ReviewHealth effects of quercetin: From antioxidant to nutraceutical
Section snippets
Origin and terminology
Free radicals are reactive molecules due to the presence of one or more unpaired electron(s). They are formed in the human body either as an essential mediator in vital processes including neurotransmission and inflammatory reactions, or as a byproduct that does not have a role in the actual process. In aerobic life forms, the reduction of oxygen is of special interest. This reduction comprises binding of most of the oxygen to hydrogen to give water, a process involved in the oxidative
Antioxidants
Fortunately, the human body comprises an elaborate antioxidant defense system to protect cellular compounds from damage induced by free radicals, ROS and other reactive species. An antioxidant has been defined as “any substance that, when present in low concentrations compared to that of an oxidizable substrate, significantly delays or inhibits the oxidation of that substrate” (Sies, 1993, Halliwell, 1995). The antioxidants that react directly with radicals or other reactive species to prevent
Oxidative stress
In normal situations, the endogenous antioxidant network as described earlier provides sufficient protection against reactive species such as ROS and RNS (Bast et al., 1991). However, when an imbalance between the production of and the protection against reactive species occurs in favor of the production, a situation called oxidative stress arises (Fig. 4). Oxidative stress may result in increased oxidative damage and can be caused either by an overproduction of free radicals and ROS or by an
Antioxidant therapy
Due to their ability to scavenge free radicals and reactive species, thereby reducing oxidative stress and associated damage, various health claims have been made regarding the use of exogenous, dietary antioxidants (Halliwell, 1996b, Diplock et al., 1998). As a result, numerous studies have been performed to examine the possible beneficial health effects of antioxidant supplementation. However, most of these studies have been conducted with healthy volunteers, i.e. people with a sufficient
Possible candidates for antioxidant therapy: flavonoids
A group of antioxidants that is often suggested to be good candidates for antioxidant therapy due to their potential role in supporting health are the flavonoids. Flavonoids are a class of naturally occurring polyphenolic compounds, ubiquitously present in photosynthesising cells (Saito, 1974, Salunkhe et al., 1982). Over 5000 different naturally occurring flavonoids have already been identified and the list is still growing (Middleton and Kandaswami, 1993, Shahidi and M., 1995). Flavonoids are
Absorption, metabolism and bio-availability
Because of the hydrophilic character of its glycosides, only quercetin without a sugar group, i.e. the aglycon, was initially suggested to be taken up in the gastro-intestinal tract by passive diffusion (Kuhnau, 1976, Griffiths, 1982). However, a study with human ileostomy volunteers showed not only that quercetin glycosides can indeed be absorbed in the small intestine, but also that this absorption surpasses that of the aglycon by far, i.e. 52% of the glycosides was absorbed versus 24% of the
Conclusions
The flavonoid quercetin has been proven to be an excellent antioxidant that also possesses anti-inflammatory, anti-proliferative and gene expression changing capacities in vitro (Fig. 9). Until now, only its antioxidative and anti-inflammatory effects have been shown in vivo as well. Interestingly, these two effects of quercetin appear to be more pronounced when the basal levels of respectively the occurring oxidative stress and inflammation are high. This indicates that the use of quercetin
Implications
As stated above, the dose-dependent safety of the (long term) use of quercetin in vivo should be examined in more detail before any recommendation regarding the use of this flavonoid as a nutraceutical can be made. Since it can be expected that antioxidant therapy will be mainly applied in patients suffering from chronic diseases that are associated with ongoing damage, chronic use of such supplementation will most likely be required. Up to date, there are no data available regarding the safety
References (195)
- et al.
Effects of alpha-tocopherol and beta-carotene supplements on cancer incidence in the alpha-tocopherol beta-carotene cancer prevention study
Am. J. Clin. Nutr.
(1995) - et al.
Polyphenols and disease risk in epidemiologic studies
Am. J. Clin. Nutr.
(2005) - et al.
A new approach to assess the total antioxidant capacity using the TEAC assay
Food Chem.
(2004) Phagocytes and oxidative stress
Am. J. Med.
(2000)- et al.
Interleukin-8, a chemotactic and inflammatory cytokine
FEBS Lett.
(1992) Is formation of reactive oxygen by cytochrome P450 perilous and predictable?
Trends Pharmacol. Sci.
(1986)- et al.
The toxicity of antioxidants and their metabolites
Environ. Toxicol. Pharmacol.
(2002) - et al.
Oxidative damage shifts from lipid peroxidation to thiol arylation by catechol-containing antioxidants
Biochim. Biophys. Acta.
(2002) - et al.
Oxidized quercetin reacts with thiols rather than with ascorbate: implication for quercetin supplementation
Biochem. Biophys. Res. Commun.
(2003) - et al.
The reversibility of the glutathionyl-quercetin adduct spreads oxidized quercetin-induced toxicity
Biochem. Biophys. Res. Commun.
(2005)