative difference in TAC, based on the stoichiometry of ascorbic acid, should have been 10% for FRAP (10). This small, but generally discernable, effect on TAC, may have been masked by the reduced sensitivity of the TAC protocols applied in this study.

In conclusion, this interesting and valuable study by Dragsted et al (1) highlights both a requirement for optimized assay conditions and the need to consider the possibility of dynamic mechanisms of control of the body’s redox defenses when designing human intervention studies with dietary antioxidants. Measurements of TAC (the sum of the parts) and of single antioxidants (parts of the sum) are useful biomonitoring tools in supplementation and health-related studies of redox balance. However, an understanding of the physiologic mechanisms of control of the body’s redox defenses is an important issue that must be addressed to clarify the role of dietary antioxidants in disease prevention.

ACKNOWLEDGMENTS

None of the authors had any conflict of interest.

REFERENCES

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