- Top of page
- 1 Introduction
- 2 Materials and methods
- 3 Results
- 4 Discussion
- 5 References
Numerous epidemiological studies have shown several important links between diet and the occurrence of chronic diseases. Evidence is available to link several nutrients, minerals, food groups, and dietary patterns with an increased, or decreased, risk of diseases []. Oxidative stress is involved in the pathogenesis of several chronic diseases []. Oxidative stress
culminates due to an imbalance between prooxidants and antioxidants and a consequent excessive production of reactive oxygen species (ROS), which contribute to oxidative damage of lipids in cellular membranes as well as in proteins and DNA []. Endogenous antioxidant enzymes can prevent intracellular ROS concentrations from reaching levels at which damage occurs. Further, important exogenous dietary antioxidants such as some vitamins, selenium, or phytochemicals are also effective against oxidation and offer protection against oxidative damage and physiological benefit accrues []. However, the evidence from animal and in vitro studies to date is not sufficient to predict such an in vivo protective effect of food or food constituents in humans []. As such, the measurement of biomarkers of oxidative damage can be used to evaluate the effects of various dietary compounds, such as polyphenols.
Polyphenols are a heterogeneous group of naturally occurring molecules distributed in fruits, vegetables, nuts, seeds, wine, tea, cocoa, and coffee. Clinical and epidemiological studies provide evidence that polyphenol-rich foods and polyphenol-rich diets have protective effects against chronic diseases []. Polyphenols have a wide range of biological activities beyond their direct antioxidant activity. Various mechanisms have been proposed to explain their biological activity, including regulation of signal transduction and modulation of redox-sensitive transcription factors []. To establish the health-benefit effects of dietary polyphenol consumption, it is essential to have quantitative information regarding their dietary intake []. A realistic option is to assay total urinary phenols as a first approach to the study of the polyphenol intake-excretion balance. The data may, then, be related to measured physiological antioxidant phenomena and, hence, to the potential health benefits of dietary-derived polyphenols [[8-10]]. The 24 h urinary total polyphenol excretion (TPE) and spot urinary TPE have been validated in intervention studies as biomarkers of dietary polyphenol-rich food ingestion [[8-10]]. The level of protection provided by dietary polyphenols would differ, depending on the population at cardiovascular disease (CVD) risk. However, this has not been well defined in a healthy population nor has the urinary TPE been explored in relation to age.
We hypothesized that polyphenol-rich food consumption results in high levels of urinary TPE which, in turn, can be used as a marker of protection against oxidative damage conferred by polyphenol intake, in healthy participants of different ages. The objective of current study was to assess, in healthy human subjects segregated with respect to age, the relationships between urinary TPE, polyphenol-rich food intake, and biomarkers of oxidative stress.
- Top of page
- 1 Introduction
- 2 Materials and methods
- 3 Results
- 4 Discussion
- 5 References
The present study confirmed our hypothesis that, in healthy subjects of different ages, a high intake of dietary polyphenols would be reflected in a high urinary excretion, which would be related to changes in oxidative stress biomarkers as indicated by a significant inverse association with urinary 8-OHdG excretion and with erythrocyte GSSG concentrations.
Polyphenols can exert their effects, in vitro, by inactivating harmful free radicals, and by chelating divalent metal ions. They are also thought to prevent oxidation of food-associated lipids prior to consumption, which could reduce the pro-oxidant burden. However, it is suggested that polyphenols in vivo do not appear to be present in the circulation at high enough concentrations to contribute significantly to total antioxidant capacity, i.e. they are present in the circulation and tissues only in nano- to low-micromolar ranges and are predominantly present as conjugated metabolites. However, recent studies indicate that polyphenols may induce up-regulation of endogenous antioxidant enzymes in vivo and exert an indirect antioxidant effect. Inductive or signaling effects may occur at concentrations much lower than required for effective radical scavenging [[6, 22]]. Polyphenols are capable of affecting two major redox-sensitive nuclear transcription factors, erythroid 2-related 2 (Nrf2) and Kappa B, which mediate antioxidant and inflammatory signaling [[6, 22, 23]]. Arola-Arnal et al. [], proposed the influence of polyphenols on microRNA expression as a new mechanism of action. Hollman et al. [], suggested that polyphenols should be considered as “versatile bioactive molecules” rather than mere antioxidants, and their role in gene transcriptional regulation need to be explored in more detail.
We observed that urinary 8-OHdG excretion was significantly, and inversely, associated with intake of polyphenols from vegetables and fermented beverages (mainly red wine). Thus, in healthy individuals, ingestion of some protective foods is associated with oxidative biomarker reduction even before these markers become relevant in assessing risk-factor status or in subclinical manifestations of CVD. However, the protective effects of polyphenols from foods would depend on their mechanisms of absorption, bioavailability, and metabolism [[6, 25]].
We need to take into account that assays for measuring oxidative stress have several shortcomings. Measurement of 8-OHdG is the commonest method of assessing DNA damage. Urinary 8-OHdG has been used to assess whole-body DNA damage. Measurements of urinary excretion rates of 8-OHdG alone should be interpreted with caution. Despite these limitations, 8-OHdG continues to be the most frequently studied biomarker in the measurement of endogenous oxidative DNA damage [[26, 27]]. Recently, an interventional study in young adult men showed that a moderate red wine intake significantly decreased 8-OHdG in DNA isolated from peripheral blood leukocytes [[28, 29]]. Di Wang et al. [] also demonstrated that dietary supplementation with polyphenol extract from black tea was, possibly, a useful agent against functional disturbance caused by environmental xenobiotics. The mechanism proposed was through maintaining DNA double-helix architecture and mitigating oxidative stress. They suggested that the most plausible molecular mechanism was that tea polyphenols can up-regulate the activation of Nrf2.
Our present study demonstrated that age is related to changes of certain oxidative stress biomarkers. Urinary 8-OHdG was highest in the youngest subjects, and decreased with increasing age. Tamura et al. [] also observed a significant inverse correlation between urinary 8-OHdG and age. However, their findings were based only on children and adolescents. Our results represent physiological changes associated with normal aging.
The glutathione system is an antioxidant system that helps to protect cells from ROS. We observed that individuals with higher TPE have lower levels of GSSG, which would indicate a greater capacity for detoxification of the glutathione system. In our study, concentrations of erythrocyte GSH also correlated positively and significantly with urinary TPE; albeit not statistically significantly on multivariate regression analysis. Urquiaga et al. [] observed that moderate red wine consumption produced an increase in erythrocyte GSH. Also, Ya-Chen Y et al. [] observed an induction of glutathione synthesis by flavonoids mediated via the Nrf2 pathway and protection against oxidative stress. Using Pearson correlation, we observed an inverse, and significant, association between urinary TPE and plasma FRAP, erythrocyte SOD, erythrocyte, and plasma GPx. In general, the antioxidant defense system seeks to maintain, or restore, redox homeostasis. To this end, endogenous enzymes and exogenous antioxidants function interactively and often synergistically []. The reduction of these enzymes could be a consequence of this attempt to regulate the body's redox balance. The studies of Covas et al. [] and Estruch et al. [] also reported that a steady consumption of a polyphenol-rich diet decreased scavenger enzymes such as SOD and GPx. The implication is that a regular diet rich in exogenous antioxidants could reduce the dependence on endogenous antioxidant defenses, probably by effects at the transcriptome level. Our data provide information on redox homeostatic regulation in a healthy population consuming its habitual diet. This is of considerable interest because, currently, there are data only on populations exposed to a high intake of polyphenols, or with a high degree of associated oxidative stress.
Our study also demonstrated that age, gender, and stress are related to erythrocyte GSSG. Further studies are needed to establish the mechanisms underlying these associations.
In the present study, the mean consumption of total polyphenol from all dietary constituents was estimated as 1564.56 mg GAE/person/day. TPI is documented as being higher than any other known dietary antioxidant, including dietary vitamin C, vitamin E, and carotenoid intake []. TPI, according to the observations of Medina-Remón et al. [], ranged from 122.96 to 3298.17 mg/day in an elderly Mediterranean population considered at high risk of CVD. Also, Saura-Calixto et al. [] estimated the TPI as 1171 mg/person/day in those consuming the Spanish Mediterranean diet. In our study participants, the mean consumption was 350.05 g/day fruits, 263.18 g/day vegetables, 103.00 g/day coffee, and 80.56 g/day fermented beverages and, in this ranking order, constituted the most important contributors to the TPI. Our results are similar to those of Medina-Remón et al. [].
Of considerable note is our novel finding that age is a factor associated with the increase in TPE. This association was also observed by Medina-Remón et al. [] but which had been established in an elderly population. Instead, the population we studied had a wide age range from 18 to 72 years. This enabled us to observe a significant increase in TPE with age. This association could be related to an increase in the consumption of vegetables, fruits, coffee, and moderate intake of fermented beverages with age, which are the greatest contributors to TPI. However, we observed that 40- to 54-year-old age group had a higher consumption of total polyphenol than the 55- to 72-year-old group while the urinary TPE excretion was higher in 55- to 72-year-old group. We observed that the 40- to 54-year-old group consumed more coffee but the 55- to 72-year-old group consumed more fermented beverages. Age alters the metabolism of common dietary polyphenols each of which has differing intrinsic activities, absorption, metabolism, and urinary elimination [].
Our participants are healthy individuals with a low risk of CVD, and represent an optimal sample to identify relationships between polyphenols, diet, and oxidative stress. Such information represents an important phase in the understanding of the role of polyphenols in oxidative stress prevention and in optimizing dietary advice for the general population. Intervention studies would be necessary to confirm these intriguing data.
One limitation of our study is that urinary TPE is an indirect biomarker of overall circulating polyphenol status. Also, the study did not focus on clinical outcomes. Our objective was to assess, in general, the relationships between polyphenol-rich foods and oxidative stress biomarkers. To obtain this insight, we needed to determine the urinary TPE rather than individual polyphenols. However, as we have found associations between food groups and oxidative stress biomarkers, future studies are warranted to determine specific polyphenols and to identify specific biomarkers of each polyphenol-rich food. Further, their roles in signaling pathways need to be identified. Such information would be a valuable in future recommendations for dietary improvements and/or fortified food selection.
In conclusion, urinary TPE increased with age and may reflect an attenuation of oxidative damage. These results could potentially explain the beneficial effects in healthy individuals of a dietary intake rich in vegetables, and moderate red wine; typical food items of the Mediterranean diet.